rdbms: allow cancelation of dangling nested atomic sections
[lhc/web/wiklou.git] / includes / libs / rdbms / database / Database.php
index 699fee7..452b4f8 100644 (file)
@@ -101,6 +101,8 @@ abstract class Database implements IDatabase, IMaintainableDatabase, LoggerAware
        protected $queryLogger;
        /** @var callback Error logging callback */
        protected $errorLogger;
+       /** @var callback Deprecation logging callback */
+       protected $deprecationLogger;
 
        /** @var resource|null Database connection */
        protected $conn = null;
@@ -141,6 +143,19 @@ abstract class Database implements IDatabase, IMaintainableDatabase, LoggerAware
        /** @var integer|null Rows affected by the last query to query() or its CRUD wrappers */
        protected $affectedRowCount;
 
+       /**
+        * @var int Transaction status
+        */
+       protected $trxStatus = self::STATUS_TRX_NONE;
+       /**
+        * @var Exception|null The last error that caused the status to become STATUS_TRX_ERROR
+        */
+       protected $trxStatusCause;
+       /**
+        * @var array|null If wasKnownStatementRollbackError() prevented trxStatus from being set,
+        *  the relevant details are stored here.
+        */
+       protected $trxStatusIgnoredCause;
        /**
         * Either 1 if a transaction is active or 0 otherwise.
         * The other Trx fields may not be meaningfull if this is 0.
@@ -197,7 +212,7 @@ abstract class Database implements IDatabase, IMaintainableDatabase, LoggerAware
        /**
         * Array of levels of atomicity within transactions
         *
-        * @var array
+        * @var array List of (name, unique ID, savepoint ID)
         */
        private $trxAtomicLevels = [];
        /**
@@ -259,6 +274,13 @@ abstract class Database implements IDatabase, IMaintainableDatabase, LoggerAware
        /** @var int */
        protected $nonNativeInsertSelectBatchSize = 10000;
 
+       /** @var int Transaction is in a error state requiring a full or savepoint rollback */
+       const STATUS_TRX_ERROR = 1;
+       /** @var int Transaction is active and in a normal state */
+       const STATUS_TRX_OK = 2;
+       /** @var int No transaction is active */
+       const STATUS_TRX_NONE = 3;
+
        /**
         * @note: exceptions for missing libraries/drivers should be thrown in initConnection()
         * @param array $params Parameters passed from Database::factory()
@@ -297,6 +319,7 @@ abstract class Database implements IDatabase, IMaintainableDatabase, LoggerAware
                $this->connLogger = $params['connLogger'];
                $this->queryLogger = $params['queryLogger'];
                $this->errorLogger = $params['errorLogger'];
+               $this->deprecationLogger = $params['deprecationLogger'];
 
                if ( isset( $params['nonNativeInsertSelectBatchSize'] ) ) {
                        $this->nonNativeInsertSelectBatchSize = $params['nonNativeInsertSelectBatchSize'];
@@ -381,6 +404,7 @@ abstract class Database implements IDatabase, IMaintainableDatabase, LoggerAware
         *      includes the agent as a SQL comment.
         *   - trxProfiler: Optional TransactionProfiler instance.
         *   - errorLogger: Optional callback that takes an Exception and logs it.
+        *   - deprecationLogger: Optional callback that takes a string and logs it.
         *   - cliMode: Whether to consider the execution context that of a CLI script.
         *   - agent: Optional name used to identify the end-user in query profiling/logging.
         *   - srvCache: Optional BagOStuff instance to an APC-style cache.
@@ -422,6 +446,11 @@ abstract class Database implements IDatabase, IMaintainableDatabase, LoggerAware
                                        trigger_error( get_class( $e ) . ': ' . $e->getMessage(), E_USER_WARNING );
                                };
                        }
+                       if ( !isset( $p['deprecationLogger'] ) ) {
+                               $p['deprecationLogger'] = function ( $msg ) {
+                                       trigger_error( $msg, E_USER_DEPRECATED );
+                               };
+                       }
 
                        /** @var Database $conn */
                        $conn = new $class( $p );
@@ -548,6 +577,14 @@ abstract class Database implements IDatabase, IMaintainableDatabase, LoggerAware
                return $this->trxLevel ? $this->trxTimestamp : null;
        }
 
