MDL-51664 externalib: clarify crucial comment

[moodle.git] / lib / externallib.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/>.


/**
 * Support for external API
 *
 * @package    core_webservice
 * @copyright  2009 Petr Skodak
 * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
 */

defined('MOODLE_INTERNAL') || die();

/**
 * Returns detailed function information
 *
 * @param string|object $function name of external function or record from external_function
 * @param int $strictness IGNORE_MISSING means compatible mode, false returned if record not found, debug message if more found;
 *                        MUST_EXIST means throw exception if no record or multiple records found
 * @return stdClass description or false if not found or exception thrown
 * @since Moodle 2.0
 */
function external_function_info($function, $strictness=MUST_EXIST) {
    global $DB, $CFG;

    if (!is_object($function)) {
        if (!$function = $DB->get_record('external_functions', array('name'=>$function), '*', $strictness)) {
            return false;
        }
    }

    // First try class autoloading.
    if (!class_exists($function->classname)) {
        // Fallback to explicit include of externallib.php.
        $function->classpath = empty($function->classpath) ? core_component::get_component_directory($function->component).'/externallib.php' : $CFG->dirroot.'/'.$function->classpath;
        if (!file_exists($function->classpath)) {
            throw new coding_exception('Cannot find file with external function implementation: ' . $function->classname);
        }
        require_once($function->classpath);
        if (!class_exists($function->classname)) {
            throw new coding_exception('Cannot find external class');
        }
    }

    $function->ajax_method = $function->methodname.'_is_allowed_from_ajax';
    $function->parameters_method = $function->methodname.'_parameters';
    $function->returns_method    = $function->methodname.'_returns';
    $function->deprecated_method = $function->methodname.'_is_deprecated';

    // make sure the implementaion class is ok
    if (!method_exists($function->classname, $function->methodname)) {
        throw new coding_exception('Missing implementation method of '.$function->classname.'::'.$function->methodname);
    }
    if (!method_exists($function->classname, $function->parameters_method)) {
        throw new coding_exception('Missing parameters description');
    }
    if (!method_exists($function->classname, $function->returns_method)) {
        throw new coding_exception('Missing returned values description');
    }
    if (method_exists($function->classname, $function->deprecated_method)) {
        if (call_user_func(array($function->classname, $function->deprecated_method)) === true) {
            $function->deprecated = true;
        }
    }
    $function->allowed_from_ajax = false;

    // fetch the parameters description
    $function->parameters_desc = call_user_func(array($function->classname, $function->parameters_method));
    if (!($function->parameters_desc instanceof external_function_parameters)) {
        throw new coding_exception('Invalid parameters description');
    }

    // fetch the return values description
    $function->returns_desc = call_user_func(array($function->classname, $function->returns_method));
    // null means void result or result is ignored
    if (!is_null($function->returns_desc) and !($function->returns_desc instanceof external_description)) {
        throw new coding_exception('Invalid return description');
    }

    //now get the function description
    //TODO MDL-31115 use localised lang pack descriptions, it would be nice to have
    //      easy to understand descriptions in admin UI,
    //      on the other hand this is still a bit in a flux and we need to find some new naming
    //      conventions for these descriptions in lang packs
    $function->description = null;
    $servicesfile = core_component::get_component_directory($function->component).'/db/services.php';
    if (file_exists($servicesfile)) {
        $functions = null;
        include($servicesfile);
        if (isset($functions[$function->name]['description'])) {
            $function->description = $functions[$function->name]['description'];
        }
        if (isset($functions[$function->name]['testclientpath'])) {
            $function->testclientpath = $functions[$function->name]['testclientpath'];
        }
        if (isset($functions[$function->name]['type'])) {
            $function->type = $functions[$function->name]['type'];
        }
        if (isset($functions[$function->name]['ajax'])) {
            $function->allowed_from_ajax = $functions[$function->name]['ajax'];
        } else if (method_exists($function->classname, $function->ajax_method)) {
            if (call_user_func(array($function->classname, $function->ajax_method)) === true) {
                debugging('External function ' . $function->ajax_method . '() function is deprecated.' .
                          'Set ajax=>true in db/service.php instead.', DEBUG_DEVELOPER);
                $function->allowed_from_ajax = true;
            }
        }
        if (isset($functions[$function->name]['loginrequired'])) {
            $function->loginrequired = $functions[$function->name]['loginrequired'];
        } else {
            $function->loginrequired = true;
        }
    }

    return $function;
}

/**
 * Exception indicating user is not allowed to use external function in the current context.
 *
 * @package    core_webservice
 * @copyright  2009 Petr Skodak
 * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
 * @since Moodle 2.0
 */
class restricted_context_exception extends moodle_exception {
    /**
     * Constructor
     *
     * @since Moodle 2.0
     */
    function __construct() {
        parent::__construct('restrictedcontextexception', 'error');
    }
}

