MDL-67594 core_lock: Modify postgres get_lock() to avoid recursion
[moodle.git] / lib / classes / lock / postgres_lock_factory.php
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1<?php
2// This file is part of Moodle - http://moodle.org/
3//
4// Moodle is free software: you can redistribute it and/or modify
5// it under the terms of the GNU General Public License as published by
6// the Free Software Foundation, either version 3 of the License, or
7// (at your option) any later version.
8//
9// Moodle is distributed in the hope that it will be useful,
10// but WITHOUT ANY WARRANTY; without even the implied warranty of
11// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
12// GNU General Public License for more details.
13//
14// You should have received a copy of the GNU General Public License
15// along with Moodle. If not, see <http://www.gnu.org/licenses/>.
16
17/**
18 * Postgres advisory locking factory.
19 *
20 * @package core
21 * @category lock
22 * @copyright Damyon Wiese 2013
23 * @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
24 */
25
26namespace core\lock;
27
28defined('MOODLE_INTERNAL') || die();
29
30/**
31 * Postgres advisory locking factory.
32 *
33 * Postgres locking implementation using advisory locks. Some important points. Postgres has
34 * 2 different forms of lock functions, some accepting a single int, and some accepting 2 ints. This implementation
35 * uses the 2 int version so that it uses a separate namespace from the session locking. The second note,
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36 * is because postgres uses integer keys for locks, we first need to map strings to a unique integer. This is done
37 * using a prefix of a sha1 hash converted to an integer.
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38 *
39 * @package core
40 * @category lock
41 * @copyright Damyon Wiese 2013
42 * @license http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
43 */
44class postgres_lock_factory implements lock_factory {
45
46 /** @var int $dblockid - used as a namespace for these types of locks (separate from session locks) */
47 protected $dblockid = -1;
48
49 /** @var array $lockidcache - static cache for string -> int conversions required for pg advisory locks. */
50 protected static $lockidcache = array();
51
34027f15 52 /** @var \moodle_database $db Hold a reference to the global $DB */
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53 protected $db;
54
55 /** @var string $type Used to prefix lock keys */
56 protected $type;
57
58 /** @var array $openlocks - List of held locks - used by auto-release */
59 protected $openlocks = array();
60
61 /**
62 * Calculate a unique instance id based on the database name and prefix.
63 * @return int.
64 */
65 protected function get_unique_db_instance_id() {
66 global $CFG;
67
68 $strkey = $CFG->dbname . ':' . $CFG->prefix;
69 $intkey = crc32($strkey);
70 // Normalize between 64 bit unsigned int and 32 bit signed ints. Php could return either from crc32.
71 if (PHP_INT_SIZE == 8) {
72 if ($intkey > 0x7FFFFFFF) {
73 $intkey -= 0x100000000;
74 }
75 }
76
77 return $intkey;
78 }
79
80 /**
81 * Almighty constructor.
82 * @param string $type - Used to prefix lock keys.
83 */
84 public function __construct($type) {
85 global $DB;
86
87 $this->type = $type;
88 $this->dblockid = $this->get_unique_db_instance_id();
89 // Save a reference to the global $DB so it will not be released while we still have open locks.
90 $this->db = $DB;
91
92 \core_shutdown_manager::register_function(array($this, 'auto_release'));
93 }
94
95 /**
96 * Is available.
97 * @return boolean - True if this lock type is available in this environment.
98 */
99 public function is_available() {
100 return $this->db->get_dbfamily() === 'postgres';
101 }
102
103 /**
104 * Return information about the blocking behaviour of the lock type on this platform.
105 * @return boolean - Defer to the DB driver.
106 */
107 public function supports_timeout() {
108 return true;
109 }
110
111 /**
112 * Will this lock type will be automatically released when a process ends.
113 *
114 * @return boolean - Via shutdown handler.
115 */
116 public function supports_auto_release() {
117 return true;
118 }
119
120 /**
83c18a2d 121 * Multiple locks for the same resource can NOT be held by a single process.
