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<?php
declare(strict_types=1);
/**
* @copyright Copyright (c) 2016, ownCloud, Inc.
*
* @author Christoph Wurst <christoph@winzerhof-wurst.at>
* @author Jaakko Salo <jaakkos@gmail.com>
* @author Robin Appelman <robin@icewind.nl>
* @author Roeland Jago Douma <roeland@famdouma.nl>
*
* @license AGPL-3.0
*
* This code is free software: you can redistribute it and/or modify
* it under the terms of the GNU Affero General Public License, version 3,
* as published by the Free Software Foundation.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU Affero General Public License for more details.
*
* You should have received a copy of the GNU Affero General Public License, version 3,
* along with this program. If not, see <http://www.gnu.org/licenses/>
*
*/
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, ?int $compare = null): void {
if (is_null($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';
}
}
}
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