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QuorumLockManager.php
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QuorumLockManager.php
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<?php
/**
* 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
*/
/**
* Base class for lock managers that use a quorum of peer servers for locks.
*
* The resource space can also be sharded into separate peer groups.
*
* See MemcLockManager and RedisLockManager.
*
* @stable to extend
* @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 doLockByType( array $pathsByType ) {
$status = StatusValue::newGood();
$pathsByTypeByBucket = []; // (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 );
$pathsByTypeByBucket[$bucket][$type][] = $path;
}
}
}
// Acquire locks in each bucket in bucket order to reduce contention. Any blocking
// mutexes during the acquisition step will not involve circular waiting on buckets.
ksort( $pathsByTypeByBucket );
$lockedPaths = []; // files locked in this attempt (type => paths)
// Attempt to acquire these locks...
foreach ( $pathsByTypeByBucket as $bucket => $bucketPathsByType ) {
// Try to acquire the locks for this bucket
$status->merge( $this->doLockingRequestBucket( $bucket, $bucketPathsByType ) );
if ( !$status->isOK() ) {
$status->merge( $this->doUnlockByType( $lockedPaths ) );
return $status;
}
// Record these locks as active
foreach ( $bucketPathsByType 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;
}
/**
* @stable to override
*
* @param array $pathsByType
*
* @return StatusValue
*/
protected function doUnlockByType( array $pathsByType ) {
$status = StatusValue::newGood();
$pathsByTypeByBucket = []; // (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 );
$pathsByTypeByBucket[$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 ( $pathsByTypeByBucket as $bucket => $bucketPathsByType ) {
$status->merge( $this->doUnlockingRequestBucket( $bucket, $bucketPathsByType ) );
}
if ( $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 yielding 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 yielding 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();
final protected function doLock( array $paths, $type ) {
// @phan-suppress-previous-line PhanPluginNeverReturnMethod
throw new LogicException( __METHOD__ . ': proxy class does not need this method.' );
}
final protected function doUnlock( array $paths, $type ) {
// @phan-suppress-previous-line PhanPluginNeverReturnMethod
throw new LogicException( __METHOD__ . ': proxy class does not need this method.' );
}
}

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