X-Git-Url: https://scripts.mit.edu/gitweb/autoinstallsdev/mediawiki.git/blobdiff_plain/19e297c21b10b1b8a3acad5e73fc71dcb35db44a..6932310fd58ebef145fa01eb76edf7150284d8ea:/includes/libs/lockmanager/QuorumLockManager.php

diff --git a/includes/libs/lockmanager/QuorumLockManager.php b/includes/libs/lockmanager/QuorumLockManager.php
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+<?php
+/**
+ * Version of LockManager that uses a quorum from peer servers for locks.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * 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 General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ * http://www.gnu.org/copyleft/gpl.html
+ *
+ * @file
+ * @ingroup LockManager
+ */
+
+/**
+ * Version of LockManager that uses a quorum from peer servers for locks.
+ * The resource space can also be sharded into separate peer groups.
+ *
+ * @ingroup LockManager
+ * @since 1.20
+ */
+abstract class QuorumLockManager extends LockManager {
+	/** @var array Map of bucket indexes to peer server lists */
+	protected $srvsByBucket = []; // (bucket index => (lsrv1, lsrv2, ...))
+
+	/** @var array Map of degraded buckets */
+	protected $degradedBuckets = []; // (bucket index => UNIX timestamp)
+
+	final protected function doLock( array $paths, $type ) {
+		return $this->doLockByType( [ $type => $paths ] );
+	}
+
+	final protected function doUnlock( array $paths, $type ) {
+		return $this->doUnlockByType( [ $type => $paths ] );
+	}
+
+	protected function doLockByType( array $pathsByType ) {
+		$status = StatusValue::newGood();
+
+		$pathsToLock = []; // (bucket => type => paths)
+		// Get locks that need to be acquired (buckets => locks)...
+		foreach ( $pathsByType as $type => $paths ) {
+			foreach ( $paths as $path ) {
+				if ( isset( $this->locksHeld[$path][$type] ) ) {
+					++$this->locksHeld[$path][$type];
+				} else {
+					$bucket = $this->getBucketFromPath( $path );
+					$pathsToLock[$bucket][$type][] = $path;
+				}
+			}
+		}
+
+		$lockedPaths = []; // files locked in this attempt (type => paths)
+		// Attempt to acquire these locks...
+		foreach ( $pathsToLock as $bucket => $pathsToLockByType ) {
+			// Try to acquire the locks for this bucket
+			$status->merge( $this->doLockingRequestBucket( $bucket, $pathsToLockByType ) );
+			if ( !$status->isOK() ) {
+				$status->merge( $this->doUnlockByType( $lockedPaths ) );
+
+				return $status;
+			}
+			// Record these locks as active
+			foreach ( $pathsToLockByType as $type => $paths ) {
+				foreach ( $paths as $path ) {
+					$this->locksHeld[$path][$type] = 1; // locked
+					// Keep track of what locks were made in this attempt
+					$lockedPaths[$type][] = $path;
+				}
+			}
+		}
+
+		return $status;
+	}
+
+	protected function doUnlockByType( array $pathsByType ) {
+		$status = StatusValue::newGood();
+
+		$pathsToUnlock = []; // (bucket => type => paths)
+		foreach ( $pathsByType as $type => $paths ) {
+			foreach ( $paths as $path ) {
+				if ( !isset( $this->locksHeld[$path][$type] ) ) {
+					$status->warning( 'lockmanager-notlocked', $path );
+				} else {
+					--$this->locksHeld[$path][$type];
+					// Reference count the locks held and release locks when zero
+					if ( $this->locksHeld[$path][$type] <= 0 ) {
+						unset( $this->locksHeld[$path][$type] );
+						$bucket = $this->getBucketFromPath( $path );
+						$pathsToUnlock[$bucket][$type][] = $path;
+					}
+					if ( !count( $this->locksHeld[$path] ) ) {
+						unset( $this->locksHeld[$path] ); // no SH or EX locks left for key
+					}
+				}
+			}
+		}
+
+		// Remove these specific locks if possible, or at least release
+		// all locks once this process is currently not holding any locks.
+		foreach ( $pathsToUnlock as $bucket => $pathsToUnlockByType ) {
+			$status->merge( $this->doUnlockingRequestBucket( $bucket, $pathsToUnlockByType ) );
+		}
+		if ( !count( $this->locksHeld ) ) {
+			$status->merge( $this->releaseAllLocks() );
+			$this->degradedBuckets = []; // safe to retry the normal quorum
+		}
+
+		return $status;
+	}
+
+	/**
+	 * Attempt to acquire locks with the peers for a bucket.
+	 * This is all or nothing; if any key is locked then this totally fails.
+	 *
+	 * @param int $bucket
+	 * @param array $pathsByType Map of LockManager::LOCK_* constants to lists of paths
+	 * @return StatusValue
+	 */
+	final protected function doLockingRequestBucket( $bucket, array $pathsByType ) {
+		return $this->collectPledgeQuorum(
+			$bucket,
+			function ( $lockSrv ) use ( $pathsByType ) {
+				return $this->getLocksOnServer( $lockSrv, $pathsByType );
+			}
+		);
+	}
+
+	/**
+	 * Attempt to release locks with the peers for a bucket
+	 *
+	 * @param int $bucket
+	 * @param array $pathsByType Map of LockManager::LOCK_* constants to lists of paths
+	 * @return StatusValue
+	 */
+	final protected function doUnlockingRequestBucket( $bucket, array $pathsByType ) {
+		return $this->releasePledges(
+			$bucket,
+			function ( $lockSrv ) use ( $pathsByType ) {
+				return $this->freeLocksOnServer( $lockSrv, $pathsByType );
+			}
+		);
+	}
+
+	/**
+	 * Attempt to acquire pledges with the peers for a bucket.
