TransactionManager.php

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<?php
namespace Wikimedia\Rdbms;
 
use Psr\Log\LoggerInterface;
use Psr\Log\NullLogger;
use RuntimeException;
use Throwable;
use UnexpectedValueException;
 
class TransactionManager {
    public const STATUS_TRX_ERROR = 1;
    public const STATUS_TRX_OK = 2;
    public const STATUS_TRX_NONE = 3;
 
    public const STATUS_SESS_ERROR = 1;
    public const STATUS_SESS_OK = 2;
 
    private const TINY_WRITE_SEC = 0.010;
    private const SLOW_WRITE_SEC = 0.500;
    private const SMALL_WRITE_ROWS = 100;
 
    private const SAVEPOINT_PREFIX = 'wikimedia_rdbms_atomic';
 
    private $trxId;
    private $trxTimestamp = null;
    private $trxRoundTripDelay = null;
    private $trxStatus = self::STATUS_TRX_NONE;
    private $trxStatusCause;
    private $trxStatusIgnoredCause;
 
    private $sessionError;
 
    private $trxWriteCallers = [];
    private $trxWriteDuration = 0.0;
    private $trxWriteQueryCount = 0;
    private $trxWriteAffectedRows = 0;
    private $trxWriteAdjDuration = 0.0;
    private $trxWriteAdjQueryCount = 0;
 
    private $trxAtomicLevels = [];
    private $trxAutomatic = false;
    private $trxAutomaticAtomic = false;
 
    private $trxFname = null;
    private $trxAtomicCounter = 0;
 
    private $trxPostCommitOrIdleCallbacks = [];
    private $trxPreCommitOrIdleCallbacks = [];
    private $trxEndCallbacks = [];
    private $trxRecurringCallbacks = [];
    private $trxEndCallbacksSuppressed = false;
 
    private $logger;
    private $profiler;
 
    public function __construct( ?LoggerInterface $logger = null, ?TransactionProfiler $profiler = null ) {
        $this->logger = $logger ?? new NullLogger();
        $this->profiler = $profiler ?? new TransactionProfiler();
    }
 
    public function trxLevel(): int {
        return $this->trxId ? 1 : 0;
    }
 
    public function getTrxId() {
        return $this->trxId;
    }
 
    private function consumeTrxId() {
        $old = $this->trxId;
        $this->trxId = null;
 
        return $old;
    }
 
    public function trxTimestamp(): ?float {
        return $this->trxLevel() ? $this->trxTimestamp : null;
    }
 
    public function trxStatus(): int {
        return $this->trxStatus;
    }
 
    public function setTrxStatusToOk() {
        $this->trxStatus = self::STATUS_TRX_OK;
        $this->trxStatusCause = null;
        $this->trxStatusIgnoredCause = null;
    }
 
    public function setTrxStatusToNone() {
        $this->trxStatus = self::STATUS_TRX_NONE;
        $this->trxStatusCause = null;
        $this->trxStatusIgnoredCause = null;
    }
 
    public function assertTransactionStatus( IDatabase $db, $deprecationLogger, $fname ) {
        if ( $this->trxStatus === self::STATUS_TRX_ERROR ) {
            throw new DBTransactionStateError(
                $db,
                "Cannot execute query from $fname while transaction status is ERROR",
                [],
                $this->trxStatusCause
            );
        } elseif ( $this->trxStatus === self::STATUS_TRX_OK && $this->trxStatusIgnoredCause ) {
            [ $iLastError, $iLastErrno, $iFname ] = $this->trxStatusIgnoredCause;
            $deprecationLogger(
                "Caller from $fname ignored an error originally raised from $iFname: " .
                "[$iLastErrno] $iLastError"
            );
            $this->trxStatusIgnoredCause = null;
        }
    }
 
