QuorumLockManager.php

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<?php
abstract class QuorumLockManager extends LockManager {
    protected $srvsByBucket = []; // (bucket index => (lsrv1, lsrv2, ...))
 
    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 ( $this->locksHeld[$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 ( $this->locksHeld === [] ) {
            $status->merge( $this->releaseAllLocks() );
            $this->degradedBuckets = []; // safe to retry the normal quorum
        }
 
        return $status;
    }
 
    final protected function doLockingRequestBucket( $bucket, array $pathsByType ) {
        return $this->collectPledgeQuorum(
            $bucket,
            function ( $lockSrv ) use ( $pathsByType ) {
                return $this->getLocksOnServer( $lockSrv, $pathsByType );
            }
        );
    }
 
    final protected function doUnlockingRequestBucket( $bucket, array $pathsByType ) {
        return $this->releasePledges(
            $bucket,
            function ( $lockSrv ) use ( $pathsByType ) {
                return $this->freeLocksOnServer( $lockSrv, $pathsByType );
            }
        );
    }
 
    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;
    }
 
    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;
    }
 
    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 $pathsByType );
 
    abstract protected function freeLocksOnServer( $lockSrv, array $pathsByType );
 
    abstract protected function releaseAllLocks();
}