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122 *
123 * @deprecated since Moodle 4.0.
83c18a2d 124 * @return boolean - false.
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125 */
126 public function supports_recursion() {
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127 debugging('The function supports_recursion() is deprecated, please do not use it anymore.',
128 DEBUG_DEVELOPER);
83c18a2d 129 return false;
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130 }
131
132 /**
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133 * This function generates the unique index for a specific lock key using
134 * a sha1 prefix converted to decimal.
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135 *
136 * @param string $key
137 * @return int
34027f15 138 * @throws \moodle_exception
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139 */
140 protected function get_index_from_key($key) {
9843e5ec 141
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142 // A prefix of 7 hex chars is chosen as fffffff is the largest hex code
143 // which when converted to decimal (268435455) fits inside a 4 byte int
144 // which is the second param to pg_try_advisory_lock().
145 $hash = substr(sha1($key), 0, 7);
146 $index = hexdec($hash);
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147 return $index;
148 }
149
150 /**
151 * Create and get a lock
83c18a2d 152 *
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153 * @param string $resource - The identifier for the lock. Should use frankenstyle prefix.
154 * @param int $timeout - The number of seconds to wait for a lock before giving up.
155 * @param int $maxlifetime - Unused by this lock type.
156 * @return boolean - true if a lock was obtained.
157 */
158 public function get_lock($resource, $timeout, $maxlifetime = 86400) {
159 $giveuptime = time() + $timeout;
160
7e086935 161 $token = $this->get_index_from_key($this->type . '_' . $resource);
9843e5ec 162
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163 if (isset($this->openlocks[$token])) {
164 return false;
165 }
166
167 $params = [
168 'locktype' => $this->dblockid,
169 'token' => $token
170 ];
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171
172 $locked = false;
173
174 do {
175 $result = $this->db->get_record_sql('SELECT pg_try_advisory_lock(:locktype, :token) AS locked', $params);
176 $locked = $result->locked === 't';
ecbe9206 177 if (!$locked && $timeout > 0) {
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178 usleep(rand(10000, 250000)); // Sleep between 10 and 250 milliseconds.
179 }
180 // Try until the giveup time.
181 } while (!$locked && time() < $giveuptime);
182
183 if ($locked) {
184 $this->openlocks[$token] = 1;
185 return new lock($token, $this);
186 }
187 return false;
188 }
189
190 /**
191 * Release a lock that was previously obtained with @lock.
192 * @param lock $lock - a lock obtained from this factory.
193 * @return boolean - true if the lock is no longer held (including if it was never held).
194 */
195 public function release_lock(lock $lock) {
196 $params = array('locktype' => $this->dblockid,
197 'token' => $lock->get_key());
198 $result = $this->db->get_record_sql('SELECT pg_advisory_unlock(:locktype, :token) AS unlocked', $params);
199 $result = $result->unlocked === 't';
200 if ($result) {
201 unset($this->openlocks[$lock->get_key()]);
202 }
203 return $result;
204 }
205
206 /**
207 * Extend a lock that was previously obtained with @lock.
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208 *
209 * @deprecated since Moodle 4.0.
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210 * @param lock $lock - a lock obtained from this factory.
211 * @param int $maxlifetime - the new lifetime for the lock (in seconds).
212 * @return boolean - true if the lock was extended.
213 */
214 public function extend_lock(lock $lock, $maxlifetime = 86400) {
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215 debugging('The function extend_lock() is deprecated, please do not use it anymore.',
216 DEBUG_DEVELOPER);
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217 // Not supported by this factory.
218 return false;
219 }
220
221 /**
222 * Auto release any open locks on shutdown.
223 * This is required, because we may be using persistent DB connections.
224 */
225 public function auto_release() {
226 // Called from the shutdown handler. Must release all open locks.
227 foreach ($this->openlocks as $key => $unused) {
228 $lock = new lock($key, $this);
894f36ab 229 $lock->release();
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230 }
231 }
232
233}