    public function assertSessionStatus( IDatabase $db, string $fname ) {
        if ( $this->sessionError ) {
            throw new DBSessionStateError(
                $db,
                "Cannot execute query from $fname while session status is ERROR",
                [],
                $this->sessionError
            );
        }
    }
 
    public function setTransactionError( Throwable $trxError ) {
        if ( $this->trxStatus !== self::STATUS_TRX_ERROR ) {
            $this->trxStatus = self::STATUS_TRX_ERROR;
            $this->trxStatusCause = $trxError;
        }
    }
 
    public function setTrxStatusIgnoredCause( ?array $trxStatusIgnoredCause ): void {
        $this->trxStatusIgnoredCause = $trxStatusIgnoredCause;
    }
 
    public function sessionStatus() {
        // Check if an unresolved error still exists
        return ( $this->sessionError ) ? self::STATUS_SESS_ERROR : self::STATUS_SESS_OK;
    }
 
    public function setSessionError( Throwable $sessionError ) {
        $this->sessionError ??= $sessionError;
    }
 
    public function clearSessionError() {
        $this->sessionError = null;
    }
 
    private function calculateLastTrxApplyTime( float $rtt ) {
        $rttAdjTotal = $this->trxWriteAdjQueryCount * $rtt;
        $applyTime = max( $this->trxWriteAdjDuration - $rttAdjTotal, 0.0 );
        // For omitted queries, make them count as something at least
        $omitted = $this->trxWriteQueryCount - $this->trxWriteAdjQueryCount;
        $applyTime += self::TINY_WRITE_SEC * $omitted;
 
        return $applyTime;
    }
 
    public function pendingWriteCallers(): array {
        return $this->trxLevel() ? $this->trxWriteCallers : [];
    }
 
    public function updateTrxWriteQueryReport( $queryVerb, $runtime, $affected, $fname ) {
        // Whether this is indicative of replica DB runtime (except for RBR or ws_repl)
        $indicativeOfReplicaRuntime = true;
        if ( $runtime > self::SLOW_WRITE_SEC ) {
            // insert(), upsert(), replace() are fast unless bulky in size or blocked on locks
            if ( $queryVerb === 'INSERT' ) {
                $indicativeOfReplicaRuntime = $affected > self::SMALL_WRITE_ROWS;
            } elseif ( $queryVerb === 'REPLACE' ) {
                $indicativeOfReplicaRuntime = $affected > self::SMALL_WRITE_ROWS / 2;
            }
        }
 
        $this->trxWriteDuration += $runtime;
        $this->trxWriteQueryCount++;
        $this->trxWriteAffectedRows += $affected;
        if ( $indicativeOfReplicaRuntime ) {
            $this->trxWriteAdjDuration += $runtime;
            $this->trxWriteAdjQueryCount++;
        }
 
        $this->trxWriteCallers[] = $fname;
    }
 
    public function pendingWriteQueryDuration( string $type = IDatabase::ESTIMATE_TOTAL ): float|false {
        if ( !$this->trxLevel() ) {
            return false;
        } elseif ( !$this->trxWriteCallers ) {
            return 0.0;
        }
 
        if ( $type === IDatabase::ESTIMATE_DB_APPLY ) {
            return $this->calculateLastTrxApplyTime( $this->trxRoundTripDelay );
        }
 
        return $this->trxWriteDuration;
    }
 
    private function flatAtomicSectionList() {
        return array_reduce( $this->trxAtomicLevels, static function ( $accum, $v ) {
            return $accum === null ? $v[0] : "$accum, " . $v[0];
        } );
    }
 
    public function resetTrxAtomicLevels() {
        $this->trxAtomicLevels = [];
        $this->trxAtomicCounter = 0;
    }
 
    public function explicitTrxActive(): bool {
        return $this->trxLevel() && ( $this->trxAtomicLevels || !$this->trxAutomatic );
    }
 