/**
 * Base class for external api methods.
 *
 * @package    core_webservice
 * @copyright  2009 Petr Skodak
 * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
 * @since Moodle 2.0
 */
class external_api {

    /** @var stdClass context where the function calls will be restricted */
    private static $contextrestriction;

    /**
     * Set context restriction for all following subsequent function calls.
     *
     * @param stdClass $context the context restriction
     * @since Moodle 2.0
     */
    public static function set_context_restriction($context) {
        self::$contextrestriction = $context;
    }

    /**
     * This method has to be called before every operation
     * that takes a longer time to finish!
     *
     * @param int $seconds max expected time the next operation needs
     * @since Moodle 2.0
     */
    public static function set_timeout($seconds=360) {
        $seconds = ($seconds < 300) ? 300 : $seconds;
        core_php_time_limit::raise($seconds);
    }

    /**
     * Validates submitted function parameters, if anything is incorrect
     * invalid_parameter_exception is thrown.
     * This is a simple recursive method which is intended to be called from
     * each implementation method of external API.
     *
     * @param external_description $description description of parameters
     * @param mixed $params the actual parameters
     * @return mixed params with added defaults for optional items, invalid_parameters_exception thrown if any problem found
     * @since Moodle 2.0
     */
    public static function validate_parameters(external_description $description, $params) {
        if ($description instanceof external_value) {
            if (is_array($params) or is_object($params)) {
                throw new invalid_parameter_exception('Scalar type expected, array or object received.');
            }

            if ($description->type == PARAM_BOOL) {
                // special case for PARAM_BOOL - we want true/false instead of the usual 1/0 - we can not be too strict here ;-)
                if (is_bool($params) or $params === 0 or $params === 1 or $params === '0' or $params === '1') {
                    return (bool)$params;
                }
            }
            $debuginfo = 'Invalid external api parameter: the value is "' . $params .
                    '", the server was expecting "' . $description->type . '" type';
            return validate_param($params, $description->type, $description->allownull, $debuginfo);

        } else if ($description instanceof external_single_structure) {
            if (!is_array($params)) {
                throw new invalid_parameter_exception('Only arrays accepted. The bad value is: \''
                        . print_r($params, true) . '\'');
            }
            $result = array();
            foreach ($description->keys as $key=>$subdesc) {
                if (!array_key_exists($key, $params)) {
                    if ($subdesc->required == VALUE_REQUIRED) {
                        throw new invalid_parameter_exception('Missing required key in single structure: '. $key);
                    }
                    if ($subdesc->required == VALUE_DEFAULT) {
                        try {
                            $result[$key] = self::validate_parameters($subdesc, $subdesc->default);
                        } catch (invalid_parameter_exception $e) {
                            //we are only interested by exceptions returned by validate_param() and validate_parameters()
                            //(in order to build the path to the faulty attribut)
                            throw new invalid_parameter_exception($key." => ".$e->getMessage() . ': ' .$e->debuginfo);
                        }
                    }
                } else {
                    try {
                        $result[$key] = self::validate_parameters($subdesc, $params[$key]);
                    } catch (invalid_parameter_exception $e) {
                        //we are only interested by exceptions returned by validate_param() and validate_parameters()
                        //(in order to build the path to the faulty attribut)
                        throw new invalid_parameter_exception($key." => ".$e->getMessage() . ': ' .$e->debuginfo);
                    }
                }
                unset($params[$key]);
            }
            if (!empty($params)) {
                throw new invalid_parameter_exception('Unexpected keys (' . implode(', ', array_keys($params)) . ') detected in parameter array.');
            }
            return $result;

        } else if ($description instanceof external_multiple_structure) {
            if (!is_array($params)) {
                throw new invalid_parameter_exception('Only arrays accepted. The bad value is: \''
                        . print_r($params, true) . '\'');
            }
            $result = array();
            foreach ($params as $param) {
                $result[] = self::validate_parameters($description->content, $param);
            }
            return $result;