+       /**
+        * @return int One of the STATUS_TRX_* class constants
+        * @since 1.31
+        */
+       public function trxStatus() {
+               return $this->trxStatus;
+       }
+
        public function tablePrefix( $prefix = null ) {
                $old = $this->tablePrefix;
                if ( $prefix !== null ) {
@@ -635,6 +672,20 @@ abstract class Database implements IDatabase, IMaintainableDatabase, LoggerAware
                );
        }
 
+       /**
+        * @return string|null
+        */
+       final protected function getTransactionRoundId() {
+               // If transaction round participation is enabled, see if one is active
+               if ( $this->getFlag( self::DBO_TRX ) ) {
+                       $id = $this->getLBInfo( 'trxRoundId' );
+
+                       return is_string( $id ) ? $id : null;
+               }
+
+               return null;
+       }
+
        public function pendingWriteQueryDuration( $type = self::ESTIMATE_TOTAL ) {
                if ( !$this->trxLevel ) {
                        return false;
@@ -690,6 +741,15 @@ abstract class Database implements IDatabase, IMaintainableDatabase, LoggerAware
                return $fnames;
        }
 
+       /**
+        * @return string
+        */
+       private function flatAtomicSectionList() {
+               return array_reduce( $this->trxAtomicLevels, function ( $accum, $v ) {
+                       return $accum === null ? $v[0] : "$accum, " . $v[0];
+               } );
+       }
+
        public function isOpen() {
                return $this->opened;
        }
@@ -832,42 +892,64 @@ abstract class Database implements IDatabase, IMaintainableDatabase, LoggerAware
                );
        }
 
-       public function close() {
+       final public function close() {
+               $exception = null; // error to throw after disconnecting
+
                if ( $this->conn ) {
                        // Resolve any dangling transaction first
-                       if ( $this->trxLevel() ) {
+                       if ( $this->trxLevel ) {
                                // Meaningful transactions should ideally have been resolved by now
                                if ( $this->writesOrCallbacksPending() ) {
                                        $this->queryLogger->warning(
                                                __METHOD__ . ": writes or callbacks still pending.",
                                                [ 'trace' => ( new RuntimeException() )->getTraceAsString() ]
                                        );
+                                       // Cannot let incomplete atomic sections be committed
+                                       if ( $this->trxAtomicLevels ) {
+                                               $levels = $this->flatAtomicSectionList();
+                                               $exception = new DBUnexpectedError(
+                                                       $this,
+                                                       __METHOD__ . ": atomic sections $levels are still open."
+                                               );
+                                       // Check if it is possible to properly commit and trigger callbacks
+                                       } elseif ( $this->trxEndCallbacksSuppressed ) {
+                                               $exception = new DBUnexpectedError(
+                                                       $this,
+                                                       __METHOD__ . ': callbacks are suppressed; cannot properly commit.'
+                                               );
+                                       }
                                }
-                               // Check if it is possible to properly commit and trigger callbacks
-                               if ( $this->trxEndCallbacksSuppressed ) {
-                                       throw new DBUnexpectedError(
-                                               $this,
-                                               __METHOD__ . ': callbacks are suppressed; cannot properly commit.'
-                                       );
+                               // Commit or rollback the changes and run any callbacks as needed
+                               if ( $this->trxStatus === self::STATUS_TRX_OK && !$exception ) {
+                                       $this->commit( __METHOD__, self::TRANSACTION_INTERNAL );
+                               } else {
+                                       $this->rollback( __METHOD__, self::TRANSACTION_INTERNAL );
                                }
-                               // Commit the changes and run any callbacks as needed
-                               $this->commit( __METHOD__, self::FLUSHING_INTERNAL );
                        }
                        // Close the actual connection in the binding handle
                        $closed = $this->closeConnection();
                        $this->conn = false;
-                       // Sanity check that no callbacks are dangling
-                       if (
-                               $this->trxIdleCallbacks || $this->trxPreCommitCallbacks || $this->trxEndCallbacks
-                       ) {
-                               throw new RuntimeException( "Transaction callbacks still pending." );
-                       }
                } else {
                        $closed = true; // already closed; nothing to do
                }
 
                $this->opened = false;
 
