registration MDL-22662 display explicit error message when ws communication fail...

[moodle.git] / lib / accesslib.php

<?php

// This file is part of Moodle - http://moodle.org/
//
// Moodle is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// Moodle is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with Moodle.  If not, see <http://www.gnu.org/licenses/>.

/**
 * This file contains functions for managing user access
 *
 * <b>Public API vs internals</b>
 *
 * General users probably only care about
 *
 * Context handling
 * - get_context_instance()
 * - get_context_instance_by_id()
 * - get_parent_contexts()
 * - get_child_contexts()
 *
 * Whether the user can do something...
 * - has_capability()
 * - has_any_capability()
 * - has_all_capabilities()
 * - require_capability()
 * - require_login() (from moodlelib)
 *
 * What courses has this user access to?
 * - get_user_courses_bycap()
 *
 * What users can do X in this context?
 * - get_users_by_capability()
 *
 * Enrol/unenrol
 * - enrol_into_course()
 * - role_assign()/role_unassign()
 *
 *
 * Advanced use
 * - load_all_capabilities()
 * - reload_all_capabilities()
 * - has_capability_in_accessdata()
 * - is_siteadmin()
 * - get_user_access_sitewide()
 * - load_subcontext()
 * - get_role_access_bycontext()
 *
 * <b>Name conventions</b>
 *
 * "ctx" means context
 *
 * <b>accessdata</b>
 *
 * Access control data is held in the "accessdata" array
 * which - for the logged-in user, will be in $USER->access
 *
 * For other users can be generated and passed around (but may also be cached
 * against userid in $ACCESSLIB_PRIVATE->accessdatabyuser.
 *
 * $accessdata is a multidimensional array, holding
 * role assignments (RAs), role-capabilities-perm sets
 * (role defs) and a list of courses we have loaded
 * data for.
 *
 * Things are keyed on "contextpaths" (the path field of
 * the context table) for fast walking up/down the tree.
 * <code>
 * $accessdata[ra][$contextpath]= array($roleid)
 *                [$contextpath]= array($roleid)
 *                [$contextpath]= array($roleid)
 * </code>
 *
 * Role definitions are stored like this
 * (no cap merge is done - so it's compact)
 *
 * <code>
 * $accessdata[rdef][$contextpath:$roleid][mod/forum:viewpost] = 1
 *                                        [mod/forum:editallpost] = -1
 *                                        [mod/forum:startdiscussion] = -1000
 * </code>
 *
 * See how has_capability_in_accessdata() walks up/down the tree.
 *
 * Normally - specially for the logged-in user, we only load
 * rdef and ra down to the course level, but not below. This
 * keeps accessdata small and compact. Below-the-course ra/rdef
 * are loaded as needed. We keep track of which courses we
 * have loaded ra/rdef in
 * <code>
 * $accessdata[loaded] = array($contextpath, $contextpath)
 * </code>
 *
 * <b>Stale accessdata</b>
 *
 * For the logged-in user, accessdata is long-lived.
 *
 * On each pageload we load $ACCESSLIB_PRIVATE->dirtycontexts which lists
 * context paths affected by changes. Any check at-or-below
 * a dirty context will trigger a transparent reload of accessdata.
 *
 * Changes at the system level will force the reload for everyone.
 *
 * <b>Default role caps</b>
 * The default role assignment is not in the DB, so we
 * add it manually to accessdata.
 *
 * This means that functions that work directly off the
 * DB need to ensure that the default role caps
 * are dealt with appropriately.
 *
 * @package   moodlecore
 * @copyright 1999 onwards Martin Dougiamas  http://dougiamas.com
 * @license   http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
 */

/** permission definitions */
define('CAP_INHERIT', 0);
/** permission definitions */
define('CAP_ALLOW', 1);
/** permission definitions */
define('CAP_PREVENT', -1);
/** permission definitions */
define('CAP_PROHIBIT', -1000);

/** context definitions */
define('CONTEXT_SYSTEM', 10);
/** context definitions */
define('CONTEXT_USER', 30);
/** context definitions */
define('CONTEXT_COURSECAT', 40);
/** context definitions */
define('CONTEXT_COURSE', 50);
/** context definitions */
define('CONTEXT_MODULE', 70);
/** context definitions */
define('CONTEXT_BLOCK', 80);

/** capability risks - see {@link http://docs.moodle.org/en/Development:Hardening_new_Roles_system} */
define('RISK_MANAGETRUST', 0x0001);
/** capability risks - see {@link http://docs.moodle.org/en/Development:Hardening_new_Roles_system} */
define('RISK_CONFIG',      0x0002);
/** capability risks - see {@link http://docs.moodle.org/en/Development:Hardening_new_Roles_system} */
define('RISK_XSS',         0x0004);
/** capability risks - see {@link http://docs.moodle.org/en/Development:Hardening_new_Roles_system} */
define('RISK_PERSONAL',    0x0008);
/** capability risks - see {@link http://docs.moodle.org/en/Development:Hardening_new_Roles_system} */
define('RISK_SPAM',        0x0010);
/** capability risks - see {@link http://docs.moodle.org/en/Development:Hardening_new_Roles_system} */
define('RISK_DATALOSS',    0x0020);

/** rolename displays - the name as defined in the role definition */
define('ROLENAME_ORIGINAL', 0);
/** rolename displays - the name as defined by a role alias */
define('ROLENAME_ALIAS', 1);
/** rolename displays - Both, like this:  Role alias (Original)*/
define('ROLENAME_BOTH', 2);
/** rolename displays - the name as defined in the role definition and the shortname in brackets*/
define('ROLENAME_ORIGINALANDSHORT', 3);
/** rolename displays - the name as defined by a role alias, in raw form suitable for editing*/
define('ROLENAME_ALIAS_RAW', 4);

/** size limit for context cache */
if (!defined('MAX_CONTEXT_CACHE_SIZE')) {
    define('MAX_CONTEXT_CACHE_SIZE', 5000);
}

/**
 * Although this looks like a global variable, it isn't really.
 *
 * It is just a private implementation detail to accesslib that MUST NOT be used elsewhere.
 * It is used to cache various bits of data between function calls for performance reasons.
 * Sadly, a PHP global variable is the only way to implement this, without rewriting everything
 * as methods of a class, instead of functions.
 *
 * @global stdClass $ACCESSLIB_PRIVATE
 * @name $ACCESSLIB_PRIVATE
 */
$ACCESSLIB_PRIVATE = new stdClass;
$ACCESSLIB_PRIVATE->contexts = array(); // Cache of context objects by level and instance
$ACCESSLIB_PRIVATE->contextsbyid = array(); // Cache of context objects by id
$ACCESSLIB_PRIVATE->systemcontext = NULL; // Used in get_system_context
$ACCESSLIB_PRIVATE->dirtycontexts = NULL; // Dirty contexts cache
$ACCESSLIB_PRIVATE->accessdatabyuser = array(); // Holds the $accessdata structure for users other than $USER
$ACCESSLIB_PRIVATE->roledefinitions = array(); // role definitions cache - helps a lot with mem usage in cron
$ACCESSLIB_PRIVATE->croncache = array(); // Used in get_role_access
$ACCESSLIB_PRIVATE->preloadedcourses = array(); // Used in preload_course_contexts.
$ACCESSLIB_PRIVATE->capabilities = NULL; // detailed information about the capabilities

/**
 * Clears accesslib's private caches. ONLY BE USED BY UNIT TESTS
 *
 * This method should ONLY BE USED BY UNIT TESTS. It clears all of
 * accesslib's private caches. You need to do this before setting up test data,
 * and also at the end of the tests.
 * @global object
 * @global object
 * @global object
 */
function accesslib_clear_all_caches_for_unit_testing() {
    global $UNITTEST, $USER, $ACCESSLIB_PRIVATE;
    if (empty($UNITTEST->running)) {
        throw new coding_exception('You must not call clear_all_caches outside of unit tests.');
    }
    $ACCESSLIB_PRIVATE->contexts = array();
    $ACCESSLIB_PRIVATE->contextsbyid = array();
    $ACCESSLIB_PRIVATE->systemcontext = NULL;
    $ACCESSLIB_PRIVATE->dirtycontexts = NULL;
    $ACCESSLIB_PRIVATE->accessdatabyuser = array();
    $ACCESSLIB_PRIVATE->roledefinitions = array();
    $ACCESSLIB_PRIVATE->croncache = array();
    $ACCESSLIB_PRIVATE->preloadedcourses = array();
    $ACCESSLIB_PRIVATE->capabilities = NULL;

    unset($USER->access);
}

/**
 * Private function. Add a context object to accesslib's caches.
 * @global object
 * @param object $context
 */
function cache_context($context) {
    global $ACCESSLIB_PRIVATE;

    // If there are too many items in the cache already, remove items until
    // there is space
    while (count($ACCESSLIB_PRIVATE->contextsbyid) >= MAX_CONTEXT_CACHE_SIZE) {
        $first = reset($ACCESSLIB_PRIVATE->contextsbyid);
        unset($ACCESSLIB_PRIVATE->contextsbyid[$first->id]);
        unset($ACCESSLIB_PRIVATE->contexts[$first->contextlevel][$first->instanceid]);
    }

    $ACCESSLIB_PRIVATE->contexts[$context->contextlevel][$context->instanceid] = $context;
    $ACCESSLIB_PRIVATE->contextsbyid[$context->id] = $context;
}

/**
 * This is really slow!!! do not use above course context level
 *
 * @global object
 * @param int $roleid
 * @param object $context
 * @return array
 */
function get_role_context_caps($roleid, $context) {
    global $DB;

    //this is really slow!!!! - do not use above course context level!
    $result = array();
    $result[$context->id] = array();

    // first emulate the parent context capabilities merging into context
    $searchcontexts = array_reverse(get_parent_contexts($context));
    array_push($searchcontexts, $context->id);
    foreach ($searchcontexts as $cid) {
        if ($capabilities = $DB->get_records('role_capabilities', array('roleid'=>$roleid, 'contextid'=>$cid))) {
            foreach ($capabilities as $cap) {
                if (!array_key_exists($cap->capability, $result[$context->id])) {
                    $result[$context->id][$cap->capability] = 0;
                }
                $result[$context->id][$cap->capability] += $cap->permission;
            }
        }
    }

    // now go through the contexts bellow given context
    $searchcontexts = array_keys(get_child_contexts($context));
    foreach ($searchcontexts as $cid) {
        if ($capabilities = $DB->get_records('role_capabilities', array('roleid'=>$roleid, 'contextid'=>$cid))) {
            foreach ($capabilities as $cap) {
                if (!array_key_exists($cap->contextid, $result)) {
                    $result[$cap->contextid] = array();
                }
                $result[$cap->contextid][$cap->capability] = $cap->permission;
            }
        }
    }

    return $result;
}

/**
 * Gets the accessdata for role "sitewide" (system down to course)
 *
 * @global object
 * @global object
 * @param int $roleid
 * @param array $accessdata defaults to NULL
 * @return array
 */
function get_role_access($roleid, $accessdata=NULL) {

    global $CFG, $DB;

    /* Get it in 1 cheap DB query...
     * - relevant role caps at the root and down
     *   to the course level - but not below
     */
    if (is_null($accessdata)) {
        $accessdata           = array(); // named list
        $accessdata['ra']     = array();
        $accessdata['rdef']   = array();
        $accessdata['loaded'] = array();
    }

    //
    // Overrides for the role IN ANY CONTEXTS
    // down to COURSE - not below -
    //
    $sql = "SELECT ctx.path,
                   rc.capability, rc.permission
              FROM {context} ctx
              JOIN {role_capabilities} rc
                   ON rc.contextid=ctx.id
             WHERE rc.roleid = ?
                   AND ctx.contextlevel <= ".CONTEXT_COURSE."
          ORDER BY ctx.depth, ctx.path";
    $params = array($roleid);

    // we need extra caching in CLI scripts and cron
    if (CLI_SCRIPT) {
        global $ACCESSLIB_PRIVATE;

        if (!isset($ACCESSLIB_PRIVATE->croncache[$roleid])) {
            $ACCESSLIB_PRIVATE->croncache[$roleid] = array();
            if ($rs = $DB->get_recordset_sql($sql, $params)) {
                foreach ($rs as $rd) {
                    $ACCESSLIB_PRIVATE->croncache[$roleid][] = $rd;
                }
                $rs->close();
            }
        }

        foreach ($ACCESSLIB_PRIVATE->croncache[$roleid] as $rd) {
            $k = "{$rd->path}:{$roleid}";
            $accessdata['rdef'][$k][$rd->capability] = $rd->permission;
        }

    } else {
        if ($rs = $DB->get_recordset_sql($sql, $params)) {
            foreach ($rs as $rd) {
                $k = "{$rd->path}:{$roleid}";
                $accessdata['rdef'][$k][$rd->capability] = $rd->permission;
            }
            unset($rd);
            $rs->close();
        }
    }

    return $accessdata;
}

/**
 * Gets the accessdata for role "sitewide" (system down to course)
 *
 * @global object
 * @global object
 * @param int $roleid
 * @param array $accessdata defaults to NULL
 * @return array
 */
function get_default_frontpage_role_access($roleid, $accessdata=NULL) {

    global $CFG, $DB;