        } else {
            throw new invalid_parameter_exception('Invalid external api description');
        }
    }

    /**
     * Clean response
     * If a response attribute is unknown from the description, we just ignore the attribute.
     * If a response attribute is incorrect, invalid_response_exception is thrown.
     * Note: this function is similar to validate parameters, however it is distinct because
     * parameters validation must be distinct from cleaning return values.
     *
     * @param external_description $description description of the return values
     * @param mixed $response the actual response
     * @return mixed response with added defaults for optional items, invalid_response_exception thrown if any problem found
     * @author 2010 Jerome Mouneyrac
     * @since Moodle 2.0
     */
    public static function clean_returnvalue(external_description $description, $response) {
        if ($description instanceof external_value) {
            if (is_array($response) or is_object($response)) {
                throw new invalid_response_exception('Scalar type expected, array or object received.');
            }

            if ($description->type == PARAM_BOOL) {
                // special case for PARAM_BOOL - we want true/false instead of the usual 1/0 - we can not be too strict here ;-)
                if (is_bool($response) or $response === 0 or $response === 1 or $response === '0' or $response === '1') {
                    return (bool)$response;
                }
            }
            $debuginfo = 'Invalid external api response: the value is "' . $response .
                    '", the server was expecting "' . $description->type . '" type';
            try {
                return validate_param($response, $description->type, $description->allownull, $debuginfo);
            } catch (invalid_parameter_exception $e) {
                //proper exception name, to be recursively catched to build the path to the faulty attribut
                throw new invalid_response_exception($e->debuginfo);
            }

        } else if ($description instanceof external_single_structure) {
            if (!is_array($response) && !is_object($response)) {
                throw new invalid_response_exception('Only arrays/objects accepted. The bad value is: \'' .
                        print_r($response, true) . '\'');
            }

            // Cast objects into arrays.
            if (is_object($response)) {
                $response = (array) $response;
            }

            $result = array();
            foreach ($description->keys as $key=>$subdesc) {
                if (!array_key_exists($key, $response)) {
                    if ($subdesc->required == VALUE_REQUIRED) {
                        throw new invalid_response_exception('Error in response - Missing following required key in a single structure: ' . $key);
                    }
                    if ($subdesc instanceof external_value) {
                        if ($subdesc->required == VALUE_DEFAULT) {
                            try {
                                    $result[$key] = self::clean_returnvalue($subdesc, $subdesc->default);
                            } catch (invalid_response_exception $e) {
                                //build the path to the faulty attribut
                                throw new invalid_response_exception($key." => ".$e->getMessage() . ': ' . $e->debuginfo);
                            }
                        }
                    }
                } else {
                    try {
                        $result[$key] = self::clean_returnvalue($subdesc, $response[$key]);
                    } catch (invalid_response_exception $e) {
                        //build the path to the faulty attribut
                        throw new invalid_response_exception($key." => ".$e->getMessage() . ': ' . $e->debuginfo);
                    }
                }
                unset($response[$key]);
            }

            return $result;

        } else if ($description instanceof external_multiple_structure) {
            if (!is_array($response)) {
                throw new invalid_response_exception('Only arrays accepted. The bad value is: \'' .
                        print_r($response, true) . '\'');
            }
            $result = array();
            foreach ($response as $param) {
                $result[] = self::clean_returnvalue($description->content, $param);
            }
            return $result;

        } else {
            throw new invalid_response_exception('Invalid external api response description');
        }
    }

    /**
     * Makes sure user may execute functions in this context.
     *
     * @param stdClass $context
     * @since Moodle 2.0
     */
    public static function validate_context($context) {
        global $CFG;

        if (empty($context)) {
            throw new invalid_parameter_exception('Context does not exist');
        }
        if (empty(self::$contextrestriction)) {
            self::$contextrestriction = context_system::instance();
        }
        $rcontext = self::$contextrestriction;

        if ($rcontext->contextlevel == $context->contextlevel) {
            if ($rcontext->id != $context->id) {
                throw new restricted_context_exception();
            }
        } else if ($rcontext->contextlevel > $context->contextlevel) {
            throw new restricted_context_exception();
        } else {
            $parents = $context->get_parent_context_ids();
            if (!in_array($rcontext->id, $parents)) {
                throw new restricted_context_exception();
            }
        }

        if ($context->contextlevel >= CONTEXT_COURSE) {
            list($context, $course, $cm) = get_context_info_array($context->id);
            require_login($course, false, $cm, false, true);
        }
    }