    public function trxCheckBeforeClose( IDatabaseForOwner $db, string $fname ): ?string {
        $error = null;
        if ( $this->trxAtomicLevels ) {
            // Cannot let incomplete atomic sections be committed
            $levels = $this->flatAtomicSectionList();
            $error = "$fname: atomic sections $levels are still open";
        } elseif ( $this->trxAutomatic ) {
            // Only the connection manager can commit non-empty DBO_TRX transactions
            // (empty ones we can silently roll back)
            if ( $db->writesOrCallbacksPending() ) {
                $error = "$fname: " .
                    "expected mass rollback of all peer transactions (DBO_TRX set)";
            }
        } else {
            // Manual transactions should have been committed or rolled
            // back, even if empty.
            $error = "$fname: transaction is still open (from {$this->trxFname})";
        }
 
        if ( $this->trxEndCallbacksSuppressed && $error === null ) {
            $error = "$fname: callbacks are suppressed; cannot properly commit";
        }
 
        return $error;
    }
 
    public function onCancelAtomicBeforeCriticalSection( IDatabase $db, string $fname ): void {
        if ( !$this->trxLevel() || !$this->trxAtomicLevels ) {
            throw new DBUnexpectedError( $db, "No atomic section is open (got $fname)" );
        }
    }
 
    public function currentAtomicSectionId(): ?AtomicSectionIdentifier {
        if ( $this->trxLevel() && $this->trxAtomicLevels ) {
            $levelInfo = end( $this->trxAtomicLevels );
 
            return $levelInfo[1];
        }
 
        return null;
    }
 
    public function addToAtomicLevels( string $fname, AtomicSectionIdentifier $sectionId, ?string $savepointId ) {
        $this->trxAtomicLevels[] = [ $fname, $sectionId, $savepointId ];
        $this->logger->debug( 'startAtomic: entering level ' .
            ( count( $this->trxAtomicLevels ) - 1 ) . " ($fname)", [ 'db_log_category' => 'trx' ] );
    }
 
    public function onBegin( IDatabase $db, string $fname ): void {
        // Protect against mismatched atomic section, transaction nesting, and snapshot loss
        if ( $this->trxLevel() ) {
            if ( $this->trxAtomicLevels ) {
                $levels = $this->flatAtomicSectionList();
                $msg = "$fname: got explicit BEGIN while atomic section(s) $levels are open";
                throw new DBUnexpectedError( $db, $msg );
            } elseif ( !$this->trxAutomatic ) {
                $msg = "$fname: explicit transaction already active (from {$this->trxFname})";
                throw new DBUnexpectedError( $db, $msg );
            } else {
                $msg = "$fname: implicit transaction already active (from {$this->trxFname})";
                throw new DBUnexpectedError( $db, $msg );
            }
        }
    }
 
    public function onCommit( IDatabase $db, $fname, $flush ): bool {
        if ( $this->trxLevel() && $this->trxAtomicLevels ) {
            // There are still atomic sections open; this cannot be ignored
            $levels = $this->flatAtomicSectionList();
            throw new DBUnexpectedError(
                $db,
                "$fname: got COMMIT while atomic sections $levels are still open"
            );
        }
 
        if ( $flush === IDatabase::FLUSHING_INTERNAL || $flush === IDatabase::FLUSHING_ALL_PEERS ) {
            if ( !$this->trxLevel() ) {
                return false; // nothing to do
            } elseif ( !$this->trxAutomatic ) {
                throw new DBUnexpectedError(
                    $db,
                    "$fname: flushing an explicit transaction, getting out of sync"
                );
            }
        } elseif ( !$this->trxLevel() ) {
            $this->logger->error(
                "$fname: no transaction to commit, something got out of sync",
                [
                    'exception' => new RuntimeException(),
                    'db_log_category' => 'trx'
                ]
            );
 
            return false; // nothing to do
        } elseif ( $this->trxAutomatic ) {
            throw new DBUnexpectedError(
                $db,
                "$fname: expected mass commit of all peer transactions (DBO_TRX set)"
            );
        }
        return true;
    }
 
