ENHANCEMENT Added Database->getLock() and Database->releaseLock() for…

… application-level advisory locks

model/Database.php

@@ -828,6 +828,59 @@ abstract function transactionRollback($savepoint=false);
 	 * Commit everything inside this transaction so far
 	 */
 	abstract function transactionEnd();
+
+	/**
+	 * Determines if the used database supports application-level locks,
+	 * which is different from table- or row-level locking.
+	 * See {@link getLock()} for details.
+	 * 
+	 * @return boolean
+	 */
+	function supportsLocks() {
+		return false;
+	}
+
+	/**
+	 * Returns if the lock is available.
+	 * See {@link supportsLocks()} to check if locking is generally supported.
+	 * 
+	 * @return Boolean
+	 */
+	function canLock($name) {
+		return false;
+	}
+
+	/** 
+	 * Sets an application-level lock so that no two processes can run at the same time,
+	 * also called a "cooperative advisory lock".
+	 * 
+	 * Return FALSE if acquiring the lock fails; otherwise return TRUE, if lock was acquired successfully.
+	 * Lock is automatically released if connection to the database is broken (either normally or abnormally),
+	 * making it less prone to deadlocks than session- or file-based locks.
+	 * Should be accompanied by a {@link releaseLock()} call after the logic requiring the lock has completed.
+	 * Can be called multiple times, in which case locks "stack" (PostgreSQL, SQL Server),
+	 * or auto-releases the previous lock (MySQL).
+	 * 
+	 * Note that this might trigger the database to wait for the lock to be released, delaying further execution.
+	 * 
+	 * @param String
+	 * @param Int Timeout in seconds
+	 * @return Boolean
+	 */
+	function getLock($name, $timeout = 5) {
+		return false;
+	}
+
+	/** 
+	 * Remove an application-level lock file to allow another process to run 
+	 * (if the execution aborts (e.g. due to an error) all locks are automatically released).
+	 * 
+	 * @param String
+	 * @return Boolean
+	 */
+	function releaseLock($name) {
+		return false;
+	}
 }
 
 /**

model/MySQLDatabase.php

@@ -1039,6 +1039,34 @@ function datetimeDifferenceClause($date1, $date2) {
 
 		return "UNIX_TIMESTAMP($date1) - UNIX_TIMESTAMP($date2)";
 	}
+
+	function supportsLocks() {
+		return true;
+	}
+
+	function canLock($name) {
+		$id = $this->getLockIdentifier($name);
+		return (bool)DB::query(sprintf("SELECT IS_FREE_LOCK('%s')", $id))->value();
+	}
+
+	function getLock($name, $timeout = 5) {
+		$id = $this->getLockIdentifier($name);
+
+		// MySQL auto-releases existing locks on subsequent GET_LOCK() calls,
+		// in contrast to PostgreSQL and SQL Server who stack the locks.
+
+		return (bool)DB::query(sprintf("SELECT GET_LOCK('%s', %d)", $id, $timeout))->value();
+	}
+
+	function releaseLock($name) {
+		$id = $this->getLockIdentifier($name);
+		return (bool)DB::query(sprintf("SELECT RELEASE_LOCK('%s')", $id))->value();
+	}
+
+	protected function getLockIdentifier($name) {
+		// Prefix with database name
+		return Convert::raw2sql($this->database . '_' . Convert::raw2sql($name));
+	}
 }
 
 /**

tests/model/DatabaseTest.php

@@ -82,7 +82,47 @@ function testHasTable() {
 		$this->assertTrue(DB::getConn()->hasTable('DatabaseTest_MyObject'));
 		$this->assertFalse(DB::getConn()->hasTable('asdfasdfasdf'));
 	}
+
+	function testGetAndReleaseLock() {
+		$db = DB::getConn();
+
+		if(!$db->supportsLocks()) {
+			return $this->markTestSkipped('Tested database doesn\'t support application locks');
+		}
 
+		$this->assertTrue($db->getLock('DatabaseTest'), 'Can aquire lock');
+		// $this->assertFalse($db->getLock('DatabaseTest'), 'Can\'t repeatedly aquire the same lock');
+		$this->assertTrue($db->getLock('DatabaseTest'), 'The same lock can be aquired multiple times in the same connection');
+
+		$this->assertTrue($db->getLock('DatabaseTestOtherLock'), 'Can aquire different lock');
+		$db->releaseLock('DatabaseTestOtherLock');
+
+		// Release potentially stacked locks from previous getLock() invocations
+		$db->releaseLock('DatabaseTest');
+		$db->releaseLock('DatabaseTest');
+
+		$this->assertTrue($db->getLock('DatabaseTest'), 'Can aquire lock after releasing it');
+		$db->releaseLock('DatabaseTest');
+	}
+
+	function testCanLock() {
+		$db = DB::getConn();
+
+		if(!$db->supportsLocks()) {
+			return $this->markTestSkipped('Database doesn\'t support locks');
+		}
+
+		if($db instanceof MSSQLDatabase) {
+			return $this->markTestSkipped('MSSQLDatabase doesn\'t support inspecting locks');
+		}
+
+		$this->assertTrue($db->canLock('DatabaseTest'), 'Can lock before first aquiring one');
+		$db->getLock('DatabaseTest');
+		$this->assertFalse($db->canLock('DatabaseTest'), 'Can\'t lock after aquiring one');
+		$db->releaseLock('DatabaseTest');
+		$this->assertTrue($db->canLock('DatabaseTest'), 'Can lock again after releasing it');
+	}
+
 }
 
 class DatabaseTest_MyObject extends DataObject implements TestOnly {