[b354002] | 1 | <?php |
---|
| 2 | |
---|
| 3 | /** |
---|
| 4 | * @file |
---|
| 5 | * A database-mediated implementation of a locking mechanism. |
---|
| 6 | */ |
---|
| 7 | |
---|
| 8 | /** |
---|
| 9 | * @defgroup lock Functions to coordinate long-running operations across requests. |
---|
| 10 | * @{ |
---|
| 11 | * In most environments, multiple Drupal page requests (a.k.a. threads or |
---|
| 12 | * processes) will execute in parallel. This leads to potential conflicts or |
---|
| 13 | * race conditions when two requests execute the same code at the same time. A |
---|
| 14 | * common example of this is a rebuild like menu_rebuild() where we invoke many |
---|
| 15 | * hook implementations to get and process data from all active modules, and |
---|
| 16 | * then delete the current data in the database to insert the new afterwards. |
---|
| 17 | * |
---|
| 18 | * This is a cooperative, advisory lock system. Any long-running operation |
---|
| 19 | * that could potentially be attempted in parallel by multiple requests should |
---|
| 20 | * try to acquire a lock before proceeding. By obtaining a lock, one request |
---|
| 21 | * notifies any other requests that a specific opertation is in progress which |
---|
| 22 | * must not be executed in parallel. |
---|
| 23 | * |
---|
| 24 | * To use this API, pick a unique name for the lock. A sensible choice is the |
---|
| 25 | * name of the function performing the operation. A very simple example use of |
---|
| 26 | * this API: |
---|
| 27 | * @code |
---|
| 28 | * function mymodule_long_operation() { |
---|
| 29 | * if (lock_acquire('mymodule_long_operation')) { |
---|
| 30 | * // Do the long operation here. |
---|
| 31 | * // ... |
---|
| 32 | * lock_release('mymodule_long_operation'); |
---|
| 33 | * } |
---|
| 34 | * } |
---|
| 35 | * @endcode |
---|
| 36 | * |
---|
| 37 | * If a function acquires a lock it should always release it when the |
---|
| 38 | * operation is complete by calling lock_release(), as in the example. |
---|
| 39 | * |
---|
| 40 | * A function that has acquired a lock may attempt to renew a lock (extend the |
---|
| 41 | * duration of the lock) by calling lock_acquire() again during the operation. |
---|
| 42 | * Failure to renew a lock is indicative that another request has acquired |
---|
| 43 | * the lock, and that the current operation may need to be aborted. |
---|
| 44 | * |
---|
| 45 | * If a function fails to acquire a lock it may either immediately return, or |
---|
| 46 | * it may call lock_wait() if the rest of the current page request requires |
---|
| 47 | * that the operation in question be complete. After lock_wait() returns, |
---|
| 48 | * the function may again attempt to acquire the lock, or may simply allow the |
---|
| 49 | * page request to proceed on the assumption that a parallel request completed |
---|
| 50 | * the operation. |
---|
| 51 | * |
---|
| 52 | * lock_acquire() and lock_wait() will automatically break (delete) a lock |
---|
| 53 | * whose duration has exceeded the timeout specified when it was acquired. |
---|
| 54 | * |
---|
| 55 | * Alternative implementations of this API (such as APC) may be substituted |
---|
| 56 | * by setting the 'lock_inc' variable to an alternate include filepath. Since |
---|
| 57 | * this is an API intended to support alternative implementations, code using |
---|
| 58 | * this API should never rely upon specific implementation details (for example |
---|
| 59 | * no code should look for or directly modify a lock in the {semaphore} table). |
---|
| 60 | */ |
---|
| 61 | |
---|
| 62 | /** |
