add method to atomically change between shared and exclusive lock

lib/private/lock/memcachelockingprovider.php

@@ -98,6 +98,26 @@ class MemcacheLockingProvider implements ILockingProvider {
 		}
 	}
 
+	/**
+	 * Change the type of an existing lock
+	 *
+	 * @param string $path
+	 * @param int $targetType self::LOCK_SHARED or self::LOCK_EXCLUSIVE
+	 * @throws \OCP\Lock\LockedException
+	 */
+	public function changeLock($path, $targetType) {
+		if ($targetType === self::LOCK_SHARED) {
+			if (!$this->memcache->cas($path, 'exclusive', 1)) {
+				throw new LockedException($path);
+			}
+		} else if ($targetType === self::LOCK_EXCLUSIVE) {
+			// we can only change a shared lock to an exclusive if there's only a single owner of the shared lock
+			if (!$this->memcache->cas($path, 1, 'exclusive')) {
+				throw new LockedException($path);
+			}
+		}
+	}
+
 	/**
 	 * release all lock acquired by this instance
 	 */

lib/public/lock/ilockingprovider.php

@@ -45,6 +45,15 @@ interface ILockingProvider {
 	 */
 	public function releaseLock($path, $type);
 
+	/**
+	 * Change the type of an existing lock
+	 *
+	 * @param string $path
+	 * @param int $targetType self::LOCK_SHARED or self::LOCK_EXCLUSIVE
+	 * @throws \OCP\Lock\LockedException
+	 */
+	public function changeLock($path, $targetType);
+
 	/**
 	 * release all lock acquired by this instance
 	 */

tests/lib/lock/lockingprovider.php

@@ -158,4 +158,57 @@ abstract class LockingProvider extends TestCase {
 			$this->assertEquals('foo', $e->getPath());
 		}
 	}
+
+	public function testChangeLockToExclusive() {
+		$this->instance->acquireLock('foo', ILockingProvider::LOCK_SHARED);
+		$this->instance->changeLock('foo', ILockingProvider::LOCK_EXCLUSIVE);
+		$this->assertFalse($this->instance->isLocked('foo', ILockingProvider::LOCK_SHARED));
+		$this->assertTrue($this->instance->isLocked('foo', ILockingProvider::LOCK_EXCLUSIVE));
+	}
+
+	public function testChangeLockToShared() {
+		$this->instance->acquireLock('foo', ILockingProvider::LOCK_EXCLUSIVE);
+		$this->instance->changeLock('foo', ILockingProvider::LOCK_SHARED);
+		$this->assertFalse($this->instance->isLocked('foo', ILockingProvider::LOCK_EXCLUSIVE));
+		$this->assertTrue($this->instance->isLocked('foo', ILockingProvider::LOCK_SHARED));
+	}
+
+	/**
+	 * @expectedException \OCP\Lock\LockedException
+	 */
+	public function testChangeLockToExclusiveDoubleShared() {
+		$this->instance->acquireLock('foo', ILockingProvider::LOCK_SHARED);
+		$this->instance->acquireLock('foo', ILockingProvider::LOCK_SHARED);
+		$this->instance->changeLock('foo', ILockingProvider::LOCK_EXCLUSIVE);
+	}
+
+	/**
+	 * @expectedException \OCP\Lock\LockedException
+	 */
+	public function testChangeLockToExclusiveNoShared() {
+		$this->instance->changeLock('foo', ILockingProvider::LOCK_EXCLUSIVE);
+	}
+
+	/**
+	 * @expectedException \OCP\Lock\LockedException
+	 */
+	public function testChangeLockToExclusiveFromExclusive() {
+		$this->instance->acquireLock('foo', ILockingProvider::LOCK_EXCLUSIVE);
+		$this->instance->changeLock('foo', ILockingProvider::LOCK_EXCLUSIVE);
+	}
+
+	/**
+	 * @expectedException \OCP\Lock\LockedException
+	 */
+	public function testChangeLockToSharedNoExclusive() {
+		$this->instance->changeLock('foo', ILockingProvider::LOCK_SHARED);
+	}
+
+	/**
+	 * @expectedException \OCP\Lock\LockedException
+	 */
+	public function testChangeLockToSharedFromShared() {
+		$this->instance->acquireLock('foo', ILockingProvider::LOCK_SHARED);
+		$this->instance->changeLock('foo', ILockingProvider::LOCK_SHARED);
+	}
 }