+               // Throw any unexpected errors after having disconnected
+               if ( $exception instanceof Exception ) {
+                       throw $exception;
+               }
+
+               // Sanity check that no callbacks are dangling
+               if (
+                       $this->trxIdleCallbacks || $this->trxPreCommitCallbacks || $this->trxEndCallbacks
+               ) {
+                       throw new RuntimeException(
+                               "Transaction callbacks are still pending:\n" .
+                               implode( ', ', $this->pendingWriteAndCallbackCallers() )
+                       );
+               }
+
                return $closed;
        }
 
@@ -991,6 +1073,11 @@ abstract class Database implements IDatabase, IMaintainableDatabase, LoggerAware
        }
 
        public function query( $sql, $fname = __METHOD__, $tempIgnore = false ) {
+               $this->assertTransactionStatus( $sql, $fname );
+
+               # Avoid fatals if close() was called
+               $this->assertOpen();
+
                $priorWritesPending = $this->writesOrCallbacksPending();
                $this->lastQuery = $sql;
 
@@ -1041,59 +1128,61 @@ abstract class Database implements IDatabase, IMaintainableDatabase, LoggerAware
                        $this->queryLogger->debug( "{$this->dbName} {$commentedSql}" );
                }
 
-               # Avoid fatals if close() was called
-               $this->assertOpen();
-
-               # Send the query to the server
+               # Send the query to the server and fetch any corresponding errors
                $ret = $this->doProfiledQuery( $sql, $commentedSql, $isNonTempWrite, $fname );
+               $lastError = $this->lastError();
+               $lastErrno = $this->lastErrno();
 
                # Try reconnecting if the connection was lost
-               if ( false === $ret && $this->wasConnectionLoss() ) {
+               if ( $ret === false && $this->wasConnectionLoss() ) {
+                       # Check if any meaningful session state was lost
                        $recoverable = $this->canRecoverFromDisconnect( $sql, $priorWritesPending );
-                       # Stash the last error values before anything might clear them
-                       $lastError = $this->lastError();
-                       $lastErrno = $this->lastErrno();
-                       # Update state tracking to reflect transaction loss due to disconnection
-                       $this->handleSessionLoss();
-                       if ( $this->reconnect() ) {
-                               $msg = __METHOD__ . ': lost connection to {dbserver}; reconnected';
-                               $params = [ 'dbserver' => $this->getServer() ];
-                               $this->connLogger->warning( $msg, $params );
-                               $this->queryLogger->warning( $msg, $params +
-                                       [ 'trace' => ( new RuntimeException() )->getTraceAsString() ] );
-
-                               if ( $recoverable ) {
-                                       # Should be safe to silently retry the query
-                                       $ret = $this->doProfiledQuery( $sql, $commentedSql, $isNonTempWrite, $fname );
-                               } else {
-                                       # Callers may catch the exception and continue to use the DB
-                                       $this->reportQueryError( $lastError, $lastErrno, $sql, $fname );
+                       # Update session state tracking and try to restore the connection
+                       $reconnected = $this->replaceLostConnection( __METHOD__ );
+                       # Silently resend the query to the server if it is safe and possible
+                       if ( $reconnected && $recoverable ) {
+                               $ret = $this->doProfiledQuery( $sql, $commentedSql, $isNonTempWrite, $fname );
+                               $lastError = $this->lastError();
+                               $lastErrno = $this->lastErrno();
+
+                               if ( $ret === false && $this->wasConnectionLoss() ) {
+                                       # Query probably causes disconnects; reconnect and do not re-run it
+                                       $this->replaceLostConnection( __METHOD__ );
                                }
-                       } else {
-                               $msg = __METHOD__ . ': lost connection to {dbserver} permanently';
-                               $this->connLogger->error( $msg, [ 'dbserver' => $this->getServer() ] );
                        }
                }
 