    $frontpagecontext = get_context_instance(CONTEXT_COURSE, SITEID);
    $base = '/'. SYSCONTEXTID .'/'. $frontpagecontext->id;

    //
    // Overrides for the role in any contexts related to the course
    //
    $sql = "SELECT ctx.path,
                   rc.capability, rc.permission
              FROM {context} ctx
              JOIN {role_capabilities} rc
                   ON rc.contextid=ctx.id
             WHERE rc.roleid = ?
                   AND (ctx.id = ".SYSCONTEXTID." OR ctx.path LIKE ?)
                   AND ctx.contextlevel <= ".CONTEXT_COURSE."
          ORDER BY ctx.depth, ctx.path";
    $params = array($roleid, "$base/%");

    if ($rs = $DB->get_recordset_sql($sql, $params)) {
        foreach ($rs as $rd) {
            $k = "{$rd->path}:{$roleid}";
            $accessdata['rdef'][$k][$rd->capability] = $rd->permission;
        }
        unset($rd);
        $rs->close();
    }

    return $accessdata;
}


/**
 * Get the default guest role
 *
 * @global object
 * @global object
 * @return object role
 */
function get_guest_role() {
    global $CFG, $DB;

    if (empty($CFG->guestroleid)) {
        if ($roles = $DB->get_records('role', array('archetype'=>'guest'))) {
            $guestrole = array_shift($roles);   // Pick the first one
            set_config('guestroleid', $guestrole->id);
            return $guestrole;
        } else {
            debugging('Can not find any guest role!');
            return false;
        }
    } else {
        if ($guestrole = $DB->get_record('role', array('id'=>$CFG->guestroleid))) {
            return $guestrole;
        } else {
            //somebody is messing with guest roles, remove incorrect setting and try to find a new one
            set_config('guestroleid', '');
            return get_guest_role();
        }
    }
}

/**
 * Check whether a user has a particular capability in a given context.
 *
 * For example::
 *      $context = get_context_instance(CONTEXT_MODULE, $cm->id);
 *      has_capability('mod/forum:replypost',$context)
 *
 * By default checks the capabilities of the current user, but you can pass a
 * different userid. By default will return true for admin users, but you can override that with the fourth argument.
 *
 * Guest and not-logged-in users can never get any dangerous capability - that is any write capability
 * or capabilities with XSS, config or data loss risks.
 *
 * @param string $capability the name of the capability to check. For example mod/forum:view
 * @param object $context the context to check the capability in. You normally get this with {@link get_context_instance}.
 * @param integer|object $user A user id or object. By default (NULL) checks the permissions of the current user.
 * @param boolean $doanything If false, ignores effect of admin role assignment
 * @return boolean true if the user has this capability. Otherwise false.
 */
function has_capability($capability, $context, $user = NULL, $doanything=true) {
    global $USER, $CFG, $DB, $SCRIPT, $ACCESSLIB_PRIVATE;

    if (during_initial_install()) {
        if ($SCRIPT === "/$CFG->admin/index.php" or $SCRIPT === "/$CFG->admin/cliupgrade.php") {
            // we are in an installer - roles can not work yet
            return true;
        } else {
            return false;
        }
    }

    if (strpos($capability, 'moodle/legacy:') === 0) {
        throw new coding_exception('Legacy capabilities can not be used any more!');
    }

    // the original $CONTEXT here was hiding serious errors
    // for security reasons do not reuse previous context
    if (empty($context)) {
        debugging('Incorrect context specified');
        return false;
    }
    if (!is_bool($doanything)) {
        throw new coding_exception('Capability parameter "doanything" is wierd ("'.$doanything.'"). This has to be fixed in code.');
    }

    // make sure there is a real user specified
    if ($user === NULL) {
        $userid = !empty($USER->id) ? $USER->id : 0;
    } else {
        $userid = !empty($user->id) ? $user->id : $user;
    }

    // capability must exist
    if (!$capinfo = get_capability_info($capability)) {
        debugging('Capability "'.$capability.'" was not found! This should be fixed in code.');
        return false;
    }
    // make sure the guest account and not-logged-in users never get any risky caps no matter what the actual settings are.
    if (($capinfo->captype === 'write') or ((int)$capinfo->riskbitmask & (RISK_XSS | RISK_CONFIG | RISK_DATALOSS))) {
        if (isguestuser($userid) or $userid == 0) {
            return false;
        }
    }

    if (is_null($context->path) or $context->depth == 0) {
        //this should not happen
        $contexts = array(SYSCONTEXTID, $context->id);
        $context->path = '/'.SYSCONTEXTID.'/'.$context->id;
        debugging('Context id '.$context->id.' does not have valid path, please use build_context_path()', DEBUG_DEVELOPER);

    } else {
        $contexts = explode('/', $context->path);
        array_shift($contexts);
    }

    if (CLI_SCRIPT && !isset($USER->access)) {
        // In cron, some modules setup a 'fake' $USER,
        // ensure we load the appropriate accessdata.
        if (isset($ACCESSLIB_PRIVATE->accessdatabyuser[$userid])) {
            $ACCESSLIB_PRIVATE->dirtycontexts = NULL; //load fresh dirty contexts
        } else {
            load_user_accessdata($userid);
            $ACCESSLIB_PRIVATE->dirtycontexts = array();
        }
        $USER->access = $ACCESSLIB_PRIVATE->accessdatabyuser[$userid];

    } else if (isset($USER->id) && ($USER->id == $userid) && !isset($USER->access)) {
        // caps not loaded yet - better to load them to keep BC with 1.8
        // not-logged-in user or $USER object set up manually first time here
        load_all_capabilities();
        $ACCESSLIB_PRIVATE->accessdatabyuser = array(); // reset the cache for other users too, the dirty contexts are empty now
        $ACCESSLIB_PRIVATE->roledefinitions = array();
    }

    // Load dirty contexts list if needed
    if (!isset($ACCESSLIB_PRIVATE->dirtycontexts)) {
        if (isset($USER->access['time'])) {
            $ACCESSLIB_PRIVATE->dirtycontexts = get_dirty_contexts($USER->access['time']);
        }
        else {
            $ACCESSLIB_PRIVATE->dirtycontexts = array();
        }
    }

    // Careful check for staleness...
    if (count($ACCESSLIB_PRIVATE->dirtycontexts) !== 0 and is_contextpath_dirty($contexts, $ACCESSLIB_PRIVATE->dirtycontexts)) {
        // reload all capabilities - preserving loginas, roleswitches, etc
        // and then cleanup any marks of dirtyness... at least from our short
        // term memory! :-)
        $ACCESSLIB_PRIVATE->accessdatabyuser = array();
        $ACCESSLIB_PRIVATE->roledefinitions = array();

        if (CLI_SCRIPT) {
            load_user_accessdata($userid);
            $USER->access = $ACCESSLIB_PRIVATE->accessdatabyuser[$userid];
            $ACCESSLIB_PRIVATE->dirtycontexts = array();

        } else {
            reload_all_capabilities();
        }
    }

    // Find out if user is admin - it is not possible to override the doanything in any way
    // and it is not possible to switch to admin role either.
    if ($doanything) {
        if (is_siteadmin($userid)) {
            return true;
        }
    }

    // divulge how many times we are called
    //// error_log("has_capability: id:{$context->id} path:{$context->path} userid:$userid cap:$capability");

    if (isset($USER->id) && ($USER->id == $userid)) { // we must accept strings and integers in $userid
        //
        // For the logged in user, we have $USER->access
        // which will have all RAs and caps preloaded for
        // course and above contexts.
        //
        // Contexts below courses && contexts that do not
        // hang from courses are loaded into $USER->access
        // on demand, and listed in $USER->access[loaded]
        //
        if ($context->contextlevel <= CONTEXT_COURSE) {
            // Course and above are always preloaded
            return has_capability_in_accessdata($capability, $context, $USER->access);
        }
        // Load accessdata for below-the-course contexts
        if (!path_inaccessdata($context->path,$USER->access)) {
            // error_log("loading access for context {$context->path} for $capability at {$context->contextlevel} {$context->id}");
            // $bt = debug_backtrace();
            // error_log("bt {$bt[0]['file']} {$bt[0]['line']}");
            load_subcontext($USER->id, $context, $USER->access);
        }
        return has_capability_in_accessdata($capability, $context, $USER->access);
    }

    if (!isset($ACCESSLIB_PRIVATE->accessdatabyuser[$userid])) {
        load_user_accessdata($userid);
    }

    if ($context->contextlevel <= CONTEXT_COURSE) {
        // Course and above are always preloaded
        return has_capability_in_accessdata($capability, $context, $ACCESSLIB_PRIVATE->accessdatabyuser[$userid]);
    }
    // Load accessdata for below-the-course contexts as needed
    if (!path_inaccessdata($context->path, $ACCESSLIB_PRIVATE->accessdatabyuser[$userid])) {
        // error_log("loading access for context {$context->path} for $capability at {$context->contextlevel} {$context->id}");
        // $bt = debug_backtrace();
        // error_log("bt {$bt[0]['file']} {$bt[0]['line']}");
        load_subcontext($userid, $context, $ACCESSLIB_PRIVATE->accessdatabyuser[$userid]);
    }
    return has_capability_in_accessdata($capability, $context, $ACCESSLIB_PRIVATE->accessdatabyuser[$userid]);
}

/**
 * Check if the user has any one of several capabilities from a list.
 *
 * This is just a utility method that calls has_capability in a loop. Try to put
 * the capabilities that most users are likely to have first in the list for best
 * performance.
 *
 * There are probably tricks that could be done to improve the performance here, for example,
 * check the capabilities that are already cached first.
 *
 * @see has_capability()
 * @param array $capabilities an array of capability names.
 * @param object $context the context to check the capability in. You normally get this with {@link get_context_instance}.
 * @param integer $userid A user id. By default (NULL) checks the permissions of the current user.
 * @param boolean $doanything If false, ignore effect of admin role assignment
 * @return boolean true if the user has any of these capabilities. Otherwise false.
 */
function has_any_capability($capabilities, $context, $userid=NULL, $doanything=true) {
    if (!is_array($capabilities)) {
        debugging('Incorrect $capabilities parameter in has_any_capabilities() call - must be an array');
        return false;
    }
    foreach ($capabilities as $capability) {
        if (has_capability($capability, $context, $userid, $doanything)) {
            return true;
        }
    }
    return false;
}

/**
 * Check if the user has all the capabilities in a list.
 *
 * This is just a utility method that calls has_capability in a loop. Try to put
 * the capabilities that fewest users are likely to have first in the list for best
 * performance.
 *
 * There are probably tricks that could be done to improve the performance here, for example,
 * check the capabilities that are already cached first.
 *
 * @see has_capability()
 * @param array $capabilities an array of capability names.
 * @param object $context the context to check the capability in. You normally get this with {@link get_context_instance}.
 * @param integer $userid A user id. By default (NULL) checks the permissions of the current user.
 * @param boolean $doanything If false, ignore effect of admin role assignment
 * @return boolean true if the user has all of these capabilities. Otherwise false.
 */
function has_all_capabilities($capabilities, $context, $userid=NULL, $doanything=true) {
    if (!is_array($capabilities)) {
        debugging('Incorrect $capabilities parameter in has_all_capabilities() call - must be an array');
        return false;
    }
    foreach ($capabilities as $capability) {
        if (!has_capability($capability, $context, $userid, $doanything)) {
            return false;
        }
    }
    return true;
}

/**
 * Check if the user is an admin at the site level.
 *
 * Please note that use of proper capabilities is always encouraged,
 * this function is supposed to be used from core or for temporary hacks.
 *
 * @param   int|object  $user_or_id user id or user object
 * @returns bool true if user is one of the administrators, false otherwise
 */
function is_siteadmin($user_or_id = NULL) {
    global $CFG, $USER;

    if ($user_or_id === NULL) {
        $user_or_id = $USER;
    }

    if (empty($user_or_id)) {
        return false;
    }
    if (!empty($user_or_id->id)) {
        // we support
        $userid = $user_or_id->id;
    } else {
        $userid = $user_or_id;
    }

    $siteadmins = explode(',', $CFG->siteadmins);
    return in_array($userid, $siteadmins);
}

/**
 * Returns true if user has at least one role assign
 * of 'coursemanager' role (is potentially listed in some course descriptions).
 * @param $userid
 * @return unknown_type
 */
function has_coursemanager_role($userid) {
    global $DB;

    if (empty($CFG->coursemanager)) {
        return false;
    }
    $sql = "SELECT 1
              FROM {role_assignments}
             WHERE userid = :userid AND roleid IN ($CFG->coursemanager)";
    return $DB->record_exists($sql, array('userid'=>$userid));
}

/**
 * @param string $path
 * @return string
 */
function get_course_from_path($path) {
    // assume that nothing is more than 1 course deep
    if (preg_match('!^(/.+)/\d+$!', $path, $matches)) {
        return $matches[1];
    }
    return false;
}

/**
 * @param string $path
 * @param array $accessdata
 * @return bool
 */
function path_inaccessdata($path, $accessdata) {
    if (empty($accessdata['loaded'])) {
        return false;
    }