    /**
     * Get context from passed parameters.
     * The passed array must either contain a contextid or a combination of context level and instance id to fetch the context.
     * For example, the context level can be "course" and instanceid can be courseid.
     *
     * See context_helper::get_all_levels() for a list of valid context levels.
     *
     * @param array $param
     * @since Moodle 2.6
     * @throws invalid_parameter_exception
     * @return context
     */
    protected static function get_context_from_params($param) {
        $levels = context_helper::get_all_levels();
        if (!empty($param['contextid'])) {
            return context::instance_by_id($param['contextid'], IGNORE_MISSING);
        } else if (!empty($param['contextlevel']) && isset($param['instanceid'])) {
            $contextlevel = "context_".$param['contextlevel'];
            if (!array_search($contextlevel, $levels)) {
                throw new invalid_parameter_exception('Invalid context level = '.$param['contextlevel']);
            }
           return $contextlevel::instance($param['instanceid'], IGNORE_MISSING);
        } else {
            // No valid context info was found.
            throw new invalid_parameter_exception('Missing parameters, please provide either context level with instance id or contextid');
        }
    }
}

/**
 * Common ancestor of all parameter description classes
 *
 * @package    core_webservice
 * @copyright  2009 Petr Skodak
 * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
 * @since Moodle 2.0
 */
abstract class external_description {
    /** @var string Description of element */
    public $desc;

    /** @var bool Element value required, null not allowed */
    public $required;

    /** @var mixed Default value */
    public $default;

    /**
     * Contructor
     *
     * @param string $desc
     * @param bool $required
     * @param mixed $default
     * @since Moodle 2.0
     */
    public function __construct($desc, $required, $default) {
        $this->desc = $desc;
        $this->required = $required;
        $this->default = $default;
    }
}

/**
 * Scalar value description class
 *
 * @package    core_webservice
 * @copyright  2009 Petr Skodak
 * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
 * @since Moodle 2.0
 */
class external_value extends external_description {

    /** @var mixed Value type PARAM_XX */
    public $type;

    /** @var bool Allow null values */
    public $allownull;

    /**
     * Constructor
     *
     * @param mixed $type
     * @param string $desc
     * @param bool $required
     * @param mixed $default
     * @param bool $allownull
     * @since Moodle 2.0
     */
    public function __construct($type, $desc='', $required=VALUE_REQUIRED,
            $default=null, $allownull=NULL_ALLOWED) {
        parent::__construct($desc, $required, $default);
        $this->type      = $type;
        $this->allownull = $allownull;
    }
}

/**
 * Associative array description class
 *
 * @package    core_webservice
 * @copyright  2009 Petr Skodak
 * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
 * @since Moodle 2.0
 */
class external_single_structure extends external_description {

     /** @var array Description of array keys key=>external_description */
    public $keys;

    /**
     * Constructor
     *
     * @param array $keys
     * @param string $desc
     * @param bool $required
     * @param array $default
     * @since Moodle 2.0
     */
    public function __construct(array $keys, $desc='',
            $required=VALUE_REQUIRED, $default=null) {
        parent::__construct($desc, $required, $default);
        $this->keys = $keys;
    }
}

/**
 * Bulk array description class.
 *
 * @package    core_webservice
 * @copyright  2009 Petr Skodak
 * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
 * @since Moodle 2.0
 */
class external_multiple_structure extends external_description {

     /** @var external_description content */
    public $content;

    /**
     * Constructor
     *
     * @param external_description $content
     * @param string $desc
     * @param bool $required
     * @param array $default
     * @since Moodle 2.0
     */
    public function __construct(external_description $content, $desc='',
            $required=VALUE_REQUIRED, $default=null) {
        parent::__construct($desc, $required, $default);
        $this->content = $content;
    }
}

/**
 * Description of top level - PHP function parameters.
 *
 * @package    core_webservice
 * @copyright  2009 Petr Skodak
 * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
 * @since Moodle 2.0
 */
class external_function_parameters extends external_single_structure {