-               if ( false === $ret ) {
-                       # Deadlocks cause the entire transaction to abort, not just the statement.
-                       # https://dev.mysql.com/doc/refman/5.7/en/innodb-error-handling.html
-                       # https://www.postgresql.org/docs/9.1/static/explicit-locking.html
-                       if ( $this->wasDeadlock() ) {
-                               if ( $this->explicitTrxActive() || $priorWritesPending ) {
-                                       $tempIgnore = false; // not recoverable
+               if ( $ret === false ) {
+                       if ( $this->trxLevel ) {
+                               if ( !$this->wasKnownStatementRollbackError() ) {
+                                       # Either the query was aborted or all queries after BEGIN where aborted.
+                                       if ( $this->explicitTrxActive() || $priorWritesPending ) {
+                                               # In the first case, the only options going forward are (a) ROLLBACK, or
+                                               # (b) ROLLBACK TO SAVEPOINT (if one was set). If the later case, the only
+                                               # option is ROLLBACK, since the snapshots would have been released.
+                                               $this->trxStatus = self::STATUS_TRX_ERROR;
+                                               $this->trxStatusCause =
+                                                       $this->makeQueryException( $lastError, $lastErrno, $sql, $fname );
+                                               $tempIgnore = false; // cannot recover
+                                       } else {
+                                               # Nothing prior was there to lose from the transaction,
+                                               # so just roll it back.
+                                               $this->doRollback( __METHOD__ . " ($fname)" );
+                                               $this->trxStatus = self::STATUS_TRX_OK;
+                                       }
+                                       $this->trxStatusIgnoredCause = null;
+                               } else {
+                                       # We're ignoring an error that caused just the current query to be aborted.
+                                       # But log the cause so we can log a deprecation notice if a
+                                       # caller actually does ignore it.
+                                       $this->trxStatusIgnoredCause = [ $lastError, $lastErrno, $fname ];
                                }
-                               # Update state tracking to reflect transaction loss
-                               $this->handleSessionLoss();
                        }
 
-                       $this->reportQueryError(
-                               $this->lastError(), $this->lastErrno(), $sql, $fname, $tempIgnore );
+                       $this->reportQueryError( $lastError, $lastErrno, $sql, $fname, $tempIgnore );
                }
 
-               $res = $this->resultObject( $ret );
-
-               return $res;
+               return $this->resultObject( $ret );
        }
 
        /**
@@ -1191,6 +1280,33 @@ abstract class Database implements IDatabase, IMaintainableDatabase, LoggerAware
                }
        }
 
+       /**
+        * @param string $sql
+        * @param string $fname
+        * @throws DBTransactionStateError
+        */
+       private function assertTransactionStatus( $sql, $fname ) {
+               if ( $this->getQueryVerb( $sql ) === 'ROLLBACK' ) { // transaction/savepoint
+                       return;
+               }
+
+               if ( $this->trxStatus < self::STATUS_TRX_OK ) {
+                       throw new DBTransactionStateError(
+                               $this,
+                               "Cannot execute query from $fname while transaction status is ERROR.",
+                               [],
+                               $this->trxStatusCause
+                       );
+               } elseif ( $this->trxStatus === self::STATUS_TRX_OK && $this->trxStatusIgnoredCause ) {
+                       list( $iLastError, $iLastErrno, $iFname ) = $this->trxStatusIgnoredCause;
+                       call_user_func( $this->deprecationLogger,
+                               "Caller from $fname ignored an error originally raised from $iFname: " .
+                               "[$iLastErrno] $iLastError"
+                       );
+                       $this->trxStatusIgnoredCause = null;
+               }
+       }
+
        /**
         * Determine whether or not it is safe to retry queries after a database
         * connection is lost
@@ -1208,12 +1324,14 @@ abstract class Database implements IDatabase, IMaintainableDatabase, LoggerAware
                # didn't matter anyway (aside from DBO_TRX snapshot loss).
                if ( $this->namedLocksHeld ) {
                        return false; // possible critical section violation
+               } elseif ( $this->sessionTempTables ) {
+                       return false; // tables might be queried latter
                } elseif ( $sql === 'COMMIT' ) {
                        return !$priorWritesPending; // nothing written anyway? (T127428)
                } elseif ( $sql === 'ROLLBACK' ) {
                        return true; // transaction lost...which is also what was requested :)
                } elseif ( $this->explicitTrxActive() ) {
-                       return false; // don't drop atomocity
+                       return false; // don't drop atomocity and explicit snapshots
                } elseif ( $priorWritesPending ) {
                        return false; // prior writes lost from implicit transaction
                }
@@ -1222,36 +1340,41 @@ abstract class Database implements IDatabase, IMaintainableDatabase, LoggerAware
        }
 