+	 * This is all or nothing; if any key is already pledged then this totally fails.
+	 *
+	 * @param int $bucket
+	 * @param callable $callback Pledge method taking a server name and yeilding a StatusValue
+	 * @return StatusValue
+	 */
+	final protected function collectPledgeQuorum( $bucket, callable $callback ) {
+		$status = StatusValue::newGood();
+
+		$yesVotes = 0; // locks made on trustable servers
+		$votesLeft = count( $this->srvsByBucket[$bucket] ); // remaining peers
+		$quorum = floor( $votesLeft / 2 + 1 ); // simple majority
+		// Get votes for each peer, in order, until we have enough...
+		foreach ( $this->srvsByBucket[$bucket] as $lockSrv ) {
+			if ( !$this->isServerUp( $lockSrv ) ) {
+				--$votesLeft;
+				$status->warning( 'lockmanager-fail-svr-acquire', $lockSrv );
+				$this->degradedBuckets[$bucket] = time();
+				continue; // server down?
+			}
+			// Attempt to acquire the lock on this peer
+			$status->merge( $callback( $lockSrv ) );
+			if ( !$status->isOK() ) {
+				return $status; // vetoed; resource locked
+			}
+			++$yesVotes; // success for this peer
+			if ( $yesVotes >= $quorum ) {
+				return $status; // lock obtained
+			}
+			--$votesLeft;
+			$votesNeeded = $quorum - $yesVotes;
+			if ( $votesNeeded > $votesLeft ) {
+				break; // short-circuit
+			}
+		}
+		// At this point, we must not have met the quorum
+		$status->setResult( false );
+
+		return $status;
+	}
+
+	/**
+	 * Attempt to release pledges with the peers for a bucket
+	 *
+	 * @param int $bucket
+	 * @param callable $callback Pledge method taking a server name and yeilding a StatusValue
+	 * @return StatusValue
+	 */
+	final protected function releasePledges( $bucket, callable $callback ) {
+		$status = StatusValue::newGood();
+
+		$yesVotes = 0; // locks freed on trustable servers
+		$votesLeft = count( $this->srvsByBucket[$bucket] ); // remaining peers
+		$quorum = floor( $votesLeft / 2 + 1 ); // simple majority
+		$isDegraded = isset( $this->degradedBuckets[$bucket] ); // not the normal quorum?
+		foreach ( $this->srvsByBucket[$bucket] as $lockSrv ) {
+			if ( !$this->isServerUp( $lockSrv ) ) {
+				$status->warning( 'lockmanager-fail-svr-release', $lockSrv );
+			} else {
+				// Attempt to release the lock on this peer
+				$status->merge( $callback( $lockSrv ) );
+				++$yesVotes; // success for this peer
+				// Normally the first peers form the quorum, and the others are ignored.
+				// Ignore them in this case, but not when an alternative quorum was used.
+				if ( $yesVotes >= $quorum && !$isDegraded ) {
+					break; // lock released
+				}
+			}
+		}
+		// Set a bad StatusValue if the quorum was not met.
+		// Assumes the same "up" servers as during the acquire step.
+		$status->setResult( $yesVotes >= $quorum );
+
+		return $status;
+	}
+
+	/**
+	 * Get the bucket for resource path.
+	 * This should avoid throwing any exceptions.
+	 *
+	 * @param string $path
+	 * @return int
+	 */
+	protected function getBucketFromPath( $path ) {
+		$prefix = substr( sha1( $path ), 0, 2 ); // first 2 hex chars (8 bits)
+		return (int)base_convert( $prefix, 16, 10 ) % count( $this->srvsByBucket );
+	}
+
+	/**
+	 * Check if a lock server is up.
+	 * This should process cache results to reduce RTT.
+	 *
+	 * @param string $lockSrv
+	 * @return bool
+	 */
+	abstract protected function isServerUp( $lockSrv );
+
+	/**
+	 * Get a connection to a lock server and acquire locks
+	 *
+	 * @param string $lockSrv
+	 * @param array $pathsByType Map of LockManager::LOCK_* constants to lists of paths
+	 * @return StatusValue
+	 */
+	abstract protected function getLocksOnServer( $lockSrv, array $pathsByType );
+
+	/**
+	 * Get a connection to a lock server and release locks on $paths.
+	 *
+	 * Subclasses must effectively implement this or releaseAllLocks().
+	 *
+	 * @param string $lockSrv
+	 * @param array $pathsByType Map of LockManager::LOCK_* constants to lists of paths
+	 * @return StatusValue
+	 */
+	abstract protected function freeLocksOnServer( $lockSrv, array $pathsByType );
+
+	/**
+	 * Release all locks that this session is holding.
+	 *
+	 * Subclasses must effectively implement this or freeLocksOnServer().
+	 *
+	 * @return StatusValue
+	 */
+	abstract protected function releaseAllLocks();
+}