    public function onEndAtomic( IDatabase $db, string $fname ): array {
        if ( !$this->trxLevel() || !$this->trxAtomicLevels ) {
            throw new DBUnexpectedError( $db, "No atomic section is open (got $fname)" );
        }
        // Check if the current section matches $fname
        $pos = count( $this->trxAtomicLevels ) - 1;
        [ $savedFname, $sectionId, $savepointId ] = $this->trxAtomicLevels[$pos];
        $this->logger->debug( "endAtomic: leaving level $pos ($fname)", [ 'db_log_category' => 'trx' ] );
 
        if ( $savedFname !== $fname ) {
            throw new DBUnexpectedError(
                $db,
                "Invalid atomic section ended (got $fname but expected $savedFname)"
            );
        }
 
        return [ $savepointId, $sectionId ];
    }
 
    public function getPositionFromSectionId( ?AtomicSectionIdentifier $sectionId = null ): ?int {
        if ( $sectionId !== null ) {
            // Find the (last) section with the given $sectionId
            $pos = -1;
            foreach ( $this->trxAtomicLevels as $i => [ , $asId, ] ) {
                if ( $asId === $sectionId ) {
                    $pos = $i;
                }
            }
        } else {
            $pos = null;
        }
 
        return $pos;
    }
 
    public function cancelAtomic( ?int $pos ): array {
        $excisedIds = [];
        $excisedFnames = [];
        $newTopSection = $this->currentAtomicSectionId();
        if ( $pos !== null ) {
            // Remove all descendant sections and re-index the array
            $len = count( $this->trxAtomicLevels );
            for ( $i = $pos + 1; $i < $len; ++$i ) {
                $excisedFnames[] = $this->trxAtomicLevels[$i][0];
                $excisedIds[] = $this->trxAtomicLevels[$i][1];
            }
            $this->trxAtomicLevels = array_slice( $this->trxAtomicLevels, 0, $pos + 1 );
            $newTopSection = $this->currentAtomicSectionId();
        }
 
        // Check if the current section matches $fname
        $pos = count( $this->trxAtomicLevels ) - 1;
        [ $savedFname, $savedSectionId, $savepointId ] = $this->trxAtomicLevels[$pos];
 
        if ( $excisedFnames ) {
            $this->logger->debug( "cancelAtomic: canceling level $pos ($savedFname) " .
                "and descendants " . implode( ', ', $excisedFnames ),
                [ 'db_log_category' => 'trx' ]
            );
        } else {
            $this->logger->debug( "cancelAtomic: canceling level $pos ($savedFname)",
                [ 'db_log_category' => 'trx' ]
            );
        }
 
        return [ $savedFname, $excisedIds, $newTopSection, $savedSectionId, $savepointId ];
    }
 
    public function popAtomicLevel() {
        array_pop( $this->trxAtomicLevels );
 
        return !$this->trxAtomicLevels && $this->trxAutomaticAtomic;
    }
 
    public function setAutomaticAtomic( bool $value ) {
        $this->trxAutomaticAtomic = $value;
    }
 
    public function turnOnAutomatic() {
        $this->trxAutomatic = true;
    }
 
    public function nextSavePointId( IDatabase $db, string $fname ): string {
        $savepointId = self::SAVEPOINT_PREFIX . ++$this->trxAtomicCounter;
        if ( strlen( $savepointId ) > 30 ) {
            // 30 == Oracle's identifier length limit (pre 12c)
            // With a 22 character prefix, that puts the highest number at 99999999.
            throw new DBUnexpectedError(
                $db,
                'There have been an excessively large number of atomic sections in a transaction'
                . " started by $this->trxFname (at $fname)"
            );
        }
 
        return $savepointId;
    }
 
    public function writesPending(): bool {
        return $this->trxLevel() && $this->trxWriteCallers;
    }
 