        /**
-        * Clean things up after transaction loss due to disconnection
-        *
-        * @return null|Exception
+        * Clean things up after session (and thus transaction) loss
         */
        private function handleSessionLoss() {
+               // Clean up tracking of session-level things...
+               // https://dev.mysql.com/doc/refman/5.7/en/implicit-commit.html
+               // https://www.postgresql.org/docs/9.2/static/sql-createtable.html (ignoring ON COMMIT)
+               $this->sessionTempTables = [];
+               // https://dev.mysql.com/doc/refman/5.7/en/miscellaneous-functions.html#function_get-lock
+               // https://www.postgresql.org/docs/9.4/static/functions-admin.html#FUNCTIONS-ADVISORY-LOCKS
+               $this->namedLocksHeld = [];
+               // Session loss implies transaction loss
+               $this->handleTransactionLoss();
+       }
+
+       /**
+        * Clean things up after transaction loss
+        */
+       private function handleTransactionLoss() {
                $this->trxLevel = 0;
                $this->trxAtomicCounter = 0;
                $this->trxIdleCallbacks = []; // T67263; transaction already lost
                $this->trxPreCommitCallbacks = []; // T67263; transaction already lost
-               $this->sessionTempTables = [];
-               $this->namedLocksHeld = [];
-
-               // Note: if callback suppression is set then some *Callbacks arrays are not cleared here
-               $e = null;
                try {
-                       // Handle callbacks in trxEndCallbacks
+                       // Handle callbacks in trxEndCallbacks, e.g. onTransactionResolution().
+                       // If callback suppression is set then the array will remain unhandled.
                        $this->runOnTransactionIdleCallbacks( self::TRIGGER_ROLLBACK );
                } catch ( Exception $ex ) {
                        // Already logged; move on...
-                       $e = $e ?: $ex;
                }
                try {
-                       // Handle callbacks in trxRecurringCallbacks
+                       // Handle callbacks in trxRecurringCallbacks, e.g. setTransactionListener()
                        $this->runTransactionListenerCallbacks( self::TRIGGER_ROLLBACK );
                } catch ( Exception $ex ) {
                        // Already logged; move on...
-                       $e = $e ?: $ex;
                }
-
-               return $e;
        }
 
        /**
@@ -1283,27 +1406,42 @@ abstract class Database implements IDatabase, IMaintainableDatabase, LoggerAware
                if ( $tempIgnore ) {
                        $this->queryLogger->debug( "SQL ERROR (ignored): $error\n" );
                } else {
-                       $sql1line = mb_substr( str_replace( "\n", "\\n", $sql ), 0, 5 * 1024 );
-                       $this->queryLogger->error(
-                               "{fname}\t{db_server}\t{errno}\t{error}\t{sql1line}",
-                               $this->getLogContext( [
-                                       'method' => __METHOD__,
-                                       'errno' => $errno,
-                                       'error' => $error,
-                                       'sql1line' => $sql1line,
-                                       'fname' => $fname,
-                               ] )
-                       );
-                       $this->queryLogger->debug( "SQL ERROR: " . $error . "\n" );
-                       $wasQueryTimeout = $this->wasQueryTimeout( $error, $errno );
-                       if ( $wasQueryTimeout ) {
-                               throw new DBQueryTimeoutError( $this, $error, $errno, $sql, $fname );
-                       } else {
-                               throw new DBQueryError( $this, $error, $errno, $sql, $fname );
-                       }
+                       $exception = $this->makeQueryException( $error, $errno, $sql, $fname );
+
+                       throw $exception;
                }
        }
 
+       /**
+        * @param string $error
+        * @param string|int $errno
+        * @param string $sql
+        * @param string $fname
+        * @return DBError
+        */
+       private function makeQueryException( $error, $errno, $sql, $fname ) {
+               $sql1line = mb_substr( str_replace( "\n", "\\n", $sql ), 0, 5 * 1024 );
+               $this->queryLogger->error(
+                       "{fname}\t{db_server}\t{errno}\t{error}\t{sql1line}",
+                       $this->getLogContext( [
+                               'method' => __METHOD__,
+                               'errno' => $errno,
+                               'error' => $error,
+                               'sql1line' => $sql1line,
+                               'fname' => $fname,
+                       ] )
+               );
+               $this->queryLogger->debug( "SQL ERROR: " . $error . "\n" );
+               $wasQueryTimeout = $this->wasQueryTimeout( $error, $errno );
+               if ( $wasQueryTimeout ) {
+                       $e = new DBQueryTimeoutError( $this, $error, $errno, $sql, $fname );
+               } else {
+                       $e = new DBQueryError( $this, $error, $errno, $sql, $fname );
+               }
+
+               return $e;
+       }
+
        public function freeResult( $res ) {
        }
 