    // assume that contexts hang from sys or from a course
    // this will only work well with stuff that hangs from a course
    if (in_array($path, $accessdata['loaded'], true)) {
            // error_log("found it!");
        return true;
    }
    $base = '/' . SYSCONTEXTID;
    while (preg_match('!^(/.+)/\d+$!', $path, $matches)) {
        $path = $matches[1];
        if ($path === $base) {
            return false;
        }
        if (in_array($path, $accessdata['loaded'], true)) {
            return true;
        }
    }
    return false;
}

/**
 * Does the user have a capability to do something?
 *
 * Walk the accessdata array and return true/false.
 * Deals with prohibits, roleswitching, aggregating
 * capabilities, etc.
 *
 * The main feature of here is being FAST and with no
 * side effects.
 *
 * Notes:
 *
 * Switch Roles exits early
 * ------------------------
 * cap checks within a switchrole need to exit early
 * in our bottom up processing so they don't "see" that
 * there are real RAs that can do all sorts of things.
 *
 * Switch Role merges with default role
 * ------------------------------------
 * If you are a teacher in course X, you have at least
 * teacher-in-X + defaultloggedinuser-sitewide. So in the
 * course you'll have techer+defaultloggedinuser.
 * We try to mimic that in switchrole.
 *
 * Permission evaluation
 * ---------------------
 * Originally there was an extremely complicated way
 * to determine the user access that dealt with
 * "locality" or role assignments and role overrides.
 * Now we simply evaluate access for each role separately
 * and then verify if user has at least one role with allow
 * and at the same time no role with prohibit.
 *
 * @param string $capability
 * @param object $context
 * @param array $accessdata
 * @return bool
 */
function has_capability_in_accessdata($capability, $context, array $accessdata) {
    global $CFG;

    if (empty($context->id)) {
        throw new coding_exception('Invalid context specified');
    }

    // Build $paths as a list of current + all parent "paths" with order bottom-to-top
    $contextids = explode('/', trim($context->path, '/'));
    $paths = array($context->path);
    while ($contextids) {
        array_pop($contextids);
        $paths[] = '/' . implode('/', $contextids);
    }
    unset($contextids);

    $roles = array();
    $switchedrole = false;

    // Find out if role switched
    if (!empty($accessdata['rsw'])) {
        // From the bottom up...
        foreach ($paths as $path) {
            if (isset($accessdata['rsw'][$path])) {
                // Found a switchrole assignment - check for that role _plus_ the default user role
                $roles = array($accessdata['rsw'][$path]=>NULL, $CFG->defaultuserroleid=>NULL);
                $switchedrole = true;
                break;
            }
        }
    }

    if (!$switchedrole) {
        // get all users roles in this context and above
        foreach ($paths as $path) {
            if (isset($accessdata['ra'][$path])) {
                foreach ($accessdata['ra'][$path] as $roleid) {
                    $roles[$roleid] = NULL;
                }
            }
        }
    }

    // Now find out what access is given to each role, going bottom-->up direction
    foreach ($roles as $roleid => $ignored) {
        foreach ($paths as $path) {
            if (isset($accessdata['rdef']["{$path}:$roleid"][$capability])) {
                $perm = (int)$accessdata['rdef']["{$path}:$roleid"][$capability];
                if ($perm === CAP_PROHIBIT or is_null($roles[$roleid])) {
                    $roles[$roleid] = $perm;
                }
            }
        }
    }
    // any CAP_PROHIBIT found means no permission for the user
    if (array_search(CAP_PROHIBIT, $roles) !== false) {
        return false;
    }

    // at least one CAP_ALLOW means the user has a permission
    return (array_search(CAP_ALLOW, $roles) !== false);
}

/**
 * @param object $context
 * @param array $accessdata
 * @return array
 */
function aggregate_roles_from_accessdata($context, $accessdata) {

    $path = $context->path;

    // build $contexts as a list of "paths" of the current
    // contexts and parents with the order top-to-bottom
    $contexts = array($path);
    while (preg_match('!^(/.+)/\d+$!', $path, $matches)) {
        $path = $matches[1];
        array_unshift($contexts, $path);
    }

    $cc = count($contexts);

    $roles = array();
    // From the bottom up...
    for ($n=$cc-1;$n>=0;$n--) {
        $ctxp = $contexts[$n];
        if (isset($accessdata['ra'][$ctxp]) && count($accessdata['ra'][$ctxp])) {
            // Found assignments on this leaf
            $addroles = $accessdata['ra'][$ctxp];
            $roles    = array_merge($roles, $addroles);
        }
    }

    return array_unique($roles);
}

/**
 * A convenience function that tests has_capability, and displays an error if
 * the user does not have that capability.
 *
 * NOTE before Moodle 2.0, this function attempted to make an appropriate
 * require_login call before checking the capability. This is no longer the case.
 * You must call require_login (or one of its variants) if you want to check the
 * user is logged in, before you call this function.
 *
 * @see has_capability()
 *
 * @param string $capability the name of the capability to check. For example mod/forum:view
 * @param object $context the context to check the capability in. You normally get this with {@link get_context_instance}.
 * @param integer $userid A user id. By default (NULL) checks the permissions of the current user.
 * @param bool $doanything If false, ignore effect of admin role assignment
 * @param string $errorstring The error string to to user. Defaults to 'nopermissions'.
 * @param string $stringfile The language file to load the error string from. Defaults to 'error'.
 * @return void terminates with an error if the user does not have the given capability.
 */
function require_capability($capability, $context, $userid = NULL, $doanything = true,
                            $errormessage = 'nopermissions', $stringfile = '') {
    if (!has_capability($capability, $context, $userid, $doanything)) {
        throw new required_capability_exception($context, $capability, $errormessage, $stringfile);
    }
}

/**
 * Get an array of courses where cap requested is available
 *
 * Get an array of courses (with magic extra bits)
 * where the accessdata and in DB enrolments show
 * that the cap requested is available.
 *
 * The main use is for get_my_courses().
 *
 * Notes
 *
 * - $fields is an array of fieldnames to ADD
 *   so name the fields you really need, which will
 *   be added and uniq'd
 *
 * - the course records have $c->context which is a fully
 *   valid context object. Saves you a query per course!
 *
 * - the course records have $c->categorypath to make
 *   category lookups cheap
 *
 * - current implementation is split in -
 *
 *   - if the user has the cap systemwide, stupidly
 *     grab *every* course for a capcheck. This eats
 *     a TON of bandwidth, specially on large sites
 *     with separate DBs...
 *
 *   - otherwise, fetch "likely" courses with a wide net
 *     that should get us _cheaply_ at least the courses we need, and some
 *     we won't - we get courses that...
 *      - are in a category where user has the cap
 *      - or where use has a role-assignment (any kind)
 *      - or where the course has an override on for this cap
 *
 *   - walk the courses recordset checking the caps on each one
 *     the checks are all in memory and quite fast
 *     (though we could implement a specialised variant of the
 *     has_capability_in_accessdata() code to speed it up)
 *
 * @global object
 * @global object
 * @param string $capability - name of the capability
 * @param array  $accessdata - accessdata session array
 * @param bool   $doanything_ignored - admin roles are completely ignored here
 * @param string $sort - sorting fields - prefix each fieldname with "c."
 * @param array  $fields - additional fields you are interested in...
 * @param int    $limit  - set if you want to limit the number of courses
 * @return array $courses - ordered array of course objects - see notes above
 */
function get_user_courses_bycap($userid, $cap, $accessdata, $doanything_ignored, $sort='c.sortorder ASC', $fields=NULL, $limit=0) {

    global $CFG, $DB;

    // Slim base fields, let callers ask for what they need...
    $basefields = array('id', 'sortorder', 'shortname', 'idnumber');

    if (!is_null($fields)) {
        $fields = array_merge($basefields, $fields);
        $fields = array_unique($fields);
    } else {
        $fields = $basefields;
    }
    // If any of the fields is '*', leave it alone, discarding the rest
    // to avoid ambiguous columns under some silly DBs. See MDL-18746 :-D
    if (in_array('*', $fields)) {
        $fields = array('*');
    }
    $coursefields = 'c.' .implode(',c.', $fields);

    $sort = trim($sort);
    if ($sort !== '') {
        $sort = "ORDER BY $sort";
    }

    $sysctx = get_context_instance(CONTEXT_SYSTEM);
    if (has_capability_in_accessdata($cap, $sysctx, $accessdata)) {
        //
        // Apparently the user has the cap sitewide, so walk *every* course
        // (the cap checks are moderately fast, but this moves massive bandwidth w the db)
        // Yuck.
        //
        $sql = "SELECT $coursefields,
                       ctx.id AS ctxid, ctx.path AS ctxpath,
                       ctx.depth AS ctxdepth, ctx.contextlevel AS ctxlevel,
                       cc.path AS categorypath
                  FROM {course} c
                  JOIN {course_categories} cc
                       ON c.category=cc.id
                  JOIN {context} ctx
                       ON (c.id=ctx.instanceid AND ctx.contextlevel=".CONTEXT_COURSE.")
                 $sort ";
        $rs = $DB->get_recordset_sql($sql);
    } else {
        //
        // narrow down where we have the caps to a few contexts
        // this will be a combination of
        // - courses    where user has an explicit enrolment
        // - courses    that have an override (any status) on that capability
        // - categories where user has the rights (granted status) on that capability
        //
        $sql = "SELECT ctx.*
                  FROM {context} ctx
                 WHERE ctx.contextlevel=".CONTEXT_COURSECAT."
              ORDER BY ctx.depth";
        $rs = $DB->get_recordset_sql($sql);
        $catpaths = array();
        foreach ($rs as $catctx) {
            if ($catctx->path != ''
                && has_capability_in_accessdata($cap, $catctx, $accessdata)) {
                $catpaths[] = $catctx->path;
            }
        }
        $rs->close();
        $catclause = '';
        $params = array();
        if (count($catpaths)) {
            $cc = count($catpaths);
            for ($n=0;$n<$cc;$n++) {
                $catpaths[$n] = "ctx.path LIKE '{$catpaths[$n]}/%'";
            }
            $catclause = 'WHERE (' . implode(' OR ', $catpaths) .')';
        }
        unset($catpaths);

        /// UNION 3 queries:
        /// - user role assignments in courses
        /// - user capability (override - any status) in courses
        /// - user right (granted status) in categories (optionally executed)
        /// Enclosing the 3-UNION into an inline_view to avoid column names conflict and making the ORDER BY cross-db
        /// and to allow selection of TEXT columns in the query (MSSQL and Oracle limitation). MDL-16209
        $sql = "
            SELECT $coursefields, ctxid, ctxpath, ctxdepth, ctxlevel, ctxinstance, categorypath
              FROM (
                    SELECT c.id,
                           ctx.id AS ctxid, ctx.path AS ctxpath,
                           ctx.depth AS ctxdepth, ctx.contextlevel AS ctxlevel, ctx.instanceid AS ctxinstance,
                           cc.path AS categorypath
                    FROM {course} c
                    JOIN {course_categories} cc
                      ON c.category=cc.id
                    JOIN {context} ctx
                      ON (c.id=ctx.instanceid AND ctx.contextlevel=".CONTEXT_COURSE.")
                    JOIN {role_assignments} ra
                      ON (ra.contextid=ctx.id AND ra.userid=:userid)
                    UNION
                    SELECT c.id,
                           ctx.id AS ctxid, ctx.path AS ctxpath,
                           ctx.depth AS ctxdepth, ctx.contextlevel AS ctxlevel, ctx.instanceid AS ctxinstance,
                           cc.path AS categorypath
                    FROM {course} c
                    JOIN {course_categories} cc
                      ON c.category=cc.id
                    JOIN {context} ctx
                      ON (c.id=ctx.instanceid AND ctx.contextlevel=".CONTEXT_COURSE.")
                    JOIN {role_capabilities} rc
                      ON (rc.contextid=ctx.id AND (rc.capability=:cap)) ";

        if (!empty($catclause)) { /// If we have found the right in categories, add child courses here too
            $sql .= "
                    UNION
                    SELECT c.id,
                           ctx.id AS ctxid, ctx.path AS ctxpath,
                           ctx.depth AS ctxdepth, ctx.contextlevel AS ctxlevel, ctx.instanceid AS ctxinstance,
                           cc.path AS categorypath
                    FROM {course} c
                    JOIN {course_categories} cc
                      ON c.category=cc.id
                    JOIN {context} ctx
                      ON (c.id=ctx.instanceid AND ctx.contextlevel=".CONTEXT_COURSE.")
                    $catclause";
        }

    /// Close the inline_view and join with courses table to get requested $coursefields
        $sql .= "
                ) inline_view
                INNER JOIN {course} c
                    ON inline_view.id = c.id";

    /// To keep cross-db we need to strip any prefix in the ORDER BY clause for queries using UNION
        $sql .= "
                " . preg_replace('/[a-z]+\./i', '', $sort); /// Add ORDER BY clause

        $params['userid'] = $userid;
        $params['cap']    = $cap;
        $rs = $DB->get_recordset_sql($sql, $params);
    }