    public function onDestruct() {
        if ( $this->trxLevel() && $this->trxWriteCallers ) {
            trigger_error( "Uncommitted DB writes (transaction from {$this->trxFname})" );
        }
    }
 
    public function transactionWritingIn( string $serverName, ?string $domainId, float $startTime ) {
        if ( !$this->trxWriteCallers ) {
            $this->profiler->transactionWritingIn(
                $serverName,
                $domainId,
                (string)$this->trxId,
                $startTime
            );
        }
    }
 
    public function transactionWritingOut( IDatabase $db, string $oldId ) {
        if ( $this->trxWriteCallers ) {
            $this->profiler->transactionWritingOut(
                $db->getServerName(),
                $db->getDomainID(),
                $oldId,
                $this->pendingWriteQueryDuration( IDatabase::ESTIMATE_TOTAL ),
                $this->trxWriteAffectedRows
            );
        }
    }
 
    public function recordQueryCompletion( $sql, $startTime, $isPermWrite, $rowCount, $serverName ) {
        $this->profiler->recordQueryCompletion(
            $sql,
            $startTime,
            $isPermWrite,
            $rowCount,
            (string)$this->trxId,
            $serverName
        );
    }
 
    public function onTransactionResolution( IDatabase $db, callable $callback, string $fname ) {
        if ( !$this->trxLevel() ) {
            throw new DBUnexpectedError( $db, "No transaction is active" );
        }
        $this->trxEndCallbacks[] = [ $callback, $fname, $this->currentAtomicSectionId() ];
    }
 
    public function addPostCommitOrIdleCallback( callable $callback, string $fname ) {
        $this->trxPostCommitOrIdleCallbacks[] = [
            $callback,
            $fname,
            $this->currentAtomicSectionId()
        ];
    }
 
    final public function addPreCommitOrIdleCallback( callable $callback, string $fname ) {
        $this->trxPreCommitOrIdleCallbacks[] = [
            $callback,
            $fname,
            $this->currentAtomicSectionId()
        ];
    }
 
    public function setTransactionListener( string $name, ?callable $callback = null ) {
        if ( $callback ) {
            $this->trxRecurringCallbacks[$name] = $callback;
        } else {
            unset( $this->trxRecurringCallbacks[$name] );
        }
    }
 
    public function setTrxEndCallbackSuppression( bool $suppress ) {
        $this->trxEndCallbacksSuppressed = $suppress;
    }
 
    public function reassignCallbacksForSection(
        AtomicSectionIdentifier $old,
        AtomicSectionIdentifier $new
    ) {
        foreach ( $this->trxPreCommitOrIdleCallbacks as $key => $info ) {
            if ( $info[2] === $old ) {
                $this->trxPreCommitOrIdleCallbacks[$key][2] = $new;
            }
        }
        foreach ( $this->trxPostCommitOrIdleCallbacks as $key => $info ) {
            if ( $info[2] === $old ) {
                $this->trxPostCommitOrIdleCallbacks[$key][2] = $new;
            }
        }
        foreach ( $this->trxEndCallbacks as $key => $info ) {
            if ( $info[2] === $old ) {
                $this->trxEndCallbacks[$key][2] = $new;
            }
        }
    }
 
    public function modifyCallbacksForCancel(
        array $excisedSectionsId,
        ?AtomicSectionIdentifier $newSectionId = null
    ) {
        // Cancel the "on commit" callbacks owned by this savepoint
        $this->trxPostCommitOrIdleCallbacks = array_filter(
            $this->trxPostCommitOrIdleCallbacks,
            static function ( $entry ) use ( $excisedSectionsId ) {
                return !in_array( $entry[2], $excisedSectionsId, true );
            }
        );
        $this->trxPreCommitOrIdleCallbacks = array_filter(
            $this->trxPreCommitOrIdleCallbacks,
            static function ( $entry ) use ( $excisedSectionsId ) {
                return !in_array( $entry[2], $excisedSectionsId, true );
            }
        );
        // Make "on resolution" callbacks owned by this savepoint to perceive a rollback
        foreach ( $this->trxEndCallbacks as $key => $entry ) {
            if ( in_array( $entry[2], $excisedSectionsId, true ) ) {
                $callback = $entry[0];
                $this->trxEndCallbacks[$key][0] = static function () use ( $callback ) {
                    return $callback( IDatabase::TRIGGER_ROLLBACK );
                };
                // This "on resolution" callback no longer belongs to a section.
                $this->trxEndCallbacks[$key][2] = null;
            }
        }
    }
 
