QuorumLockManager.php

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<?php
abstract class QuorumLockManager extends LockManager {
        protected $srvsByBucket = array(); // (bucket index => (lsrv1, lsrv2, ...))

        final protected function doLock( array $paths, $type ) {
                $status = Status::newGood();

                $pathsToLock = array(); // (bucket => paths)
                // Get locks that need to be acquired (buckets => locks)...
                foreach ( $paths as $path ) {
                        if ( isset( $this->locksHeld[$path][$type] ) ) {
                                ++$this->locksHeld[$path][$type];
                        } else {
                                $bucket = $this->getBucketFromPath( $path );
                                $pathsToLock[$bucket][] = $path;
                        }
                }

                $lockedPaths = array(); // files locked in this attempt
                // Attempt to acquire these locks...
                foreach ( $pathsToLock as $bucket => $paths ) {
                        // Try to acquire the locks for this bucket
                        $status->merge( $this->doLockingRequestBucket( $bucket, $paths, $type ) );
                        if ( !$status->isOK() ) {
                                $status->merge( $this->doUnlock( $lockedPaths, $type ) );
                                return $status;
                        }
                        // Record these locks as active
                        foreach ( $paths as $path ) {
                                $this->locksHeld[$path][$type] = 1; // locked
                        }
                        // Keep track of what locks were made in this attempt
                        $lockedPaths = array_merge( $lockedPaths, $paths );
                }

                return $status;
        }

        final protected function doUnlock( array $paths, $type ) {
                $status = Status::newGood();

                $pathsToUnlock = array();
                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][] = $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 => $paths ) {
                        $status->merge( $this->doUnlockingRequestBucket( $bucket, $paths, $type ) );
                }
                if ( !count( $this->locksHeld ) ) {
                        $status->merge( $this->releaseAllLocks() );
                }

                return $status;
        }

        final protected function doLockingRequestBucket( $bucket, array $paths, $type ) {
                $status = Status::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 );
                                continue; // server down?
                        }
                        // Attempt to acquire the lock on this peer
                        $status->merge( $this->getLocksOnServer( $lockSrv, $paths, $type ) );
                        if ( !$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;
        }

        final protected function doUnlockingRequestBucket( $bucket, array $paths, $type ) {
                $status = Status::newGood();

                foreach ( $this->srvsByBucket[$bucket] as $lockSrv ) {
                        if ( !$this->isServerUp( $lockSrv ) ) {
                                $status->fatal( 'lockmanager-fail-svr-release', $lockSrv );
                        // Attempt to release the lock on this peer
                        } else {
                                $status->merge( $this->freeLocksOnServer( $lockSrv, $paths, $type ) );
                        }
                }

                return $status;
        }

        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 );
        }

        abstract protected function isServerUp( $lockSrv );

        abstract protected function getLocksOnServer( $lockSrv, array $paths, $type );

        abstract protected function freeLocksOnServer( $lockSrv, array $paths, $type );

        abstract protected function releaseAllLocks();
}