@@ -3010,6 +3148,16 @@ abstract class Database implements IDatabase, IMaintainableDatabase, LoggerAware
                return false;
        }
 
+       /**
+        * @return bool Whether it is safe to assume the given error only caused statement rollback
+        * @note This is for backwards compatibility for callers catching DBError exceptions in
+        *   order to ignore problems like duplicate key errors or foriegn key violations
+        * @since 1.31
+        */
+       protected function wasKnownStatementRollbackError() {
+               return false; // don't know; it could have caused a transaction rollback
+       }
+
        public function deadlockLoop() {
                $args = func_get_args();
                $function = array_shift( $args );
@@ -3073,6 +3221,12 @@ abstract class Database implements IDatabase, IMaintainableDatabase, LoggerAware
        }
 
        final public function onTransactionIdle( callable $callback, $fname = __METHOD__ ) {
+               if ( !$this->trxLevel && $this->getTransactionRoundId() ) {
+                       // Start an implicit transaction similar to how query() does
+                       $this->begin( __METHOD__, self::TRANSACTION_INTERNAL );
+                       $this->trxAutomatic = true;
+               }
+
                $this->trxIdleCallbacks[] = [ $callback, $fname ];
                if ( !$this->trxLevel ) {
                        $this->runOnTransactionIdleCallbacks( self::TRIGGER_IDLE );
@@ -3080,10 +3234,13 @@ abstract class Database implements IDatabase, IMaintainableDatabase, LoggerAware
        }
 
        final public function onTransactionPreCommitOrIdle( callable $callback, $fname = __METHOD__ ) {
-               if ( $this->trxLevel || $this->getFlag( self::DBO_TRX ) ) {
-                       // As long as DBO_TRX is set, writes will accumulate until the load balancer issues
-                       // an implicit commit of all peer databases. This is true even if a transaction has
-                       // not yet been triggered by writes; make sure $callback runs *after* any such writes.
+               if ( !$this->trxLevel && $this->getTransactionRoundId() ) {
+                       // Start an implicit transaction similar to how query() does
+                       $this->begin( __METHOD__, self::TRANSACTION_INTERNAL );
+                       $this->trxAutomatic = true;
+               }
+
+               if ( $this->trxLevel ) {
                        $this->trxPreCommitCallbacks[] = [ $callback, $fname ];
                } else {
                        // No transaction is active nor will start implicitly, so make one for this callback
@@ -3296,56 +3453,104 @@ abstract class Database implements IDatabase, IMaintainableDatabase, LoggerAware
                        $this->doSavepoint( $savepointId, $fname );
                }
 
-               $this->trxAtomicLevels[] = [ $fname, $savepointId ];
+               $sectionId = new AtomicSectionIdentifier;
+               $this->trxAtomicLevels[] = [ $fname, $sectionId, $savepointId ];
+
+               return $sectionId;
        }
 
        final public function endAtomic( $fname = __METHOD__ ) {
-               if ( !$this->trxLevel ) {
-                       throw new DBUnexpectedError( $this, "No atomic transaction is open (got $fname)." );
+               if ( !$this->trxLevel || !$this->trxAtomicLevels ) {
+                       throw new DBUnexpectedError( $this, "No atomic section is open (got $fname)." );
                }
 
-               list( $savedFname, $savepointId ) = $this->trxAtomicLevels
-                       ? array_pop( $this->trxAtomicLevels ) : [ null, null ];
+               // Check if the current section matches $fname
+               $pos = count( $this->trxAtomicLevels ) - 1;
+               list( $savedFname, , $savepointId ) = $this->trxAtomicLevels[$pos];
+
                if ( $savedFname !== $fname ) {
-                       throw new DBUnexpectedError( $this, "Invalid atomic section ended (got $fname)." );
+                       throw new DBUnexpectedError(
+                               $this,
+                               "Invalid atomic section ended (got $fname but expected $savedFname)."
+                       );
                }
 