/// Confirm rights (granted capability) for each course returned
    $courses = array();
    $cc = 0; // keep count
    if ($rs) {
        foreach ($rs as $c) {
            // build the context obj
            context_instance_preload($c);
            $context = get_context_instance(CONTEXT_COURSE, $c->id);

            if (has_capability_in_accessdata($cap, $context, $accessdata)) {
                if ($limit > 0 && $cc >= $limit) {
                    break;
                }

                $courses[] = $c;
                $cc++;
            }
        }
        $rs->close();
    }

    return $courses;
}


/**
 * Return a nested array showing role assignments
 * all relevant role capabilities for the user at
 * site/metacourse/course_category/course levels
 *
 * We do _not_ delve deeper than courses because the number of
 * overrides at the module/block levels is HUGE.
 *
 * [ra]   => [/path/][]=roleid
 * [rdef] => [/path/:roleid][capability]=permission
 * [loaded] => array('/path', '/path')
 *
 * @global object
 * @global object
 * @param $userid integer - the id of the user
 */
function get_user_access_sitewide($userid) {

    global $CFG, $DB;

    /* Get in 3 cheap DB queries...
     * - role assignments
     * - relevant role caps
     *   - above and within this user's RAs
     *   - below this user's RAs - limited to course level
     */

    $accessdata           = array(); // named list
    $accessdata['ra']     = array();
    $accessdata['rdef']   = array();
    $accessdata['loaded'] = array();

    //
    // Role assignments
    //
    $sql = "SELECT ctx.path, ra.roleid
              FROM {role_assignments} ra
              JOIN {context} ctx ON ctx.id=ra.contextid
             WHERE ra.userid = ? AND ctx.contextlevel <= ".CONTEXT_COURSE;
    $params = array($userid);
    $rs = $DB->get_recordset_sql($sql, $params);

    //
    // raparents collects paths & roles we need to walk up
    // the parenthood to build the rdef
    //
    $raparents = array();
    if ($rs) {
        foreach ($rs as $ra) {
            // RAs leafs are arrays to support multi
            // role assignments...
            if (!isset($accessdata['ra'][$ra->path])) {
                $accessdata['ra'][$ra->path] = array();
            }
            array_push($accessdata['ra'][$ra->path], $ra->roleid);

            // Concatenate as string the whole path (all related context)
            // for this role. This is damn faster than using array_merge()
            // Will unique them later
            if (isset($raparents[$ra->roleid])) {
                $raparents[$ra->roleid] .= $ra->path;
            } else {
                $raparents[$ra->roleid] = $ra->path;
            }
        }
        unset($ra);
        $rs->close();
    }

    // Walk up the tree to grab all the roledefs
    // of interest to our user...
    //
    // NOTE: we use a series of IN clauses here - which
    // might explode on huge sites with very convoluted nesting of
    // categories... - extremely unlikely that the number of categories
    // and roletypes is so large that we hit the limits of IN()
    $clauses = '';
    $cparams = array();
    foreach ($raparents as $roleid=>$strcontexts) {
        $contexts = implode(',', array_unique(explode('/', trim($strcontexts, '/'))));
        if ($contexts ==! '') {
            if ($clauses) {
                $clauses .= ' OR ';
            }
            $clauses .= "(roleid=? AND contextid IN ($contexts))";
            $cparams[] = $roleid;
        }
    }

    if ($clauses !== '') {
        $sql = "SELECT ctx.path, rc.roleid, rc.capability, rc.permission
                FROM {role_capabilities} rc
                JOIN {context} ctx
                  ON rc.contextid=ctx.id
                WHERE $clauses";

        unset($clauses);
        $rs = $DB->get_recordset_sql($sql, $cparams);

        if ($rs) {
            foreach ($rs as $rd) {
                $k = "{$rd->path}:{$rd->roleid}";
                $accessdata['rdef'][$k][$rd->capability] = $rd->permission;
            }
            unset($rd);
            $rs->close();
        }
    }

    //
    // Overrides for the role assignments IN SUBCONTEXTS
    // (though we still do _not_ go below the course level.
    //
    // NOTE that the JOIN w sctx is with 3-way triangulation to
    // catch overrides to the applicable role in any subcontext, based
    // on the path field of the parent.
    //
    $sql = "SELECT sctx.path, ra.roleid,
                   ctx.path AS parentpath,
                   rco.capability, rco.permission
              FROM {role_assignments} ra
              JOIN {context} ctx
                   ON ra.contextid=ctx.id
              JOIN {context} sctx
                   ON (sctx.path LIKE " . $DB->sql_concat('ctx.path',"'/%'"). " )
              JOIN {role_capabilities} rco
                   ON (rco.roleid=ra.roleid AND rco.contextid=sctx.id)
             WHERE ra.userid = ?
               AND ctx.contextlevel <= ".CONTEXT_COURSECAT."
               AND sctx.contextlevel <= ".CONTEXT_COURSE."
          ORDER BY sctx.depth, sctx.path, ra.roleid";
    $params = array($userid);
    $rs = $DB->get_recordset_sql($sql, $params);
    if ($rs) {
        foreach ($rs as $rd) {
            $k = "{$rd->path}:{$rd->roleid}";
            $accessdata['rdef'][$k][$rd->capability] = $rd->permission;
        }
        unset($rd);
        $rs->close();
    }
    return $accessdata;
}

/**
 * Add to the access ctrl array the data needed by a user for a given context
 *
 * @global object
 * @global object
 * @param integer $userid the id of the user
 * @param object $context needs path!
 * @param array $accessdata accessdata array
 */
function load_subcontext($userid, $context, &$accessdata) {

    global $CFG, $DB;

    /* Get the additional RAs and relevant rolecaps
     * - role assignments - with role_caps
     * - relevant role caps
     *   - above this user's RAs
     *   - below this user's RAs - limited to course level
     */

    $base = "/" . SYSCONTEXTID;

    //
    // Replace $context with the target context we will
    // load. Normally, this will be a course context, but
    // may be a different top-level context.
    //
    // We have 3 cases
    //
    // - Course
    // - BLOCK/PERSON/USER/COURSE(sitecourse) hanging from SYSTEM
    // - BLOCK/MODULE/GROUP hanging from a course
    //
    // For course contexts, we _already_ have the RAs
    // but the cost of re-fetching is minimal so we don't care.
    //
    if ($context->contextlevel !== CONTEXT_COURSE
        && $context->path !== "$base/{$context->id}") {
        // Case BLOCK/MODULE/GROUP hanging from a course
        // Assumption: the course _must_ be our parent
        // If we ever see stuff nested further this needs to
        // change to do 1 query over the exploded path to
        // find out which one is the course
        $courses = explode('/',get_course_from_path($context->path));
        $targetid = array_pop($courses);
        $context = get_context_instance_by_id($targetid);

    }

    //
    // Role assignments in the context and below
    //
    $sql = "SELECT ctx.path, ra.roleid
              FROM {role_assignments} ra
              JOIN {context} ctx
                   ON ra.contextid=ctx.id
             WHERE ra.userid = ?
                   AND (ctx.path = ? OR ctx.path LIKE ?)
          ORDER BY ctx.depth, ctx.path, ra.roleid";
    $params = array($userid, $context->path, $context->path."/%");
    $rs = $DB->get_recordset_sql($sql, $params);

    //
    // Read in the RAs, preventing duplicates
    //
    if ($rs) {
        $localroles = array();
        $lastseen  = '';
        foreach ($rs as $ra) {
            if (!isset($accessdata['ra'][$ra->path])) {
                $accessdata['ra'][$ra->path] = array();
            }
            // only add if is not a repeat caused
            // by capability join...
            // (this check is cheaper than in_array())
            if ($lastseen !== $ra->path.':'.$ra->roleid) {
                $lastseen = $ra->path.':'.$ra->roleid;
                array_push($accessdata['ra'][$ra->path], $ra->roleid);
                array_push($localroles,           $ra->roleid);
            }
        }
        $rs->close();
    }

    //
    // Walk up and down the tree to grab all the roledefs
    // of interest to our user...
    //
    // NOTES
    // - we use IN() but the number of roles is very limited.
    //
    $courseroles    = aggregate_roles_from_accessdata($context, $accessdata);

    // Do we have any interesting "local" roles?
    $localroles = array_diff($localroles,$courseroles); // only "new" local roles
    $wherelocalroles='';
    if (count($localroles)) {
        // Role defs for local roles in 'higher' contexts...
        $contexts = substr($context->path, 1); // kill leading slash
        $contexts = str_replace('/', ',', $contexts);
        $localroleids = implode(',',$localroles);
        $wherelocalroles="OR (rc.roleid IN ({$localroleids})
                              AND ctx.id IN ($contexts))" ;
    }

    // We will want overrides for all of them
    $whereroles = '';
    if ($roleids  = implode(',',array_merge($courseroles,$localroles))) {
        $whereroles = "rc.roleid IN ($roleids) AND";
    }
    $sql = "SELECT ctx.path, rc.roleid, rc.capability, rc.permission
              FROM {role_capabilities} rc
              JOIN {context} ctx
                   ON rc.contextid=ctx.id
             WHERE ($whereroles
                    (ctx.id=? OR ctx.path LIKE ?))
                   $wherelocalroles
          ORDER BY ctx.depth ASC, ctx.path DESC, rc.roleid ASC ";
    $params = array($context->id, $context->path."/%");

    $newrdefs = array();
    if ($rs = $DB->get_recordset_sql($sql, $params)) {
        foreach ($rs as $rd) {
            $k = "{$rd->path}:{$rd->roleid}";
            if (!array_key_exists($k, $newrdefs)) {
                $newrdefs[$k] = array();
            }
            $newrdefs[$k][$rd->capability] = $rd->permission;
        }
        $rs->close();
    } else {
        debugging('Bad SQL encountered!');
    }

    compact_rdefs($newrdefs);
    foreach ($newrdefs as $key=>$value) {
        $accessdata['rdef'][$key] =& $newrdefs[$key];
    }

    // error_log("loaded {$context->path}");
    $accessdata['loaded'][] = $context->path;
}

/**
 * Add to the access ctrl array the data needed by a role for a given context.
 *
 * The data is added in the rdef key.
 *
 * This role-centric function is useful for role_switching
 * and to get an overview of what a role gets under a
 * given context and below...
 *
 * @global object
 * @global object
 * @param integer $roleid the id of the user
 * @param object $context needs path!
 * @param array $accessdata accessdata array NULL by default
 * @return array
 */
function get_role_access_bycontext($roleid, $context, $accessdata=NULL) {

    global $CFG, $DB;

    /* Get the relevant rolecaps into rdef
     * - relevant role caps
     *   - at ctx and above
     *   - below this ctx
     */

    if (is_null($accessdata)) {
        $accessdata           = array(); // named list
        $accessdata['ra']     = array();
        $accessdata['rdef']   = array();
        $accessdata['loaded'] = array();
    }

    $contexts = substr($context->path, 1); // kill leading slash
    $contexts = str_replace('/', ',', $contexts);

    //
    // Walk up and down the tree to grab all the roledefs
    // of interest to our role...
    //
    // NOTE: we use an IN clauses here - which
    // might explode on huge sites with very convoluted nesting of
    // categories... - extremely unlikely that the number of nested
    // categories is so large that we hit the limits of IN()
    //
    $sql = "SELECT ctx.path, rc.capability, rc.permission
              FROM {role_capabilities} rc
              JOIN {context} ctx
                   ON rc.contextid=ctx.id
             WHERE rc.roleid=? AND
                   ( ctx.id IN ($contexts) OR
                    ctx.path LIKE ? )
          ORDER BY ctx.depth ASC, ctx.path DESC, rc.roleid ASC ";
    $params = array($roleid, $context->path."/%");

    if ($rs = $DB->get_recordset_sql($sql, $params)) {
        foreach ($rs as $rd) {
            $k = "{$rd->path}:{$roleid}";
            $accessdata['rdef'][$k][$rd->capability] = $rd->permission;
        }
        $rs->close();
    }

    return $accessdata;
}

/**
 * Load accessdata for a user into the $ACCESSLIB_PRIVATE->accessdatabyuser global
 *
 * Used by has_capability() - but feel free
 * to call it if you are about to run a BIG
 * cron run across a bazillion users.
 *
 * @global object
 * @global object
 * @param int $userid
 * @return array returns ACCESSLIB_PRIVATE->accessdatabyuser[userid]
 */
function load_user_accessdata($userid) {
    global $CFG, $ACCESSLIB_PRIVATE;

    $base = '/'.SYSCONTEXTID;

    $accessdata = get_user_access_sitewide($userid);
    $frontpagecontext = get_context_instance(CONTEXT_COURSE, SITEID);
    //
    // provide "default role" & set 'dr'
    //
    if (!empty($CFG->defaultuserroleid)) {
        $accessdata = get_role_access($CFG->defaultuserroleid, $accessdata);
        if (!isset($accessdata['ra'][$base])) {
            $accessdata['ra'][$base] = array($CFG->defaultuserroleid);
        } else {
            array_push($accessdata['ra'][$base], $CFG->defaultuserroleid);
        }
        $accessdata['dr'] = $CFG->defaultuserroleid;
    }