    /**
     * Constructor - does extra checking to prevent top level optional parameters.
     *
     * @param array $keys
     * @param string $desc
     * @param bool $required
     * @param array $default
     */
    public function __construct(array $keys, $desc='', $required=VALUE_REQUIRED, $default=null) {
        global $CFG;

        if ($CFG->debugdeveloper) {
            foreach ($keys as $key => $value) {
                if ($value instanceof external_value) {
                    if ($value->required == VALUE_OPTIONAL) {
                        debugging('External function parameters: invalid OPTIONAL value specified.', DEBUG_DEVELOPER);
                        break;
                    }
                }
            }
        }
        parent::__construct($keys, $desc, $required, $default);
    }
}

/**
 * Generate a token
 *
 * @param string $tokentype EXTERNAL_TOKEN_EMBEDDED|EXTERNAL_TOKEN_PERMANENT
 * @param stdClass|int $serviceorid service linked to the token
 * @param int $userid user linked to the token
 * @param stdClass|int $contextorid
 * @param int $validuntil date when the token expired
 * @param string $iprestriction allowed ip - if 0 or empty then all ips are allowed
 * @return string generated token
 * @author  2010 Jamie Pratt
 * @since Moodle 2.0
 */
function external_generate_token($tokentype, $serviceorid, $userid, $contextorid, $validuntil=0, $iprestriction=''){
    global $DB, $USER;
    // make sure the token doesn't exist (even if it should be almost impossible with the random generation)
    $numtries = 0;
    do {
        $numtries ++;
        $generatedtoken = md5(uniqid(rand(),1));
        if ($numtries > 5){
            throw new moodle_exception('tokengenerationfailed');
        }
    } while ($DB->record_exists('external_tokens', array('token'=>$generatedtoken)));
    $newtoken = new stdClass();
    $newtoken->token = $generatedtoken;
    if (!is_object($serviceorid)){
        $service = $DB->get_record('external_services', array('id' => $serviceorid));
    } else {
        $service = $serviceorid;
    }
    if (!is_object($contextorid)){
        $context = context::instance_by_id($contextorid, MUST_EXIST);
    } else {
        $context = $contextorid;
    }
    if (empty($service->requiredcapability) || has_capability($service->requiredcapability, $context, $userid)) {
        $newtoken->externalserviceid = $service->id;
    } else {
        throw new moodle_exception('nocapabilitytousethisservice');
    }
    $newtoken->tokentype = $tokentype;
    $newtoken->userid = $userid;
    if ($tokentype == EXTERNAL_TOKEN_EMBEDDED){
        $newtoken->sid = session_id();
    }

    $newtoken->contextid = $context->id;
    $newtoken->creatorid = $USER->id;
    $newtoken->timecreated = time();
    $newtoken->validuntil = $validuntil;
    if (!empty($iprestriction)) {
        $newtoken->iprestriction = $iprestriction;
    }
    $DB->insert_record('external_tokens', $newtoken);
    return $newtoken->token;
}

/**
 * Create and return a session linked token. Token to be used for html embedded client apps that want to communicate
 * with the Moodle server through web services. The token is linked to the current session for the current page request.
 * It is expected this will be called in the script generating the html page that is embedding the client app and that the
 * returned token will be somehow passed into the client app being embedded in the page.
 *
 * @param string $servicename name of the web service. Service name as defined in db/services.php
 * @param int $context context within which the web service can operate.
 * @return int returns token id.
 * @since Moodle 2.0
 */
function external_create_service_token($servicename, $context){
    global $USER, $DB;
    $service = $DB->get_record('external_services', array('name'=>$servicename), '*', MUST_EXIST);
    return external_generate_token(EXTERNAL_TOKEN_EMBEDDED, $service, $USER->id, $context, 0);
}

/**
 * Delete all pre-built services (+ related tokens) and external functions information defined in the specified component.
 *
 * @param string $component name of component (moodle, mod_assignment, etc.)
 */
function external_delete_descriptions($component) {
    global $DB;

    $params = array($component);

    $DB->delete_records_select('external_tokens',
            "externalserviceid IN (SELECT id FROM {external_services} WHERE component = ?)", $params);
    $DB->delete_records_select('external_services_users',
            "externalserviceid IN (SELECT id FROM {external_services} WHERE component = ?)", $params);
    $DB->delete_records_select('external_services_functions',
            "functionname IN (SELECT name FROM {external_functions} WHERE component = ?)", $params);
    $DB->delete_records('external_services', array('component'=>$component));
    $DB->delete_records('external_functions', array('component'=>$component));
}