    public function consumeEndCallbacks(): array {
        $callbackEntries = array_merge(
            $this->trxPostCommitOrIdleCallbacks,
            $this->trxEndCallbacks
        );
        $this->trxPostCommitOrIdleCallbacks = []; // consumed (and recursion guard)
        $this->trxEndCallbacks = []; // consumed (recursion guard)
 
        return $callbackEntries;
    }
 
    public function runOnTransactionPreCommitCallbacks(): int {
        $count = 0;
 
        // Drain the queues of transaction "precommit" callbacks until it is empty
        do {
            $callbackEntries = $this->trxPreCommitOrIdleCallbacks;
            $this->trxPreCommitOrIdleCallbacks = []; // consumed (and recursion guard)
            $count += count( $callbackEntries );
            foreach ( $callbackEntries as $entry ) {
                try {
                    $entry[0]();
                } catch ( Throwable $trxError ) {
                    $this->setTransactionError( $trxError );
                    throw $trxError;
                }
            }
            // @phan-suppress-next-line PhanImpossibleConditionInLoop
        } while ( $this->trxPreCommitOrIdleCallbacks );
 
        return $count;
    }
 
    public function clearPreEndCallbacks() {
        $this->trxPostCommitOrIdleCallbacks = [];
        $this->trxPreCommitOrIdleCallbacks = [];
    }
 
    public function clearEndCallbacks() {
        $this->trxEndCallbacks = []; // don't copy
    }
 
    public function writesOrCallbacksPending(): bool {
        return $this->trxLevel() && (
                $this->trxWriteCallers ||
                $this->trxPostCommitOrIdleCallbacks ||
                $this->trxPreCommitOrIdleCallbacks ||
                $this->trxEndCallbacks
            );
    }
 
    public function pendingWriteAndCallbackCallers(): array {
        $fnames = $this->pendingWriteCallers();
        foreach ( [
            $this->trxPostCommitOrIdleCallbacks,
            $this->trxPreCommitOrIdleCallbacks,
            $this->trxEndCallbacks
        ] as $callbacks ) {
            foreach ( $callbacks as $callback ) {
                $fnames[] = $callback[1];
            }
        }
 
        return $fnames;
    }
 
    public function pendingPreCommitCallbackCallers(): array {
        $fnames = $this->pendingWriteCallers();
        foreach ( $this->trxPreCommitOrIdleCallbacks as $callback ) {
            $fnames[] = $callback[1];
        }
 
        return $fnames;
    }
 
    public function isEndCallbacksSuppressed(): bool {
        return $this->trxEndCallbacksSuppressed;
    }
 
    public function getRecurringCallbacks(): array {
        return $this->trxRecurringCallbacks;
    }
 
    public function countPostCommitOrIdleCallbacks(): int {
        return count( $this->trxPostCommitOrIdleCallbacks );
    }
 