    //
    // provide "default frontpage role"
    //
    if (!empty($CFG->defaultfrontpageroleid)) {
        $base = '/'. SYSCONTEXTID .'/'. $frontpagecontext->id;
        $accessdata = get_default_frontpage_role_access($CFG->defaultfrontpageroleid, $accessdata);
        if (!isset($accessdata['ra'][$base])) {
            $accessdata['ra'][$base] = array($CFG->defaultfrontpageroleid);
        } else {
            array_push($accessdata['ra'][$base], $CFG->defaultfrontpageroleid);
        }
    }
    // for dirty timestamps in cron
    $accessdata['time'] = time();

    $ACCESSLIB_PRIVATE->accessdatabyuser[$userid] = $accessdata;
    compact_rdefs($ACCESSLIB_PRIVATE->accessdatabyuser[$userid]['rdef']);

    return $ACCESSLIB_PRIVATE->accessdatabyuser[$userid];
}

/**
 * Use shared copy of role definitions stored in ACCESSLIB_PRIVATE->roledefinitions;
 *
 * @global object
 * @param array $rdefs array of role definitions in contexts
 */
function compact_rdefs(&$rdefs) {
    global $ACCESSLIB_PRIVATE;

    /*
     * This is a basic sharing only, we could also
     * use md5 sums of values. The main purpose is to
     * reduce mem in cron jobs - many users in $ACCESSLIB_PRIVATE->accessdatabyuser array.
     */

    foreach ($rdefs as $key => $value) {
        if (!array_key_exists($key, $ACCESSLIB_PRIVATE->roledefinitions)) {
            $ACCESSLIB_PRIVATE->roledefinitions[$key] = $rdefs[$key];
        }
        $rdefs[$key] =& $ACCESSLIB_PRIVATE->roledefinitions[$key];
    }
}

/**
 * A convenience function to completely load all the capabilities
 * for the current user.   This is what gets called from complete_user_login()
 * for example. Call it only _after_ you've setup $USER and called
 * check_enrolment_plugins();
 * @see check_enrolment_plugins()
 *
 * @global object
 * @global object
 * @global object
 */
function load_all_capabilities() {
    global $USER, $CFG, $ACCESSLIB_PRIVATE;

    // roles not installed yet - we are in the middle of installation
    if (during_initial_install()) {
        return;
    }

    $base = '/'.SYSCONTEXTID;

    if (isguestuser()) {
        $guest = get_guest_role();

        // Load the rdefs
        $USER->access = get_role_access($guest->id);
        // Put the ghost enrolment in place...
        $USER->access['ra'][$base] = array($guest->id);


    } else if (isloggedin()) {

        $accessdata = get_user_access_sitewide($USER->id);

        //
        // provide "default role" & set 'dr'
        //
        if (!empty($CFG->defaultuserroleid)) {
            $accessdata = get_role_access($CFG->defaultuserroleid, $accessdata);
            if (!isset($accessdata['ra'][$base])) {
                $accessdata['ra'][$base] = array($CFG->defaultuserroleid);
            } else {
                array_push($accessdata['ra'][$base], $CFG->defaultuserroleid);
            }
            $accessdata['dr'] = $CFG->defaultuserroleid;
        }

        $frontpagecontext = get_context_instance(CONTEXT_COURSE, SITEID);

        //
        // provide "default frontpage role"
        //
        if (!empty($CFG->defaultfrontpageroleid)) {
            $base = '/'. SYSCONTEXTID .'/'. $frontpagecontext->id;
            $accessdata = get_default_frontpage_role_access($CFG->defaultfrontpageroleid, $accessdata);
            if (!isset($accessdata['ra'][$base])) {
                $accessdata['ra'][$base] = array($CFG->defaultfrontpageroleid);
            } else {
                array_push($accessdata['ra'][$base], $CFG->defaultfrontpageroleid);
            }
        }
        $USER->access = $accessdata;

    } else if (!empty($CFG->notloggedinroleid)) {
        $USER->access = get_role_access($CFG->notloggedinroleid);
        $USER->access['ra'][$base] = array($CFG->notloggedinroleid);
    }

    // Timestamp to read dirty context timestamps later
    $USER->access['time'] = time();
    $ACCESSLIB_PRIVATE->dirtycontexts = array();

    // Clear to force a refresh
    unset($USER->mycourses);
}

/**
 * A convenience function to completely reload all the capabilities
 * for the current user when roles have been updated in a relevant
 * context -- but PRESERVING switchroles and loginas.
 *
 * That is - completely transparent to the user.
 *
 * Note: rewrites $USER->access completely.
 *
 * @global object
 * @global object
 */
function reload_all_capabilities() {
    global $USER, $DB;

    // error_log("reloading");
    // copy switchroles
    $sw = array();
    if (isset($USER->access['rsw'])) {
        $sw = $USER->access['rsw'];
        // error_log(print_r($sw,1));
    }

    unset($USER->access);
    unset($USER->mycourses);

    load_all_capabilities();

    foreach ($sw as $path => $roleid) {
        $context = $DB->get_record('context', array('path'=>$path));
        role_switch($roleid, $context);
    }

}

/**
 * Adds a temp role to an accessdata array.
 *
 * Useful for the "temporary guest" access
 * we grant to logged-in users.
 *
 * Note - assumes a course context!
 *
 * @global object
 * @global object
 * @param object $content
 * @param int $roleid
 * @param array $accessdata
 * @return array Returns access data
 */
function load_temp_role($context, $roleid, $accessdata) {

    global $CFG, $DB;

    //
    // Load rdefs for the role in -
    // - this context
    // - all the parents
    // - and below - IOWs overrides...
    //

    // turn the path into a list of context ids
    $contexts = substr($context->path, 1); // kill leading slash
    $contexts = str_replace('/', ',', $contexts);

    $sql = "SELECT ctx.path, rc.capability, rc.permission
              FROM {context} ctx
              JOIN {role_capabilities} rc
                   ON rc.contextid=ctx.id
             WHERE (ctx.id IN ($contexts)
                    OR ctx.path LIKE ?)
                   AND rc.roleid = ?
          ORDER BY ctx.depth, ctx.path";
    $params = array($context->path."/%", $roleid);
    if ($rs = $DB->get_recordset_sql($sql, $params)) {
        foreach ($rs as $rd) {
            $k = "{$rd->path}:{$roleid}";
            $accessdata['rdef'][$k][$rd->capability] = $rd->permission;
        }
        $rs->close();
    }

    //
    // Say we loaded everything for the course context
    // - which we just did - if the user gets a proper
    // RA in this session, this data will need to be reloaded,
    // but that is handled by the complete accessdata reload
    //
    array_push($accessdata['loaded'], $context->path);

    //
    // Add the ghost RA
    //
    if (isset($accessdata['ra'][$context->path])) {
        array_push($accessdata['ra'][$context->path], $roleid);
    } else {
        $accessdata['ra'][$context->path] = array($roleid);
    }

    return $accessdata;
}


/**
 * Check all the login enrolment information for the given user object
 * by querying the enrolment plugins
 *
 * @global object
 * @param object $user
 * @return void
 */
function check_enrolment_plugins(&$user) {
    global $CFG;

    if (empty($user->id) or isguestuser($user)) {
        // shortcut - there is no enrolment work for guests and not-logged-in users
        return;
    }

    static $inprogress = array();  // To prevent this function being called more than once in an invocation

    if (!empty($inprogress[$user->id])) {
        return;
    }

    $inprogress[$user->id] = true;  // Set the flag

    require_once($CFG->dirroot .'/enrol/enrol.class.php');

    if (!($plugins = explode(',', $CFG->enrol_plugins_enabled))) {
        $plugins = array($CFG->enrol);
    }

    foreach ($plugins as $plugin) {
        $enrol = enrolment_factory::factory($plugin);
        if (method_exists($enrol, 'setup_enrolments')) {  /// Plugin supports Roles (Moodle 1.7 and later)
            $enrol->setup_enrolments($user);
        } else {                                          /// Run legacy enrolment methods
            if (method_exists($enrol, 'get_student_courses')) {
                $enrol->get_student_courses($user);
            }
            if (method_exists($enrol, 'get_teacher_courses')) {
                $enrol->get_teacher_courses($user);
            }

        /// deal with $user->students and $user->teachers stuff
            unset($user->student);
            unset($user->teacher);
        }
        unset($enrol);
    }

    unset($inprogress[$user->id]);  // Unset the flag
}

/**
 * Returns array of all role archetypes.
 *
 * @return array
 */
function get_role_archetypes() {
    return array(
        'manager'        => 'manager',
        'coursecreator'  => 'coursecreator',
        'editingteacher' => 'editingteacher',
        'teacher'        => 'teacher',
        'student'        => 'student',
        'guest'          => 'guest',
        'user'           => 'user',
        'frontpage'      => 'frontpage'
    );
}

/**
 * Assign the defaults found in this capability definition to roles that have
 * the corresponding legacy capabilities assigned to them.
 *
 * @param string $capability
 * @param array $legacyperms an array in the format (example):
 *                      'guest' => CAP_PREVENT,
 *                      'student' => CAP_ALLOW,
 *                      'teacher' => CAP_ALLOW,
 *                      'editingteacher' => CAP_ALLOW,
 *                      'coursecreator' => CAP_ALLOW,
 *                      'manager' => CAP_ALLOW
 * @return boolean success or failure.
 */
function assign_legacy_capabilities($capability, $legacyperms) {

    $archetypes = get_role_archetypes();

    foreach ($legacyperms as $type => $perm) {

        $systemcontext = get_context_instance(CONTEXT_SYSTEM);
        if ($type === 'admin') {
            debugging('Legacy type admin in access.php was renamed to manager, please update the code.');
            $type = 'manager';
        }

        if (!array_key_exists($type, $archetypes)) {
            print_error('invalidlegacy', '', '', $type);
        }

        if ($roles = get_archetype_roles($type)) {
            foreach ($roles as $role) {
                // Assign a site level capability.
                if (!assign_capability($capability, $perm, $role->id, $systemcontext->id)) {
                    return false;
                }
            }
        }
    }
    return true;
}

/**
 * @param object $capability a capability - a row from the capabilities table.
 * @return boolean whether this capability is safe - that is, whether people with the
 *      safeoverrides capability should be allowed to change it.
 */
function is_safe_capability($capability) {
    return !((RISK_DATALOSS | RISK_MANAGETRUST | RISK_CONFIG | RISK_XSS | RISK_PERSONAL) & $capability->riskbitmask);
}

/**********************************
 * Context Manipulation functions *
 **********************************/

/**
 * Context creation - internal implementation.
 *
 * Create a new context record for use by all roles-related stuff
 * assumes that the caller has done the homework.
 *
 * DO NOT CALL THIS DIRECTLY, instead use {@link get_context_instance}!
 *
 * @param int $contextlevel
 * @param int $instanceid
 * @param int $strictness
 * @return object newly created context
 */
function create_context($contextlevel, $instanceid, $strictness=IGNORE_MISSING) {

    global $CFG, $DB;

    if ($contextlevel == CONTEXT_SYSTEM) {
        return create_system_context();
    }

    $context = new object();
    $context->contextlevel = $contextlevel;
    $context->instanceid = $instanceid;

    // Define $context->path based on the parent
    // context. In other words... Who is your daddy?
    $basepath  = '/' . SYSCONTEXTID;
    $basedepth = 1;

    $result = true;
    $error_message = NULL;

    switch ($contextlevel) {
        case CONTEXT_COURSECAT:
            $sql = "SELECT ctx.path, ctx.depth
                      FROM {context}           ctx
                      JOIN {course_categories} cc
                           ON (cc.parent=ctx.instanceid AND ctx.contextlevel=".CONTEXT_COURSECAT.")
                     WHERE cc.id=?";
            $params = array($instanceid);
            if ($p = $DB->get_record_sql($sql, $params)) {
                $basepath  = $p->path;
                $basedepth = $p->depth;
            } else if ($category = $DB->get_record('course_categories', array('id'=>$instanceid), '*', $strictness)) {
                if (empty($category->parent)) {
                    // ok - this is a top category
                } else if ($parent = get_context_instance(CONTEXT_COURSECAT, $category->parent)) {
                    $basepath  = $parent->path;
                    $basedepth = $parent->depth;
                } else {
                    // wrong parent category - no big deal, this can be fixed later
                    $basepath  = NULL;
                    $basedepth = 0;
                }
            } else {
                // incorrect category id
                $error_message = "incorrect course category id ($instanceid)";
                $result = false;
            }
            break;

        case CONTEXT_COURSE:
            $sql = "SELECT ctx.path, ctx.depth
                      FROM {context} ctx
                      JOIN {course}  c
                           ON (c.category=ctx.instanceid AND ctx.contextlevel=".CONTEXT_COURSECAT.")
                     WHERE c.id=? AND c.id !=" . SITEID;
            $params = array($instanceid);
            if ($p = $DB->get_record_sql($sql, $params)) {
                $basepath  = $p->path;
                $basedepth = $p->depth;
            } else if ($course = $DB->get_record('course', array('id'=>$instanceid), '*', $strictness)) {
                if ($course->id == SITEID) {
                    //ok - no parent category
                } else if ($parent = get_context_instance(CONTEXT_COURSECAT, $course->category)) {
                    $basepath  = $parent->path;
                    $basedepth = $parent->depth;
                } else {
                    // wrong parent category of course - no big deal, this can be fixed later
                    $basepath  = NULL;
                    $basedepth = 0;
                }
            } else if ($instanceid == SITEID) {
                // no errors for missing site course during installation
                return false;
            } else {
                // incorrect course id
                $error_message = "incorrect course id ($instanceid)";
                $result = false;
            }
            break;

