1 files changed, 181 insertions, 0 deletions
diff --git a/lib/private/Lock/MemcacheLockingProvider.php b/lib/private/Lock/MemcacheLockingProvider.php
new file mode 100644
index 00000000000..8cc0c27bbf9
--- /dev/null
+++ b/lib/private/Lock/MemcacheLockingProvider.php
@@ -0,0 +1,181 @@
+<?php
+
+declare(strict_types=1);
+/**
+ * SPDX-FileCopyrightText: 2016-2024 Nextcloud GmbH and Nextcloud contributors
+ * SPDX-FileCopyrightText: 2016 ownCloud, Inc.
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+namespace OC\Lock;
+
+use OCP\AppFramework\Utility\ITimeFactory;
+use OCP\IMemcache;
+use OCP\IMemcacheTTL;
+use OCP\Lock\LockedException;
+
+class MemcacheLockingProvider extends AbstractLockingProvider {
+	/** @var array<string, array{time: int, ttl: int}> */
+	private array $oldTTLs = [];
+
+	public function __construct(
+		private IMemcache $memcache,
+		private ITimeFactory $timeFactory,
+		int $ttl = 3600,
+	) {
+		parent::__construct($ttl);
+	}
+
+	private function setTTL(string $path, ?int $ttl = null, mixed $compare = null): void {
+		if (is_null($ttl)) {
+			$ttl = $this->ttl;
+		}
+		if ($this->memcache instanceof IMemcacheTTL) {
+			if ($compare !== null) {
+				$this->memcache->compareSetTTL($path, $compare, $ttl);
+			} else {
+				$this->memcache->setTTL($path, $ttl);
+			}
+		}
+	}
+
+	private function getTTL(string $path): int {
+		if ($this->memcache instanceof IMemcacheTTL) {
+			$ttl = $this->memcache->getTTL($path);
+			return $ttl === false ? -1 : $ttl;
+		} else {
+			return -1;
+		}
+	}
+
+	public function isLocked(string $path, int $type): bool {
+		$lockValue = $this->memcache->get($path);
+		if ($type === self::LOCK_SHARED) {
+			return is_int($lockValue) && $lockValue > 0;
+		} elseif ($type === self::LOCK_EXCLUSIVE) {
+			return $lockValue === 'exclusive';
+		} else {
+			return false;
+		}
+	}
+
+	public function acquireLock(string $path, int $type, ?string $readablePath = null): void {
+		if ($type === self::LOCK_SHARED) {
+			// save the old TTL to for `restoreTTL`
+			$this->oldTTLs[$path] = [
+				'ttl' => $this->getTTL($path),
+				'time' => $this->timeFactory->getTime()
+			];
+			if (!$this->memcache->inc($path)) {
+				throw new LockedException($path, null, $this->getExistingLockForException($path), $readablePath);
+			}
+		} else {
+			// when getting exclusive locks, we know there are no old TTLs to restore
+			$this->memcache->add($path, 0);
+			// ttl is updated automatically when the `set` succeeds
+			if (!$this->memcache->cas($path, 0, 'exclusive')) {
+				throw new LockedException($path, null, $this->getExistingLockForException($path), $readablePath);
+			}
+			unset($this->oldTTLs[$path]);
+		}
+		$this->setTTL($path);
+		$this->markAcquire($path, $type);
+	}
+
+	public function releaseLock(string $path, int $type): void {
+		if ($type === self::LOCK_SHARED) {
+			$ownSharedLockCount = $this->getOwnSharedLockCount($path);
+			$newValue = 0;
+			if ($ownSharedLockCount === 0) { // if we are not holding the lock, don't try to release it
+				return;
+			}
+			if ($ownSharedLockCount === 1) {
+				$removed = $this->memcache->cad($path, 1); // if we're the only one having a shared lock we can remove it in one go
+				if (!$removed) { //someone else also has a shared lock, decrease only
+					$newValue = $this->memcache->dec($path);
+				}
+			} else {
+				// if we own more than one lock ourselves just decrease
+				$newValue = $this->memcache->dec($path);
+			}
+
+			if ($newValue > 0) {
+				$this->restoreTTL($path);
+			} else {
+				unset($this->oldTTLs[$path]);
+			}
+
+			// if we somehow release more locks then exists, reset the lock
+			if ($newValue < 0) {
+				$this->memcache->cad($path, $newValue);
+			}
+		} elseif ($type === self::LOCK_EXCLUSIVE) {
+			$this->memcache->cad($path, 'exclusive');
+		}
+		$this->markRelease($path, $type);
+	}
+
+	public function changeLock(string $path, int $targetType): void {
+		if ($targetType === self::LOCK_SHARED) {
+			if (!$this->memcache->cas($path, 'exclusive', 1)) {
+				throw new LockedException($path, null, $this->getExistingLockForException($path));
+			}
+		} elseif ($targetType === self::LOCK_EXCLUSIVE) {
+			// we can only change a shared lock to an exclusive if there's only a single owner of the shared lock
+			if (!$this->memcache->cas($path, 1, 'exclusive')) {
+				$this->restoreTTL($path);
+				throw new LockedException($path, null, $this->getExistingLockForException($path));
+			}
+			unset($this->oldTTLs[$path]);
+		}
+		$this->setTTL($path);
+		$this->markChange($path, $targetType);
+	}
+
+	/**
+	 * With shared locks, each time the lock is acquired, the ttl for the path is reset.
+	 *
+	 * Due to this "ttl extension" when a shared lock isn't freed correctly for any reason
+	 * the lock won't expire until no shared locks are required for the path for 1h.
+	 * This can lead to a client repeatedly trying to upload a file, and failing forever
+	 * because the lock never gets the opportunity to expire.
+	 *
+	 * To help the lock expire in this case, we lower the TTL back to what it was before we
+	 * took the shared lock *only* if nobody else got a shared lock after we did.
+	 *
+	 * This doesn't handle all cases where multiple requests are acquiring shared locks
+	 * but it should handle some of the more common ones and not hurt things further
+	 */
+	private function restoreTTL(string $path): void {
+		if (isset($this->oldTTLs[$path])) {
+			$saved = $this->oldTTLs[$path];
+			$elapsed = $this->timeFactory->getTime() - $saved['time'];
+
+			// old value to compare to when setting ttl in case someone else changes the lock in the middle of this function
+			$value = $this->memcache->get($path);
+
+			$currentTtl = $this->getTTL($path);
+
+			// what the old ttl would be given the time elapsed since we acquired the lock
+			// note that if this gets negative the key will be expired directly when we set the ttl
+			$remainingOldTtl = $saved['ttl'] - $elapsed;
+			// what the currently ttl would be if nobody else acquired a lock since we did (+1 to cover rounding errors)
+			$expectedTtl = $this->ttl - $elapsed + 1;
+
+			// check if another request has acquired a lock (and didn't release it yet)
+			if ($currentTtl <= $expectedTtl) {
+				$this->setTTL($path, $remainingOldTtl, $value);
+			}
+		}
+	}
+
+	private function getExistingLockForException(string $path): string {
+		$existing = $this->memcache->get($path);
+		if (!$existing) {
+			return 'none';
+		} elseif ($existing === 'exclusive') {
+			return $existing;
+		} else {
+			return $existing . ' shared locks';
+		}
+	}
+}