    public function onBeginInCriticalSection( $mode, $fname, $rtt ) {
        $this->trxId = new TransactionIdentifier();
        $this->setTrxStatusToOk();
        $this->resetTrxAtomicLevels();
        $this->trxWriteCallers = [];
        $this->trxWriteDuration = 0.0;
        $this->trxWriteQueryCount = 0;
        $this->trxWriteAffectedRows = 0;
        $this->trxWriteAdjDuration = 0.0;
        $this->trxWriteAdjQueryCount = 0;
        // T147697: make explicitTrxActive() return true until begin() finishes. This way,
        // no caller triggered by getApproximateLagStatus() will think its OK to muck around
        // with the transaction just because startAtomic() has not yet finished updating the
        // tracking fields (e.g. trxAtomicLevels).
        $this->trxAutomatic = ( $mode === IDatabase::TRANSACTION_INTERNAL );
        $this->trxAutomaticAtomic = false;
        $this->trxFname = $fname;
        $this->trxTimestamp = microtime( true );
        $this->trxRoundTripDelay = $rtt;
    }
 
    public function onRollbackInCriticalSection( IDatabase $db ) {
        $oldTrxId = $this->consumeTrxId();
        $this->setTrxStatusToNone();
        $this->resetTrxAtomicLevels();
        $this->clearPreEndCallbacks();
        $this->transactionWritingOut( $db, (string)$oldTrxId );
    }
 
    public function onCommitInCriticalSection( IDatabase $db ): ?float {
        $lastWriteTime = null;
 
        $oldTrxId = $this->consumeTrxId();
        $this->setTrxStatusToNone();
        if ( $this->trxWriteCallers ) {
            $lastWriteTime = microtime( true );
            $this->transactionWritingOut( $db, (string)$oldTrxId );
        }
 
        return $lastWriteTime;
    }
 
    public function onSessionLoss( IDatabase $db ) {
        $oldTrxId = $this->consumeTrxId();
        $this->clearPreEndCallbacks();
        $this->transactionWritingOut( $db, (string)$oldTrxId );
    }
 
    public function onEndAtomicInCriticalSection( AtomicSectionIdentifier $sectionId ) {
        // Hoist callback ownership for callbacks in the section that just ended;
        // all callbacks should have an owner that is present in trxAtomicLevels.
        $currentSectionId = $this->currentAtomicSectionId();
        if ( $currentSectionId ) {
            $this->reassignCallbacksForSection( $sectionId, $currentSectionId );
        }
    }
 
    public function onFlushSnapshot( IDatabase $db, string $fname, string $flush, ?string $trxRoundFname ) {
        if ( $this->explicitTrxActive() ) {
            // Committing this transaction would break callers that assume it is still open
            throw new DBUnexpectedError(
                $db,
                "$fname: Cannot flush snapshot; " .
                "explicit transaction '{$this->trxFname}' is still open"
            );
        } elseif ( $this->writesOrCallbacksPending() ) {
            // This only flushes transactions to clear snapshots, not to write data
            $fnames = implode( ', ', $this->pendingWriteAndCallbackCallers() );
            throw new DBUnexpectedError(
                $db,
                "$fname: Cannot flush snapshot; " .
                "writes from transaction {$this->trxFname} are still pending ($fnames)"
            );
        } elseif (
            $this->trxLevel() &&
            $trxRoundFname !== null &&
            $flush !== IDatabase::FLUSHING_INTERNAL &&
            $flush !== IDatabase::FLUSHING_ALL_PEERS
        ) {
            $this->logger->warning(
                "$fname: Expected mass snapshot flush of all peer transactions " .
                "in the explicit transactions round '{$trxRoundFname}'",
                [
                    'exception' => new RuntimeException(),
                    'db_log_category' => 'trx'
                ]
            );
        }
    }
 
    public function onGetScopedLockAndFlush( IDatabase $db, string $fname ) {
        if ( $this->writesOrCallbacksPending() ) {
            // This only flushes transactions to clear snapshots, not to write data
            $fnames = implode( ', ', $this->pendingWriteAndCallbackCallers() );
            throw new DBUnexpectedError(
                $db,
                "$fname: Cannot flush pre-lock snapshot; " .
                "writes from transaction {$this->trxFname} are still pending ($fnames)"
            );
        }
    }
}