        case CONTEXT_MODULE:
            $sql = "SELECT ctx.path, ctx.depth
                      FROM {context}        ctx
                      JOIN {course_modules} cm
                           ON (cm.course=ctx.instanceid AND ctx.contextlevel=".CONTEXT_COURSE.")
                     WHERE cm.id=?";
            $params = array($instanceid);
            if ($p = $DB->get_record_sql($sql, $params)) {
                $basepath  = $p->path;
                $basedepth = $p->depth;
            } else if ($cm = $DB->get_record('course_modules', array('id'=>$instanceid), '*', $strictness)) {
                if ($parent = get_context_instance(CONTEXT_COURSE, $cm->course, $strictness)) {
                    $basepath  = $parent->path;
                    $basedepth = $parent->depth;
                } else {
                    // course does not exist - modules can not exist without a course
                    $error_message = "course does not exist ($cm->course) - modules can not exist without a course";
                    $result = false;
                }
            } else {
                // cm does not exist
                $error_message = "cm with id $instanceid does not exist";
                $result = false;
            }
            break;

        case CONTEXT_BLOCK:
            $sql = "SELECT ctx.path, ctx.depth
                      FROM {context} ctx
                      JOIN {block_instances} bi ON (bi.parentcontextid=ctx.id)
                     WHERE bi.id = ?";
            $params = array($instanceid, CONTEXT_COURSE);
            if ($p = $DB->get_record_sql($sql, $params, '*', $strictness)) {
                $basepath  = $p->path;
                $basedepth = $p->depth;
            } else {
                // block does not exist
                $error_message = 'block or parent context does not exist';
                $result = false;
            }
            break;
        case CONTEXT_USER:
            // default to basepath
            break;
    }

    // if grandparents unknown, maybe rebuild_context_path() will solve it later
    if ($basedepth != 0) {
        $context->depth = $basedepth+1;
    }

    if (!$result) {
        debugging('Error: could not insert new context level "'.
                  s($contextlevel).'", instance "'.
                  s($instanceid).'". ' . $error_message);

        return false;
    }

    $id = $DB->insert_record('context', $context);
    // can't set the full path till we know the id!
    if ($basedepth != 0 and !empty($basepath)) {
        $DB->set_field('context', 'path', $basepath.'/'. $id, array('id'=>$id));
    }
    return get_context_instance_by_id($id);
}

/**
 * Returns system context or NULL if can not be created yet.
 *
 * @todo can not use get_record() because we do not know if query failed :-(
 * switch to get_record() later
 *
 * @global object
 * @global object
 * @param bool $cache use caching
 * @return mixed system context or NULL
 */
function get_system_context($cache=true) {
    global $DB, $ACCESSLIB_PRIVATE;
    if ($cache and defined('SYSCONTEXTID')) {
        if (is_null($ACCESSLIB_PRIVATE->systemcontext)) {
            $ACCESSLIB_PRIVATE->systemcontext = new object();
            $ACCESSLIB_PRIVATE->systemcontext->id           = SYSCONTEXTID;
            $ACCESSLIB_PRIVATE->systemcontext->contextlevel = CONTEXT_SYSTEM;
            $ACCESSLIB_PRIVATE->systemcontext->instanceid   = 0;
            $ACCESSLIB_PRIVATE->systemcontext->path         = '/'.SYSCONTEXTID;
            $ACCESSLIB_PRIVATE->systemcontext->depth        = 1;
        }
        return $ACCESSLIB_PRIVATE->systemcontext;
    }
    try {
        $context = $DB->get_record('context', array('contextlevel'=>CONTEXT_SYSTEM));
    } catch (dml_exception $e) {
        //table does not exist yet, sorry
        return NULL;
    }

    if (!$context) {
        $context = new object();
        $context->contextlevel = CONTEXT_SYSTEM;
        $context->instanceid   = 0;
        $context->depth        = 1;
        $context->path         = NULL; //not known before insert

        try {
            $context->id = $DB->insert_record('context', $context);
        } catch (dml_exception $e) {
            // can not create context yet, sorry
            return NULL;
        }
    }

    if (!isset($context->depth) or $context->depth != 1 or $context->instanceid != 0 or $context->path != '/'.$context->id) {
        $context->instanceid   = 0;
        $context->path         = '/'.$context->id;
        $context->depth        = 1;
        $DB->update_record('context', $context);
    }

    if (!defined('SYSCONTEXTID')) {
        define('SYSCONTEXTID', $context->id);
    }

    $ACCESSLIB_PRIVATE->systemcontext = $context;
    return $ACCESSLIB_PRIVATE->systemcontext;
}

/**
 * Remove a context record and any dependent entries,
 * removes context from static context cache too
 *
 * @param int $level
 * @param int $instanceid
 * @return bool returns true or throws an exception
 */
function delete_context($contextlevel, $instanceid) {
    global $DB, $ACCESSLIB_PRIVATE, $CFG;

    // do not use get_context_instance(), because the related object might not exist,
    // or the context does not exist yet and it would be created now
    if ($context = $DB->get_record('context', array('contextlevel'=>$contextlevel, 'instanceid'=>$instanceid))) {
        $DB->delete_records('role_assignments', array('contextid'=>$context->id));
        $DB->delete_records('role_capabilities', array('contextid'=>$context->id));
        $DB->delete_records('context', array('id'=>$context->id));
        $DB->delete_records('role_names', array('contextid'=>$context->id));

        // delete all files attached to this context
        $fs = get_file_storage();
        $fs->delete_area_files($context->id);

        // do not mark dirty contexts if parents unknown
        if (!is_null($context->path) and $context->depth > 0) {
            mark_context_dirty($context->path);
        }

        // purge static context cache if entry present
        unset($ACCESSLIB_PRIVATE->contexts[$contextlevel][$instanceid]);
        unset($ACCESSLIB_PRIVATE->contextsbyid[$context->id]);

        blocks_delete_all_for_context($context->id);
        filter_delete_all_for_context($context->id);
    }

    return true;
}

/**
 * Precreates all contexts including all parents
 *
 * @global object
 * @param int $contextlevel empty means all
 * @param bool $buildpaths update paths and depths
 * @return void
 */
function create_contexts($contextlevel=NULL, $buildpaths=true) {
    global $DB;

    //make sure system context exists
    $syscontext = get_system_context(false);

    if (empty($contextlevel) or $contextlevel == CONTEXT_COURSECAT
                             or $contextlevel == CONTEXT_COURSE
                             or $contextlevel == CONTEXT_MODULE
                             or $contextlevel == CONTEXT_BLOCK) {
        $sql = "INSERT INTO {context} (contextlevel, instanceid)
                SELECT ".CONTEXT_COURSECAT.", cc.id
                  FROM {course}_categories cc
                 WHERE NOT EXISTS (SELECT 'x'
                                     FROM {context} cx
                                    WHERE cc.id = cx.instanceid AND cx.contextlevel=".CONTEXT_COURSECAT.")";
        $DB->execute($sql);

    }

    if (empty($contextlevel) or $contextlevel == CONTEXT_COURSE
                             or $contextlevel == CONTEXT_MODULE
                             or $contextlevel == CONTEXT_BLOCK) {
        $sql = "INSERT INTO {context} (contextlevel, instanceid)
                SELECT ".CONTEXT_COURSE.", c.id
                  FROM {course} c
                 WHERE NOT EXISTS (SELECT 'x'
                                     FROM {context} cx
                                    WHERE c.id = cx.instanceid AND cx.contextlevel=".CONTEXT_COURSE.")";
        $DB->execute($sql);

    }

    if (empty($contextlevel) or $contextlevel == CONTEXT_MODULE
                             or $contextlevel == CONTEXT_BLOCK) {
        $sql = "INSERT INTO {context} (contextlevel, instanceid)
                SELECT ".CONTEXT_MODULE.", cm.id
                  FROM {course}_modules cm
                 WHERE NOT EXISTS (SELECT 'x'
                                     FROM {context} cx
                                    WHERE cm.id = cx.instanceid AND cx.contextlevel=".CONTEXT_MODULE.")";
        $DB->execute($sql);
    }

    if (empty($contextlevel) or $contextlevel == CONTEXT_USER
                             or $contextlevel == CONTEXT_BLOCK) {
        $sql = "INSERT INTO {context} (contextlevel, instanceid)
                SELECT ".CONTEXT_USER.", u.id
                  FROM {user} u
                 WHERE u.deleted=0
                   AND NOT EXISTS (SELECT 'x'
                                     FROM {context} cx
                                    WHERE u.id = cx.instanceid AND cx.contextlevel=".CONTEXT_USER.")";
        $DB->execute($sql);

    }

    if (empty($contextlevel) or $contextlevel == CONTEXT_BLOCK) {
        $sql = "INSERT INTO {context} (contextlevel, instanceid)
                SELECT ".CONTEXT_BLOCK.", bi.id
                  FROM {block_instances} bi
                 WHERE NOT EXISTS (SELECT 'x'
                                     FROM {context} cx
                                    WHERE bi.id = cx.instanceid AND cx.contextlevel=".CONTEXT_BLOCK.")";
        $DB->execute($sql);
    }

    if ($buildpaths) {
        build_context_path(false);
    }
}

/**
 * Remove stale context records
 *
 * @global object
 * @return bool
 */
function cleanup_contexts() {
    global $DB;

    $sql = "  SELECT c.contextlevel,
                     c.instanceid AS instanceid
                FROM {context} c
                LEFT OUTER JOIN {course}_categories t
                     ON c.instanceid = t.id
               WHERE t.id IS NULL AND c.contextlevel = ".CONTEXT_COURSECAT."
            UNION
              SELECT c.contextlevel,
                     c.instanceid
                FROM {context} c
                LEFT OUTER JOIN {course} t
                     ON c.instanceid = t.id
               WHERE t.id IS NULL AND c.contextlevel = ".CONTEXT_COURSE."
            UNION
              SELECT c.contextlevel,
                     c.instanceid
                FROM {context} c
                LEFT OUTER JOIN {course}_modules t
                     ON c.instanceid = t.id
               WHERE t.id IS NULL AND c.contextlevel = ".CONTEXT_MODULE."
            UNION
              SELECT c.contextlevel,
                     c.instanceid
                FROM {context} c
                LEFT OUTER JOIN {user} t
                     ON c.instanceid = t.id
               WHERE t.id IS NULL AND c.contextlevel = ".CONTEXT_USER."
            UNION
              SELECT c.contextlevel,
                     c.instanceid
                FROM {context} c
                LEFT OUTER JOIN {block_instances} t
                     ON c.instanceid = t.id
               WHERE t.id IS NULL AND c.contextlevel = ".CONTEXT_BLOCK."
           ";

    // transactions used only for performance reasons here
    $transaction = $DB->start_delegated_transaction();

    if ($rs = $DB->get_recordset_sql($sql)) {
        foreach ($rs as $ctx) {
            delete_context($ctx->contextlevel, $ctx->instanceid);
        }
        $rs->close();
    }

    $transaction->allow_commit();
    return true;
}

/**
 * Preloads all contexts relating to a course: course, modules. Block contexts
 * are no longer loaded here. The contexts for all the blocks on the current
 * page are now efficiently loaded by {@link block_manager::load_blocks()}.
 *
 * @param int $courseid Course ID
 * @return void
 */
function preload_course_contexts($courseid) {
    global $DB, $ACCESSLIB_PRIVATE;

    // Users can call this multiple times without doing any harm
    global $ACCESSLIB_PRIVATE;
    if (array_key_exists($courseid, $ACCESSLIB_PRIVATE->preloadedcourses)) {
        return;
    }

    $params = array($courseid, $courseid, $courseid);
    $sql = "SELECT x.instanceid, x.id, x.contextlevel, x.path, x.depth
              FROM {course_modules} cm
              JOIN {context} x ON x.instanceid=cm.id
             WHERE cm.course=? AND x.contextlevel=".CONTEXT_MODULE."