/**
 * Standard Moodle web service warnings
 *
 * @package    core_webservice
 * @copyright  2012 Jerome Mouneyrac
 * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
 * @since Moodle 2.3
 */
class external_warnings extends external_multiple_structure {

    /**
     * Constructor
     *
     * @since Moodle 2.3
     */
    public function __construct($itemdesc = 'item', $itemiddesc = 'item id',
        $warningcodedesc = 'the warning code can be used by the client app to implement specific behaviour') {

        parent::__construct(
            new external_single_structure(
                array(
                    'item' => new external_value(PARAM_TEXT, $itemdesc, VALUE_OPTIONAL),
                    'itemid' => new external_value(PARAM_INT, $itemiddesc, VALUE_OPTIONAL),
                    'warningcode' => new external_value(PARAM_ALPHANUM, $warningcodedesc),
                    'message' => new external_value(PARAM_TEXT,
                            'untranslated english message to explain the warning')
                ), 'warning'),
            'list of warnings', VALUE_OPTIONAL);
    }
}

/**
 * A pre-filled external_value class for text format.
 *
 * Default is FORMAT_HTML
 * This should be used all the time in external xxx_params()/xxx_returns functions
 * as it is the standard way to implement text format param/return values.
 *
 * @package    core_webservice
 * @copyright  2012 Jerome Mouneyrac
 * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
 * @since Moodle 2.3
 */
class external_format_value extends external_value {

    /**
     * Constructor
     *
     * @param string $textfieldname Name of the text field
     * @param int $required if VALUE_REQUIRED then set standard default FORMAT_HTML
     * @since Moodle 2.3
     */
    public function __construct($textfieldname, $required = VALUE_REQUIRED) {

        $default = ($required == VALUE_DEFAULT) ? FORMAT_HTML : null;

        $desc = $textfieldname . ' format (' . FORMAT_HTML . ' = HTML, '
                . FORMAT_MOODLE . ' = MOODLE, '
                . FORMAT_PLAIN . ' = PLAIN or '
                . FORMAT_MARKDOWN . ' = MARKDOWN)';

        parent::__construct(PARAM_INT, $desc, $required, $default);
    }
}

/**
 * Validate text field format against known FORMAT_XXX
 *
 * @param array $format the format to validate
 * @return the validated format
 * @throws coding_exception
 * @since Moodle 2.3
 */
function external_validate_format($format) {
    $allowedformats = array(FORMAT_HTML, FORMAT_MOODLE, FORMAT_PLAIN, FORMAT_MARKDOWN);
    if (!in_array($format, $allowedformats)) {
        throw new moodle_exception('formatnotsupported', 'webservice', '' , null,
                'The format with value=' . $format . ' is not supported by this Moodle site');
    }
    return $format;
}

/**
 * Format the string to be returned properly as requested by the either the web service server,
 * either by an internally call.
 * The caller can change the format (raw) with the external_settings singleton
 * All web service servers must set this singleton when parsing the $_GET and $_POST.
 *
 * @param string $str The string to be filtered. Should be plain text, expect
 * possibly for multilang tags.
 * @param boolean $striplinks To strip any link in the result text. Moodle 1.8 default changed from false to true! MDL-8713
 * @param int $contextid The id of the context for the string (affects filters).
 * @param array $options options array/object or courseid
 * @return string text
 * @since Moodle 3.0
 */
function external_format_string($str, $contextid, $striplinks = true, $options = array()) {

    // Get settings (singleton).
    $settings = external_settings::get_instance();
    if (empty($contextid)) {
        throw new coding_exception('contextid is required');
    }

    if (!$settings->get_raw()) {
        $context = context::instance_by_id($contextid);
        $options['context'] = $context;
        $options['filter'] = $settings->get_filter();
        $str = format_string($str, $striplinks, $options);
    }

    return $str;
}

/**
 * Format the text to be returned properly as requested by the either the web service server,
 * either by an internally call.
 * The caller can change the format (raw, filter, file, fileurl) with the external_settings singleton
 * All web service servers must set this singleton when parsing the $_GET and $_POST.
 *
 * @param string $text The content that may contain ULRs in need of rewriting.
 * @param int $textformat The text format.
 * @param int $contextid This parameter and the next two identify the file area to use.
 * @param string $component
 * @param string $filearea helps identify the file area.
 * @param int $itemid helps identify the file area.
 * @return array text + textformat
 * @since Moodle 2.3
 */
function external_format_text($text, $textformat, $contextid, $component, $filearea, $itemid) {
    global $CFG;