+               // Remove the last section and re-index the array
+               $this->trxAtomicLevels = array_slice( $this->trxAtomicLevels, 0, $pos );
+
                if ( !$this->trxAtomicLevels && $this->trxAutomaticAtomic ) {
                        $this->commit( $fname, self::FLUSHING_INTERNAL );
-               } elseif ( $savepointId && $savepointId !== 'n/a' ) {
+               } elseif ( $savepointId !== null && $savepointId !== 'n/a' ) {
                        $this->doReleaseSavepoint( $savepointId, $fname );
                }
        }
 
-       final public function cancelAtomic( $fname = __METHOD__ ) {
-               if ( !$this->trxLevel ) {
-                       throw new DBUnexpectedError( $this, "No atomic transaction is open (got $fname)." );
+       final public function cancelAtomic(
+               $fname = __METHOD__, AtomicSectionIdentifier $sectionId = null
+       ) {
+               if ( !$this->trxLevel || !$this->trxAtomicLevels ) {
+                       throw new DBUnexpectedError( $this, "No atomic section is open (got $fname)." );
                }
 
-               list( $savedFname, $savepointId ) = $this->trxAtomicLevels
-                       ? array_pop( $this->trxAtomicLevels ) : [ null, null ];
-               if ( $savedFname !== $fname ) {
-                       throw new DBUnexpectedError( $this, "Invalid atomic section ended (got $fname)." );
+               if ( $sectionId !== null ) {
+                       // Find the (last) section with the given $sectionId
+                       $pos = -1;
+                       foreach ( $this->trxAtomicLevels as $i => list( $asFname, $asId, $spId ) ) {
+                               if ( $asId === $sectionId ) {
+                                       $pos = $i;
+                               }
+                       }
+                       if ( $pos < 0 ) {
+                               throw new DBUnexpectedError( "Atomic section not found (for $fname)" );
+                       }
+                       // Remove all descendant sections and re-index the array
+                       $this->trxAtomicLevels = array_slice( $this->trxAtomicLevels, 0, $pos + 1 );
                }
-               if ( !$savepointId ) {
-                       throw new DBUnexpectedError( $this, "Uncancelable atomic section canceled (got $fname)." );
+
+               // Check if the current section matches $fname
+               $pos = count( $this->trxAtomicLevels ) - 1;
+               list( $savedFname, , $savepointId ) = $this->trxAtomicLevels[$pos];
+
+               if ( $savedFname !== $fname ) {
+                       throw new DBUnexpectedError(
+                               $this,
+                               "Invalid atomic section ended (got $fname but expected $savedFname)."
+                       );
                }
 
-               if ( !$this->trxAtomicLevels && $this->trxAutomaticAtomic ) {
-                       $this->rollback( $fname, self::FLUSHING_INTERNAL );
-               } elseif ( $savepointId !== 'n/a' ) {
-                       $this->doRollbackToSavepoint( $savepointId, $fname );
+               // Remove the last section and re-index the array
+               $this->trxAtomicLevels = array_slice( $this->trxAtomicLevels, 0, $pos );
+
+               if ( $savepointId !== null ) {
+                       // Rollback the transaction to the state just before this atomic section
+                       if ( $savepointId === 'n/a' ) {
+                               $this->rollback( $fname, self::FLUSHING_INTERNAL );
+                       } else {
+                               $this->doRollbackToSavepoint( $savepointId, $fname );
+                               $this->trxStatus = self::STATUS_TRX_OK; // no exception; recovered
+                               $this->trxStatusIgnoredCause = null;
+                       }
+               } elseif ( $this->trxStatus > self::STATUS_TRX_ERROR ) {
+                       // Put the transaction into an error state if it's not already in one
+                       $this->trxStatus = self::STATUS_TRX_ERROR;
+                       $this->trxStatusCause = new DBUnexpectedError(
+                               $this,
+                               "Uncancelable atomic section canceled (got $fname)."
+                       );
                }
 