         UNION ALL

            SELECT x.instanceid, x.id, x.contextlevel, x.path, x.depth
              FROM {context} x
             WHERE x.instanceid=? AND x.contextlevel=".CONTEXT_COURSE."";

    $rs = $DB->get_recordset_sql($sql, $params);
    foreach($rs as $context) {
        cache_context($context);
    }
    $rs->close();
    $ACCESSLIB_PRIVATE->preloadedcourses[$courseid] = true;
}

/**
 * Get the context instance as an object. This function will create the
 * context instance if it does not exist yet.
 *
 * @todo Remove code branch from previous fix MDL-9016 which is no longer needed
 *
 * @param integer $level The context level, for example CONTEXT_COURSE, or CONTEXT_MODULE.
 * @param integer $instance The instance id. For $level = CONTEXT_COURSE, this would be $course->id,
 *      for $level = CONTEXT_MODULE, this would be $cm->id. And so on. Defaults to 0
 * @param int $strictness IGNORE_MISSING means compatible mode, false returned if record not found, debug message if more found;
 *      MUST_EXIST means throw exception if no record or multiple records found
 * @return object The context object.
 */
function get_context_instance($contextlevel, $instance=0, $strictness=IGNORE_MISSING) {

    global $DB, $ACCESSLIB_PRIVATE;
    static $allowed_contexts = array(CONTEXT_SYSTEM, CONTEXT_USER, CONTEXT_COURSECAT, CONTEXT_COURSE, CONTEXT_MODULE, CONTEXT_BLOCK);

    if ($contextlevel === 'clearcache') {
        // TODO: Remove for v2.0
        // No longer needed, but we'll catch it to avoid erroring out on custom code.
        // This used to be a fix for MDL-9016
        // "Restoring into existing course, deleting first
        //  deletes context and doesn't recreate it"
        return false;
    }

/// System context has special cache
    if ($contextlevel == CONTEXT_SYSTEM) {
        return get_system_context();
    }

/// check allowed context levels
    if (!in_array($contextlevel, $allowed_contexts)) {
        // fatal error, code must be fixed - probably typo or switched parameters
        print_error('invalidcourselevel');
    }

    if (!is_array($instance)) {
    /// Check the cache
        if (isset($ACCESSLIB_PRIVATE->contexts[$contextlevel][$instance])) {  // Already cached
            return $ACCESSLIB_PRIVATE->contexts[$contextlevel][$instance];
        }

    /// Get it from the database, or create it
        if (!$context = $DB->get_record('context', array('contextlevel'=>$contextlevel, 'instanceid'=>$instance))) {
            $context = create_context($contextlevel, $instance, $strictness);
        }

    /// Only add to cache if context isn't empty.
        if (!empty($context)) {
            cache_context($context);
        }

        return $context;
    }


/// ok, somebody wants to load several contexts to save some db queries ;-)
    $instances = $instance;
    $result = array();

    foreach ($instances as $key=>$instance) {
    /// Check the cache first
        if (isset($ACCESSLIB_PRIVATE->contexts[$contextlevel][$instance])) {  // Already cached
            $result[$instance] = $ACCESSLIB_PRIVATE->contexts[$contextlevel][$instance];
            unset($instances[$key]);
            continue;
        }
    }

    if ($instances) {
        list($instanceids, $params) = $DB->get_in_or_equal($instances, SQL_PARAMS_QM);
        array_unshift($params, $contextlevel);
        $sql = "SELECT instanceid, id, contextlevel, path, depth
                  FROM {context}
                 WHERE contextlevel=? AND instanceid $instanceids";

        if (!$contexts = $DB->get_records_sql($sql, $params)) {
            $contexts = array();
        }

        foreach ($instances as $instance) {
            if (isset($contexts[$instance])) {
                $context = $contexts[$instance];
            } else {
                $context = create_context($contextlevel, $instance);
            }

            if (!empty($context)) {
                cache_context($context);
            }

            $result[$instance] = $context;
        }
    }

    return $result;
}


/**
 * Get a context instance as an object, from a given context id.
 *
 * @param mixed $id a context id or array of ids.
 * @param int $strictness IGNORE_MISSING means compatible mode, false returned if record not found, debug message if more found;
 *                        MUST_EXIST means throw exception if no record or multiple records found
 * @return mixed object, array of the context object, or false.
 */
function get_context_instance_by_id($id, $strictness=IGNORE_MISSING) {
    global $DB, $ACCESSLIB_PRIVATE;

    if ($id == SYSCONTEXTID) {
        return get_system_context();
    }

    if (isset($ACCESSLIB_PRIVATE->contextsbyid[$id])) {  // Already cached
        return $ACCESSLIB_PRIVATE->contextsbyid[$id];
    }

    if ($context = $DB->get_record('context', array('id'=>$id), '*', $strictness)) {
        cache_context($context);
        return $context;
    }

    return false;
}


/**
 * Get the local override (if any) for a given capability in a role in a context
 *
 * @global object
 * @param int $roleid
 * @param int $contextid
 * @param string $capability
 */
function get_local_override($roleid, $contextid, $capability) {
    global $DB;
    return $DB->get_record('role_capabilities', array('roleid'=>$roleid, 'capability'=>$capability, 'contextid'=>$contextid));
}

/**
 * Returns context instance plus related course and cm instances
 * @param int $contextid
 * @return array of ($context, $course, $cm)
 */
function get_context_info_array($contextid) {
    global $DB;

    $context = get_context_instance_by_id($contextid, MUST_EXIST);
    $course  = NULL;
    $cm      = NULL;

    if ($context->contextlevel == CONTEXT_COURSE) {
        $course = $DB->get_record('course', array('id'=>$context->instanceid), '*', MUST_EXIST);

    } else if ($context->contextlevel == CONTEXT_MODULE) {
        $cm = get_coursemodule_from_id('', $context->instanceid, 0, false, MUST_EXIST);
        $course = $DB->get_record('course', array('id'=>$cm->course), '*', MUST_EXIST);

    } else if ($context->contextlevel == CONTEXT_BLOCK) {
        $parentcontexts = get_parent_contexts($context, false);
        $parent = reset($parentcontexts);
        $parent = get_context_instance_by_id($parent);

        if ($parent->contextlevel == CONTEXT_COURSE) {
            $course = $DB->get_record('course', array('id'=>$parent->instanceid), '*', MUST_EXIST);
        } else if ($parent->contextlevel == CONTEXT_MODULE) {
            $cm = get_coursemodule_from_id('', $parent->instanceid, 0, false, MUST_EXIST);
            $course = $DB->get_record('course', array('id'=>$cm->course), '*', MUST_EXIST);
        }
    }

    return array($context, $course, $cm);
}


//////////////////////////////////////
//    DB TABLE RELATED FUNCTIONS    //
//////////////////////////////////////

/**
 * function that creates a role
 *
 * @global object
 * @param string $name role name
 * @param string $shortname role short name
 * @param string $description role description
 * @param string $archetype
 * @return mixed id or dml_exception
 */
function create_role($name, $shortname, $description, $archetype='') {
    global $DB;

    if (strpos($archetype, 'moodle/legacy:') !== false) {
        throw new coding_exception('Use new role archetype parameter in create_role() instead of old legacy capabilities.');
    }

    // verify role archetype actually exists
    $archetypes = get_role_archetypes();
    if (empty($archetypes[$archetype])) {
        $archetype = '';
    }

    // Get the system context.
    $context = get_context_instance(CONTEXT_SYSTEM);

    // Insert the role record.
    $role = new object();
    $role->name        = $name;
    $role->shortname   = $shortname;
    $role->description = $description;
    $role->archetype   = $archetype;

    //find free sortorder number
    $role->sortorder = $DB->get_field('role', 'MAX(sortorder) + 1', array());
    if (empty($role->sortorder)) {
        $role->sortorder = 1;
    }
    $id = $DB->insert_record('role', $role);

    return $id;
}

/**
 * Function that deletes a role and cleanups up after it
 *
 * @param int $roleid id of role to delete
 * @return bool always true
 */
function delete_role($roleid) {
    global $CFG, $DB;

    // first unssign all users
    role_unassign($roleid);

    // cleanup all references to this role, ignore errors
    $DB->delete_records('role_capabilities',   array('roleid'=>$roleid));
    $DB->delete_records('role_allow_assign',   array('roleid'=>$roleid));
    $DB->delete_records('role_allow_assign',   array('allowassign'=>$roleid));
    $DB->delete_records('role_allow_override', array('roleid'=>$roleid));
    $DB->delete_records('role_allow_override', array('allowoverride'=>$roleid));
    $DB->delete_records('role_names',          array('roleid'=>$roleid));
    $DB->delete_records('role_context_levels', array('roleid'=>$roleid));

    // finally delete the role itself
    // get this before the name is gone for logging
    $rolename = $DB->get_field('role', 'name', array('id'=>$roleid));

    $DB->delete_records('role', array('id'=>$roleid));

    add_to_log(SITEID, 'role', 'delete', 'admin/roles/action=delete&roleid='.$roleid, $rolename, '');

    return true;
}

/**
 * Function to write context specific overrides, or default capabilities.
 *
 * @global object
 * @global object
 * @param string module string name
 * @param string capability string name
 * @param int contextid context id
 * @param int roleid role id
 * @param int permission int 1,-1 or -1000 should not be writing if permission is 0
 * @return bool
 */
function assign_capability($capability, $permission, $roleid, $contextid, $overwrite=false) {
    global $USER, $DB;

    if (empty($permission) || $permission == CAP_INHERIT) { // if permission is not set
        unassign_capability($capability, $roleid, $contextid);
        return true;
    }

    $existing = $DB->get_record('role_capabilities', array('contextid'=>$contextid, 'roleid'=>$roleid, 'capability'=>$capability));

    if ($existing and !$overwrite) {   // We want to keep whatever is there already
        return true;
    }

    $cap = new object;
    $cap->contextid = $contextid;
    $cap->roleid = $roleid;
    $cap->capability = $capability;
    $cap->permission = $permission;
    $cap->timemodified = time();
    $cap->modifierid = empty($USER->id) ? 0 : $USER->id;

    if ($existing) {
        $cap->id = $existing->id;
        $DB->update_record('role_capabilities', $cap);
    } else {
        $c = $DB->get_record('context', array('id'=>$contextid));
        $DB->insert_record('role_capabilities', $cap);
    }
    return true;
}

/**
 * Unassign a capability from a role.
 *
 * @global object
 * @param int $roleid the role id
 * @param string $capability the name of the capability
 * @return boolean success or failure
 */
function unassign_capability($capability, $roleid, $contextid=NULL) {
    global $DB;

    if (!empty($contextid)) {
        // delete from context rel, if this is the last override in this context
        $DB->delete_records('role_capabilities', array('capability'=>$capability, 'roleid'=>$roleid, 'contextid'=>$contextid));
    } else {
        $DB->delete_records('role_capabilities', array('capability'=>$capability, 'roleid'=>$roleid));
    }
    return true;
}


/**
 * Get the roles that have a given capability assigned to it
 * Get the roles that have a given capability assigned to it. This function
 * does not resolve the actual permission of the capability. It just checks
 * for assignment only.
 *
 * @global object
 * @global object
 * @param string $capability - capability name (string)
 * @param string $permission - optional, the permission defined for this capability
 *                      either CAP_ALLOW, CAP_PREVENT or CAP_PROHIBIT. Defaults to NULL
 * @param object $contect
 * @return mixed array or role objects
 */
function get_roles_with_capability($capability, $permission=NULL, $context=NULL) {
    global $CFG, $DB;

    $params = array();

    if ($context) {
        if ($contexts = get_parent_contexts($context)) {
            $listofcontexts = '('.implode(',', $contexts).')';
        } else {
            $sitecontext = get_context_instance(CONTEXT_SYSTEM);
            $listofcontexts = '('.$sitecontext->id.')'; // must be site
        }
        $contextstr = "AND (rc.contextid = ? OR  rc.contextid IN $listofcontexts)";
        $params[] = $context->id;
    } else {
        $contextstr = '';
    }

    $selectroles = "SELECT r.*
                      FROM {role} r,
                           {role_capabilities} rc
                     WHERE rc.capability = ?
                           AND rc.roleid = r.id $contextstr";

    array_unshift($params, $capability);

    if (isset($permission)) {
        $selectroles .= " AND rc.permission = ?";
        $params[] = $permission;
    }
    return $DB->get_records_sql($selectroles, $params);
}


/**
 * This function makes a role-assignment (a role for a user or group in a particular context)
 *
 * @param int $roleid the role of the id
 * @param int $userid userid
 * @param int $groupid_ignored - never implemented!
 * @param int $contextid id of the context
 * @param int $timestart time this assignment becomes effective defaults to 0
 * @param int $timeend time this assignment ceases to be effective defaults to 0
 * @param int $hidden_ignored - use roels with moodle/course:view capability or enrolment instead
 * @param string $enrol defaults to 'manual'
 * @param string $timemodified defaults to ''
 * @return int new id of the assignment
 */
function role_assign($roleid, $userid, $groupid_ignored, $contextid, $timestart=0, $timeend=0, $hidden_ignored=0, $enrol='manual',$timemodified='') {
    global $USER, $CFG, $DB;

/// Do some data validation

    if (empty($roleid)) {
        debugging('Role ID not provided');
        return false;
    }

    if (empty($userid)) {
        debugging('Userid or groupid must be provided');
        return false;
    }

    if ($userid && !$DB->record_exists('user', array('id'=>$userid))) {
        debugging('User ID '.intval($userid).' does not exist!');
        return false;
    }

    if (!$context = get_context_instance_by_id($contextid)) {
        debugging('Context ID '.intval($contextid).' does not exist!');
        return false;
    }

    if (($timestart and $timeend) and ($timestart > $timeend)) {
        debugging('The end time can not be earlier than the start time');
        return false;
    }

    if (!$timemodified) {
        $timemodified = time();
    }

/// Check for existing entry
    $ra = $DB->get_record('role_assignments', array('roleid'=>$roleid, 'contextid'=>$context->id, 'userid'=>$userid));

    if (empty($ra)) {             // Create a new entry
        $ra = new object();
        $ra->roleid = $roleid;
        $ra->contextid = $context->id;
        $ra->userid = $userid;
        $ra->enrol = $enrol;
    /// Always round timestart downto 100 secs to help DBs to use their own caching algorithms
    /// by repeating queries with the same exact parameters in a 100 secs time window
        $ra->timestart = round($timestart, -2);
        $ra->timeend = $timeend;
        $ra->timemodified = $timemodified;
        $ra->modifierid = empty($USER->id) ? 0 : $USER->id;