    // Get settings (singleton).
    $settings = external_settings::get_instance();

    if ($settings->get_fileurl()) {
        require_once($CFG->libdir . "/filelib.php");
        $text = file_rewrite_pluginfile_urls($text, $settings->get_file(), $contextid, $component, $filearea, $itemid);
    }

    if (!$settings->get_raw()) {
        $context = context::instance_by_id($contextid);
        $text = format_text($text, $textformat, array('para' => false, 'filter' => $settings->get_filter(), 'context' => $context));
        $textformat = FORMAT_HTML; // Once converted to html (from markdown, plain... lets inform consumer this is already HTML).
    }

    return array($text, $textformat);
}

/**
 * Singleton to handle the external settings.
 *
 * We use singleton to encapsulate the "logic"
 *
 * @package    core_webservice
 * @copyright  2012 Jerome Mouneyrac
 * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
 * @since Moodle 2.3
 */
class external_settings {

    /** @var object the singleton instance */
    public static $instance = null;

    /** @var boolean Should the external function return raw text or formatted */
    private $raw = false;

    /** @var boolean Should the external function filter the text */
    private $filter = false;

    /** @var boolean Should the external function rewrite plugin file url */
    private $fileurl = true;

    /** @var string In which file should the urls be rewritten */
    private $file = 'webservice/pluginfile.php';

    /**
     * Constructor - protected - can not be instanciated
     */
    protected function __construct() {
        if (!defined('AJAX_SCRIPT') && !defined('CLI_SCRIPT') && !defined('WS_SERVER')) {
            // For normal pages, the default should match the default for format_text.
            $this->filter = true;
        }
    }

    /**
     * Clone - private - can not be cloned
     */
    private final function __clone() {
    }

    /**
     * Return only one instance
     *
     * @return object
     */
    public static function get_instance() {
        if (self::$instance === null) {
            self::$instance = new external_settings;
        }

        return self::$instance;
    }

    /**
     * Set raw
     *
     * @param boolean $raw
     */
    public function set_raw($raw) {
        $this->raw = $raw;
    }

    /**
     * Get raw
     *
     * @return boolean
     */
    public function get_raw() {
        return $this->raw;
    }

    /**
     * Set filter
     *
     * @param boolean $filter
     */
    public function set_filter($filter) {
        $this->filter = $filter;
    }

    /**
     * Get filter
     *
     * @return boolean
     */
    public function get_filter() {
        return $this->filter;
    }

    /**
     * Set fileurl
     *
     * @param boolean $fileurl
     */
    public function set_fileurl($fileurl) {
        $this->fileurl = $fileurl;
    }

    /**
     * Get fileurl
     *
     * @return boolean
     */
    public function get_fileurl() {
        return $this->fileurl;
    }

    /**
     * Set file
     *
     * @param string $file
     */
    public function set_file($file) {
        $this->file = $file;
    }

    /**
     * Get file
     *
     * @return string
     */
    public function get_file() {
        return $this->file;
    }
}

/**
 * Utility functions for the external API.
 *
 * @package    core_webservice
 * @copyright  2015 Juan Leyva
 * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
 * @since Moodle 3.0
 */
class external_util {

    /**
     * Validate a list of courses, returning the complete course objects for valid courses.
     *
     * @param  array $courseids A list of course ids
     * @param  array $courses   An array of courses already pre-fetched, indexed by course id.
     * @return array            An array of courses and the validation warnings
     */
    public static function validate_courses($courseids, $courses = array()) {
        // Delete duplicates.
        $courseids = array_unique($courseids);
        $warnings = array();

        // Remove courses which are not even requested.
        $courses =  array_intersect_key($courses, array_flip($courseids));

        foreach ($courseids as $cid) {
            // Check the user can function in this context.
            try {
                $context = context_course::instance($cid);
                external_api::validate_context($context);

                if (!isset($courses[$cid])) {
                    $courses[$cid] = get_course($cid);
                }
            } catch (Exception $e) {
                unset($courses[$cid]);
                $warnings[] = array(
                    'item' => 'course',
                    'itemid' => $cid,
                    'warningcode' => '1',
                    'message' => 'No access rights in course context'
                );
            }
        }

        return array($courses, $warnings);
    }

}