---|
| 63 | * Initialize the locking system. |
---|
| 64 | */ |
---|
| 65 | function lock_init() { |
---|
| 66 | global $locks; |
---|
| 67 | |
---|
| 68 | $locks = array(); |
---|
| 69 | } |
---|
| 70 | |
---|
| 71 | /** |
---|
| 72 | * Helper function to get this request's unique id. |
---|
| 73 | */ |
---|
| 74 | function _lock_id() { |
---|
| 75 | static $lock_id; |
---|
| 76 | |
---|
| 77 | if (!isset($lock_id)) { |
---|
| 78 | // Assign a unique id. |
---|
| 79 | $lock_id = uniqid(mt_rand(), TRUE); |
---|
| 80 | // We only register a shutdown function if a lock is used. |
---|
| 81 | register_shutdown_function('lock_release_all', $lock_id); |
---|
| 82 | } |
---|
| 83 | return $lock_id; |
---|
| 84 | } |
---|
| 85 | |
---|
| 86 | /** |
---|
| 87 | * Acquire (or renew) a lock, but do not block if it fails. |
---|
| 88 | * |
---|
| 89 | * @param $name |
---|
| 90 | * The name of the lock. |
---|
| 91 | * @param $timeout |
---|
| 92 | * A number of seconds (float) before the lock expires. |
---|
| 93 | * @return |
---|
| 94 | * TRUE if the lock was acquired, FALSE if it failed. |
---|
| 95 | */ |
---|
| 96 | function lock_acquire($name, $timeout = 30.0) { |
---|
| 97 | global $locks; |
---|
| 98 | |
---|
| 99 | // Insure that the timeout is at least 1 ms. |
---|
| 100 | $timeout = max($timeout, 0.001); |
---|
| 101 | list($usec, $sec) = explode(' ', microtime()); |
---|
| 102 | $expire = (float)$usec + (float)$sec + $timeout; |
---|
| 103 | if (isset($locks[$name])) { |
---|
| 104 | // Try to extend the expiration of a lock we already acquired. |
---|
[d7a822e] | 105 | db_query("UPDATE {semaphore} SET expire = %f WHERE name = '%s' AND value = '%s'", $expire, $name, _lock_id()); |
---|
| 106 | if (!db_affected_rows()) { |
---|
[b354002] | 107 | // The lock was broken. |
---|
| 108 | unset($locks[$name]); |
---|
| 109 | } |
---|
| 110 | } |
---|
| 111 | else { |
---|
| 112 | // Optimistically try to acquire the lock, then retry once if it fails. |
---|
| 113 | // The first time through the loop cannot be a retry. |
---|
| 114 | $retry = FALSE; |
---|
| 115 | // We always want to do this code at least once. |
---|
| 116 | do { |
---|
| 117 | if (@db_query("INSERT INTO {semaphore} (name, value, expire) VALUES ('%s', '%s', %f)", $name, _lock_id(), $expire)) { |
---|
| 118 | // We track all acquired locks in the global variable. |
---|
| 119 | $locks[$name] = TRUE; |
---|
| 120 | // We never need to try again. |
---|
| 121 | $retry = FALSE; |
---|
| 122 | } |
---|
| 123 | else { |
---|
| 124 | // Suppress the error. If this is our first pass through the loop, |
---|
| 125 | // then $retry is FALSE. In this case, the insert must have failed |
---|
| 126 | // meaning some other request acquired the lock but did not release it. |
---|
| 127 | // We decide whether to retry by checking lock_may_be_available() |
---|
| 128 | // Since this will break the lock in case it is expired. |
---|
| 129 | $retry = $retry ? FALSE : lock_may_be_available($name); |
---|
| 130 | } |
---|
| 131 | // We only retry in case the first attempt failed, but we then broke |
---|
| 132 | // an expired lock. |
---|
| 133 | } while ($retry); |
---|
| 134 | } |
---|
| 135 | return isset($locks[$name]); |
---|
| 136 | } |
---|
| 137 | |
---|
| 138 | /** |
---|
| 139 | * Check if lock acquired by a different process may be available. |
---|
| 140 | * |
---|
| 141 | * If an existing lock has expired, it is removed. |
---|
| 142 | * |
---|
| 143 | * @param $name |
---|
| 144 | * The name of the lock. |
---|
| 145 | * @return |
---|