        $ra->id = $DB->insert_record('role_assignments', $ra);

    } else {                      // We already have one, just update it
        $ra->id = $ra->id;
        $ra->enrol = $enrol;
    /// Always round timestart downto 100 secs to help DBs to use their own caching algorithms
    /// by repeating queries with the same exact parameters in a 100 secs time window
        $ra->timestart = round($timestart, -2);
        $ra->timeend = $timeend;
        $ra->timemodified = $timemodified;
        $ra->modifierid = empty($USER->id) ? 0 : $USER->id;

        $DB->update_record('role_assignments', $ra);
    }

/// mark context as dirty - modules might use has_capability() in xxx_role_assing()
/// again expensive, but needed
    mark_context_dirty($context->path);

    if (!empty($USER->id) && $USER->id == $userid) {
/// If the user is the current user, then do full reload of capabilities too.
        load_all_capabilities();
    }

/// Ask all the modules if anything needs to be done for this user
    $mods = get_plugin_list('mod');
    foreach ($mods as $mod => $moddir) {
        include_once($moddir.'/lib.php');
        $functionname = $mod.'_role_assign';
        if (function_exists($functionname)) {
            $functionname($userid, $context, $roleid);
        }
    }

    /// now handle metacourse role assignments if in course context
    if ($context->contextlevel == CONTEXT_COURSE) {
        if ($parents = $DB->get_records('course_meta', array('child_course'=>$context->instanceid))) {
            foreach ($parents as $parent) {
                sync_metacourse($parent->parent_course);
            }
        }
    }

    events_trigger('role_assigned', $ra);

    return $ra->id;
}


/**
 * Deletes one or more role assignments.   You must specify at least one parameter.
 *
 * @global object
 * @global object
 * @global object
 * @param int $roleid defaults to 0
 * @param int $userid defaults to 0
 * @param int $groupid_ignored never implemented
 * @param int $contextid defaults to 0
 * @param mixed $enrol unassign only if enrolment type matches, NULL means anything. Defaults to NULL
 * @return boolean success or failure
 */
function role_unassign($roleid=0, $userid=0, $groupid_ignored=0, $contextid=0, $enrol=NULL) {

    global $USER, $CFG, $DB;
    require_once($CFG->dirroot.'/group/lib.php');

    $args = array('roleid', 'userid', 'contextid');
    $select = array();
    $params = array();

    foreach ($args as $arg) {
        if ($$arg) {
            $select[] = "$arg = ?";
            $params[] = $$arg;
        }
    }
    if (!empty($enrol)) {
        $select[] = "enrol=?";
        $params[] = $enrol;
    }

    if ($select) {
        if ($ras = $DB->get_records_select('role_assignments', implode(' AND ', $select), $params)) {
            $mods = get_plugin_list('mod');
            foreach($ras as $ra) {
                /// infinite loop protection when deleting recursively
                if (!$ra = $DB->get_record('role_assignments', array('id'=>$ra->id))) {
                    continue;
                }
                $DB->delete_records('role_assignments', array('id'=>$ra->id));

                if (!$context = get_context_instance_by_id($ra->contextid)) {
                    // strange error, not much to do
                    continue;
                }

                /* mark contexts as dirty here, because we need the refreshed
                 * caps bellow to delete group membership and user_lastaccess!
                 * and yes, this is very expensive for bulk operations :-(
                 */
                mark_context_dirty($context->path);

                /// If the user is the current user, then do full reload of capabilities too.
                if (!empty($USER->id) && $USER->id == $ra->userid) {
                    load_all_capabilities();
                }

                /// Ask all the modules if anything needs to be done for this user
                foreach ($mods as $mod=>$moddir) {
                    include_once($moddir.'/lib.php');
                    $functionname = $mod.'_role_unassign';
                    if (function_exists($functionname)) {
                        $functionname($ra->userid, $context); // watch out, $context might be NULL if something goes wrong
                    }
                }

                /// now handle metacourse role unassigment and removing from groups if in course context
                if ($context->contextlevel == CONTEXT_COURSE) {

                    // cleanup leftover course groups/subscriptions etc when user has
                    // no capability to view course
                    // this may be slow, but this is the proper way of doing it
                    if (!has_capability('moodle/course:participate', $context, $ra->userid)) {
                        // remove from groups
                        groups_delete_group_members($context->instanceid, $ra->userid);

                        // delete lastaccess records
                        $DB->delete_records('user_lastaccess', array('userid'=>$ra->userid, 'courseid'=>$context->instanceid));
                    }

                    //unassign roles in metacourses if needed
                    if ($parents = $DB->get_records('course_meta', array('child_course'=>$context->instanceid))) {
                        foreach ($parents as $parent) {
                            sync_metacourse($parent->parent_course);
                        }
                    }
                }

                events_trigger('role_unassigned', $ra);
            }
        }
    }

    return true;
}

/**
 * Enrol someone without using the default role in a course
 *
 * A convenience function to take care of the common case where you
 * just want to enrol someone using the default role into a course
 *
 * @param object $course
 * @param object $user
 * @param string $enrol the plugin used to do this enrolment
 * @return bool
 */
function enrol_into_course($course, $user, $enrol) {

    $timestart = time();
    // remove time part from the timestamp and keep only the date part
    $timestart = make_timestamp(date('Y', $timestart), date('m', $timestart), date('d', $timestart), 0, 0, 0);
    if ($course->enrolperiod) {
        $timeend = $timestart + $course->enrolperiod;
    } else {
        $timeend = 0;
    }

    if ($role = get_default_course_role($course)) {

        $context = get_context_instance(CONTEXT_COURSE, $course->id);

        if (!role_assign($role->id, $user->id, 0, $context->id, $timestart, $timeend, 0, $enrol)) {
            return false;
        }

        // force accessdata refresh for users visiting this context...
        mark_context_dirty($context->path);

        email_welcome_message_to_user($course, $user);

        add_to_log($course->id, 'course', 'enrol',
                'view.php?id='.$course->id, $course->id);

        return true;
    }

    return false;
}

/**
 * Determines if a user is currently logged in
 *
 * @return bool
 */
function isloggedin() {
    global $USER;

    return (!empty($USER->id));
}

/**
 * Determines if a user is logged in as real guest user with username 'guest'.
 *
 * @param int $user mixed user object or id, $USER if not specified
 * @return bool true if user is the real guest user, false if not logged in or other user
 */
function isguestuser($user = NULL) {
    global $USER, $DB, $CFG;

    // make sure we have the user id cached in config table, because we are going to use it a lot
    if (empty($CFG->siteguest)) {
        if (!$guestid = $DB->get_field('user', 'id', array('username'=>'guest', 'mnethostid'=>$CFG->mnet_localhost_id))) {
            // guest does not exist yet, weird
            return false;
        }
        set_config('siteguest', $guestid);
    }
    if ($user === NULL) {
        $user = $USER;
    }

    if ($user === NULL) {
        // happens when setting the $USER
        return false;

    } else if (is_numeric($user)) {
        return ($CFG->siteguest == $user);

    } else if (is_object($user)) {
        if (empty($user->id)) {
            return false; // not logged in means is not be guest
        } else {
            return ($CFG->siteguest == $user->id);
        }

    } else {
        throw new coding_exception('Invalid user parameter supplied for isguestuser() function!');
    }
}

/**
 * Does user have a (temporary or real) guest access to course?
 *
 * @param object $context
 * @param object|int $user
 * @return bool
 */
function is_guest($context, $user = NULL) {
    // first find the course context
    $coursecontext = get_course_context($context);

    // make sure there is a real user specified
    if ($user === NULL) {
        $userid = !empty($USER->id) ? $USER->id : 0;
    } else {
        $userid = !empty($user->id) ? $user->id : $user;
    }

    if (isguestuser($userid)) {
        // can not inspect or be enrolled
        return true;
    }

    if (has_capability('moodle/course:view', $coursecontext, $user)) {
        // viewing users appear out of nowhere, they are neither guests nor participants
        return false;
    }

    if (has_capability('moodle/course:participate', $coursecontext, $userid, false)) {
        return false;
    }

    return true;
}


/**
 * Returns true if user has course:inspect capability in course,
 * this is intended for admins, managers (aka small admins), inspectors, etc.
 *
 * @param object $context
 * @param int|object $user, if NULL $USER is used
 * @param string $withcapability extra capability name
 * @return bool
 */
function is_viewing($context, $user = NULL, $withcapability = '') {
    global $USER;

    // first find the course context
    $coursecontext = get_course_context($context);

    if (isguestuser($user)) {
        // can not inspect
        return false;
    }

    if (!has_capability('moodle/course:view', $coursecontext, $user)) {
        // admins are allowed to inspect courses
        return false;
    }

    if ($withcapability and !has_capability($withcapability, $context, $user)) {
        // site admins always have the capability, but the enrolment above blocks
        return false;
    }

    return true;
}

/**
 * Returns true if user is enrolled (is participating) in course
 * this is intended for students and teachers.
 *
 * @param object $context
 * @param int|object $user, if NULL $USER is used, otherwise user object or id expected
 * @param string $withcapability extra capability name
 * @return bool
 */
function is_enrolled($context, $user = NULL, $withcapability = '') {
    global $USER;

    // first find the course context
    $coursecontext = get_course_context($context);

    // make sure there is a real user specified
    if ($user === NULL) {
        $userid = !empty($USER->id) ? $USER->id : 0;
    } else {
        $userid = !empty($user->id) ? $user->id : $user;
    }

    if (empty($userid)) {
        // not-logged-in!
        return false;
    } else if (isguestuser($userid)) {
        // guest account can not be enrolled anywhere
        return false;
    }

    if ($coursecontext->instanceid != SITEID and !has_capability('moodle/course:participate', $coursecontext, $userid, false)) {
        // admins are not enrolled, everybody is "enrolled" in the frontpage course
        return false;
    }

    if ($withcapability and !has_capability($withcapability, $context, $userid)) {
        return false;
    }

    return true;
}

/**
 * Returns array with sql code and parameters returning all ids
 * of users enrolled into course.
 * @param object $context
 * @param string $withcapability
 * @param int $groupid 0 means ignore groups, any other value limits the result by group id
 * @param string $prefix used for alias of user table, parameter names and in aliases of other used tables
 * @return array list($sql, $params)
 */
function get_enrolled_sql($context, $withcapability = '', $groupid = 0, $prefix = 'eu') {
    global $DB;

    if ($context->contextlevel < CONTEXT_COURSE) {
        throw new coding_exception('get_enrolled_sql() expects course context and bellow!');
    }

    // first find the course context
    if ($context->contextlevel == CONTEXT_COURSE) {
        $coursecontext = $context;

    } else if ($context->contextlevel == CONTEXT_MODULE) {
        $coursecontext = get_context_instance_by_id(get_parent_contextid($context, MUST_EXIST));

    } else if ($context->contextlevel == CONTEXT_BLOCK) {
        $parentcontext = get_context_instance_by_id(get_parent_contextid($context, MUST_EXIST));
        if ($parentcontext->contextlevel == CONTEXT_COURSE) {
            $coursecontext = $parentcontext;
        } else if ($parentcontext->contextlevel == CONTEXT_MODULE) {
            $coursecontext = get_context_instance_by_id(get_parent_contextid($parentcontext, MUST_EXIST));
        } else {
            throw new coding_exception('Invalid context supplied to get_enrolled_sql()!');
        }

    } else {
        throw new coding_exception('Invalid context supplied to get_enrolled_sql()!');
    }

    list($contextids, $contextpaths) = get_context_info_list($context);
    list($coursecontextids, $coursecontextpaths) = get_context_info_list($coursecontext);

    // get all relevant capability info for all roles
    if ($withcapability) {
        list($incontexts, $params) = $DB->get_in_or_equal($contextids, SQL_PARAMS_NAMED, 'con00');
        $incaps = "IN (:participate, :withcap)";
        $params['participate'] = 'moodle/course:participate';
        $params['withcap']     = $withcapability;
    } else {
        list($incontexts, $params) = $DB->get_in_or_equal($coursecontextids, SQL_PARAMS_NAMED, 'con00');
        $incaps = "= :participate";
        $params['participate'] = 'moodle/course:participate';
    }
    $defs = array();
    $sql = "SELECT rc.id, rc.roleid, rc.permission, rc.capability, ctx.path
              FROM {role_capabilities} rc
              JOIN {context} ctx on rc.contextid = ctx.id
             WHERE rc.contextid $incontexts AND rc.capability $incaps";
    $rcs = $DB->get_records_sql($sql, $params);
    foreach ($rcs as $rc) {
        $defs[$rc->capability][$rc->path][$rc->roleid] = $rc->permission;
    }

    $courseaccess = array();
    if (!empty($defs['moodle/course:participate'])) {