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201 lines
6.7 KiB
PHP
201 lines
6.7 KiB
PHP
<?php
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declare(strict_types=1);
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/**
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* @copyright Copyright (c) 2016, ownCloud, Inc.
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*
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* @author Christoph Wurst <christoph@winzerhof-wurst.at>
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* @author Jaakko Salo <jaakkos@gmail.com>
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* @author Robin Appelman <robin@icewind.nl>
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* @author Roeland Jago Douma <roeland@famdouma.nl>
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*
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* @license AGPL-3.0
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*
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* This code is free software: you can redistribute it and/or modify
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* it under the terms of the GNU Affero General Public License, version 3,
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* as published by the Free Software Foundation.
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*
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* This program is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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* GNU Affero General Public License for more details.
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*
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* You should have received a copy of the GNU Affero General Public License, version 3,
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* along with this program. If not, see <http://www.gnu.org/licenses/>
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*
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*/
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namespace OC\Lock;
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use OCP\AppFramework\Utility\ITimeFactory;
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use OCP\IMemcache;
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use OCP\IMemcacheTTL;
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use OCP\Lock\LockedException;
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class MemcacheLockingProvider extends AbstractLockingProvider {
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/** @var array<string, array{time: int, ttl: int}> */
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private array $oldTTLs = [];
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public function __construct(
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private IMemcache $memcache,
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private ITimeFactory $timeFactory,
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int $ttl = 3600,
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) {
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parent::__construct($ttl);
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}
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private function setTTL(string $path, int $ttl = null, ?int $compare = null): void {
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if (is_null($ttl)) {
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$ttl = $this->ttl;
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}
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if ($this->memcache instanceof IMemcacheTTL) {
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if ($compare !== null) {
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$this->memcache->compareSetTTL($path, $compare, $ttl);
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} else {
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$this->memcache->setTTL($path, $ttl);
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}
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}
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}
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private function getTTL(string $path): int {
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if ($this->memcache instanceof IMemcacheTTL) {
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$ttl = $this->memcache->getTTL($path);
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return $ttl === false ? -1 : $ttl;
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} else {
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return -1;
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}
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}
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public function isLocked(string $path, int $type): bool {
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$lockValue = $this->memcache->get($path);
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if ($type === self::LOCK_SHARED) {
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return is_int($lockValue) && $lockValue > 0;
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} elseif ($type === self::LOCK_EXCLUSIVE) {
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return $lockValue === 'exclusive';
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} else {
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return false;
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}
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}
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public function acquireLock(string $path, int $type, ?string $readablePath = null): void {
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if ($type === self::LOCK_SHARED) {
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// save the old TTL to for `restoreTTL`
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$this->oldTTLs[$path] = [
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"ttl" => $this->getTTL($path),
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"time" => $this->timeFactory->getTime()
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];
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if (!$this->memcache->inc($path)) {
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throw new LockedException($path, null, $this->getExistingLockForException($path), $readablePath);
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}
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} else {
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// when getting exclusive locks, we know there are no old TTLs to restore
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$this->memcache->add($path, 0);
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// ttl is updated automatically when the `set` succeeds
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if (!$this->memcache->cas($path, 0, 'exclusive')) {
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throw new LockedException($path, null, $this->getExistingLockForException($path), $readablePath);
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}
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unset($this->oldTTLs[$path]);
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}
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$this->setTTL($path);
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$this->markAcquire($path, $type);
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}
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public function releaseLock(string $path, int $type): void {
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if ($type === self::LOCK_SHARED) {
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$ownSharedLockCount = $this->getOwnSharedLockCount($path);
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$newValue = 0;
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if ($ownSharedLockCount === 0) { // if we are not holding the lock, don't try to release it
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return;
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}
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if ($ownSharedLockCount === 1) {
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$removed = $this->memcache->cad($path, 1); // if we're the only one having a shared lock we can remove it in one go
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if (!$removed) { //someone else also has a shared lock, decrease only
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$newValue = $this->memcache->dec($path);
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}
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} else {
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// if we own more than one lock ourselves just decrease
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$newValue = $this->memcache->dec($path);
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}
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if ($newValue > 0) {
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$this->restoreTTL($path);
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} else {
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unset($this->oldTTLs[$path]);
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}
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// if we somehow release more locks then exists, reset the lock
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if ($newValue < 0) {
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$this->memcache->cad($path, $newValue);
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}
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} elseif ($type === self::LOCK_EXCLUSIVE) {
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$this->memcache->cad($path, 'exclusive');
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}
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$this->markRelease($path, $type);
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}
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public function changeLock(string $path, int $targetType): void {
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if ($targetType === self::LOCK_SHARED) {
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if (!$this->memcache->cas($path, 'exclusive', 1)) {
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throw new LockedException($path, null, $this->getExistingLockForException($path));
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}
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} elseif ($targetType === self::LOCK_EXCLUSIVE) {
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// we can only change a shared lock to an exclusive if there's only a single owner of the shared lock
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if (!$this->memcache->cas($path, 1, 'exclusive')) {
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$this->restoreTTL($path);
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throw new LockedException($path, null, $this->getExistingLockForException($path));
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}
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unset($this->oldTTLs[$path]);
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}
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$this->setTTL($path);
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$this->markChange($path, $targetType);
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}
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/**
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* With shared locks, each time the lock is acquired, the ttl for the path is reset.
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*
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* Due to this "ttl extension" when a shared lock isn't freed correctly for any reason
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* the lock won't expire until no shared locks are required for the path for 1h.
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* This can lead to a client repeatedly trying to upload a file, and failing forever
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* because the lock never gets the opportunity to expire.
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*
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* To help the lock expire in this case, we lower the TTL back to what it was before we
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* took the shared lock *only* if nobody else got a shared lock after we did.
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*
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* This doesn't handle all cases where multiple requests are acquiring shared locks
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* but it should handle some of the more common ones and not hurt things further
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*/
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private function restoreTTL(string $path): void {
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if (isset($this->oldTTLs[$path])) {
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$saved = $this->oldTTLs[$path];
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$elapsed = $this->timeFactory->getTime() - $saved['time'];
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// old value to compare to when setting ttl in case someone else changes the lock in the middle of this function
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$value = $this->memcache->get($path);
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$currentTtl = $this->getTTL($path);
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// what the old ttl would be given the time elapsed since we acquired the lock
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// note that if this gets negative the key will be expired directly when we set the ttl
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$remainingOldTtl = $saved['ttl'] - $elapsed;
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// what the currently ttl would be if nobody else acquired a lock since we did (+1 to cover rounding errors)
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$expectedTtl = $this->ttl - $elapsed + 1;
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// check if another request has acquired a lock (and didn't release it yet)
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if ($currentTtl <= $expectedTtl) {
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$this->setTTL($path, $remainingOldTtl, $value);
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}
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}
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}
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private function getExistingLockForException(string $path): string {
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$existing = $this->memcache->get($path);
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if (!$existing) {
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return 'none';
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} elseif ($existing === 'exclusive') {
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return $existing;
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} else {
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return $existing . ' shared locks';
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}
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}
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}
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