| 146 | * TRUE if there is no lock or it was removed, FALSE otherwise. |
---|
| 147 | */ |
---|
| 148 | function lock_may_be_available($name) { |
---|
| 149 | $lock = db_fetch_array(db_query("SELECT expire, value FROM {semaphore} WHERE name = '%s'", $name)); |
---|
| 150 | if (!$lock) { |
---|
| 151 | return TRUE; |
---|
| 152 | } |
---|
| 153 | $expire = (float) $lock['expire']; |
---|
| 154 | list($usec, $sec) = explode(' ', microtime()); |
---|
| 155 | $now = (float)$usec + (float)$sec; |
---|
| 156 | if ($now > $lock['expire']) { |
---|
| 157 | // We check two conditions to prevent a race condition where another |
---|
| 158 | // request acquired the lock and set a new expire time. We add a small |
---|
| 159 | // number to $expire to avoid errors with float to string conversion. |
---|
| 160 | db_query("DELETE FROM {semaphore} WHERE name = '%s' AND value = '%s' AND expire <= %f", $name, $lock['value'], 0.0001 + $expire); |
---|
| 161 | return (bool)db_affected_rows(); |
---|
| 162 | } |
---|
| 163 | return FALSE; |
---|
| 164 | } |
---|
| 165 | |
---|
| 166 | /** |
---|
| 167 | * Wait for a lock to be available. |
---|
| 168 | * |
---|
| 169 | * This function may be called in a request that fails to acquire a desired |
---|
| 170 | * lock. This will block further execution until the lock is available or the |
---|
| 171 | * specified delay in seconds is reached. This should not be used with locks |
---|
| 172 | * that are acquired very frequently, since the lock is likely to be acquired |
---|
| 173 | * again by a different request during the sleep(). |
---|
| 174 | * |
---|
| 175 | * @param $name |
---|
| 176 | * The name of the lock. |
---|
| 177 | * @param $delay |
---|
| 178 | * The maximum number of seconds to wait, as an integer. |
---|
| 179 | * @return |
---|
| 180 | * TRUE if the lock holds, FALSE if it is available. |
---|
| 181 | */ |
---|
| 182 | function lock_wait($name, $delay = 30) { |
---|
| 183 | |
---|
| 184 | while ($delay--) { |
---|
| 185 | // This function should only be called by a request that failed to get a |
---|
| 186 | // lock, so we sleep first to give the parallel request a chance to finish |
---|
| 187 | // and release the lock. |
---|
| 188 | sleep(1); |
---|
| 189 | if (lock_may_be_available($name)) { |
---|
| 190 | // No longer need to wait. |
---|
| 191 | return FALSE; |
---|
| 192 | } |
---|
| 193 | } |
---|
| 194 | // The caller must still wait longer to get the lock. |
---|
| 195 | return TRUE; |
---|
| 196 | } |
---|
| 197 | |
---|
| 198 | /** |
---|
| 199 | * Release a lock previously acquired by lock_acquire(). |
---|
| 200 | * |
---|
| 201 | * This will release the named lock if it is still held by the current request. |
---|
| 202 | * |
---|
| 203 | * @param $name |
---|
| 204 | * The name of the lock. |
---|
| 205 | */ |
---|
| 206 | function lock_release($name) { |
---|
| 207 | global $locks; |
---|
| 208 | |
---|
| 209 | unset($locks[$name]); |
---|
| 210 | db_query("DELETE FROM {semaphore} WHERE name = '%s' AND value = '%s'", $name, _lock_id()); |
---|
| 211 | } |
---|
| 212 | |
---|
| 213 | /** |
---|
| 214 | * Release all previously acquired locks. |
---|
| 215 | */ |
---|
| 216 | function lock_release_all($lock_id = NULL) { |
---|
| 217 | global $locks; |
---|
| 218 | |
---|
| 219 | $locks = array(); |
---|
| 220 | if (empty($lock_id)) { |
---|
| 221 | $lock_id = _lock_id(); |
---|
| 222 | } |
---|
| 223 | |
---|
| 224 | db_query("DELETE FROM {semaphore} WHERE value = '%s'", _lock_id()); |
---|
| 225 | } |
---|
| 226 | |
---|
| 227 | /** |
---|
| 228 | * @} End of "defgroup locks". |
---|
| 229 | */ |
---|