                $this->affectedRowCount = 0; // for the sake of consistency
        }
 
-       final public function doAtomicSection( $fname, callable $callback ) {
-               $this->startAtomic( $fname, self::ATOMIC_CANCELABLE );
+       final public function doAtomicSection(
+               $fname, callable $callback, $cancelable = self::ATOMIC_NOT_CANCELABLE
+       ) {
+               $sectionId = $this->startAtomic( $fname, $cancelable );
                try {
                        $res = call_user_func_array( $callback, [ $this, $fname ] );
                } catch ( Exception $e ) {
-                       $this->cancelAtomic( $fname );
+                       $this->cancelAtomic( $fname, $sectionId );
+
                        throw $e;
                }
                $this->endAtomic( $fname );
@@ -3357,9 +3562,7 @@ abstract class Database implements IDatabase, IMaintainableDatabase, LoggerAware
                // Protect against mismatched atomic section, transaction nesting, and snapshot loss
                if ( $this->trxLevel ) {
                        if ( $this->trxAtomicLevels ) {
-                               $levels = array_reduce( $this->trxAtomicLevels, function ( $accum, $v ) {
-                                       return $accum === null ? $v[0] : "$accum, " . $v[0];
-                               } );
+                               $levels = $this->flatAtomicSectionList();
                                $msg = "$fname: Got explicit BEGIN while atomic section(s) $levels are open.";
                                throw new DBUnexpectedError( $this, $msg );
                        } elseif ( !$this->trxAutomatic ) {
@@ -3378,6 +3581,8 @@ abstract class Database implements IDatabase, IMaintainableDatabase, LoggerAware
                $this->assertOpen();
 
                $this->doBegin( $fname );
+               $this->trxStatus = self::STATUS_TRX_OK;
+               $this->trxStatusIgnoredCause = null;
                $this->trxAtomicCounter = 0;
                $this->trxTimestamp = microtime( true );
                $this->trxFname = $fname;
@@ -3393,6 +3598,7 @@ abstract class Database implements IDatabase, IMaintainableDatabase, LoggerAware
                $this->trxWriteCallers = [];
                // First SELECT after BEGIN will establish the snapshot in REPEATABLE-READ.
                // Get an estimate of the replication lag before any such queries.
+               $this->trxReplicaLag = null; // clear cached value first
                $this->trxReplicaLag = $this->getApproximateLagStatus()['lag'];
                // T147697: make explicitTrxActive() return true until begin() finishes. This way, no
                // caller will think its OK to muck around with the transaction just because startAtomic()
@@ -3414,9 +3620,7 @@ abstract class Database implements IDatabase, IMaintainableDatabase, LoggerAware
        final public function commit( $fname = __METHOD__, $flush = '' ) {
                if ( $this->trxLevel && $this->trxAtomicLevels ) {
                        // There are still atomic sections open. This cannot be ignored
-                       $levels = array_reduce( $this->trxAtomicLevels, function ( $accum, $v ) {
-                               return $accum === null ? $v[0] : "$accum, " . $v[0];
-                       } );
+                       $levels = $this->flatAtomicSectionList();
                        throw new DBUnexpectedError(
                                $this,
                                "$fname: Got COMMIT while atomic sections $levels are still open."
@@ -3451,6 +3655,7 @@ abstract class Database implements IDatabase, IMaintainableDatabase, LoggerAware
                $this->runOnTransactionPreCommitCallbacks();
                $writeTime = $this->pendingWriteQueryDuration( self::ESTIMATE_DB_APPLY );
                $this->doCommit( $fname );
+               $this->trxStatus = self::STATUS_TRX_NONE;
                if ( $this->trxDoneWrites ) {
                        $this->lastWriteTime = microtime( true );
                        $this->trxProfiler->transactionWritingOut(
@@ -3496,6 +3701,7 @@ abstract class Database implements IDatabase, IMaintainableDatabase, LoggerAware
                        $this->assertOpen();
 
                        $this->doRollback( $fname );
+                       $this->trxStatus = self::STATUS_TRX_NONE;
                        $this->trxAtomicLevels = [];
                        if ( $this->trxDoneWrites ) {
                                $this->trxProfiler->transactionWritingOut(
@@ -3646,11 +3852,12 @@ abstract class Database implements IDatabase, IMaintainableDatabase, LoggerAware
        }
 
        /**
-        * Close existing database connection and open a new connection
+        * Close any existing (dead) database connection and open a new connection
         *
+        * @param string $fname
         * @return bool True if new connection is opened successfully, false if error
         */
-       protected function reconnect() {
+       protected function replaceLostConnection( $fname ) {
                $this->closeConnection();
                $this->opened = false;
                $this->conn = false;
@@ -3658,15 +3865,30 @@ abstract class Database implements IDatabase, IMaintainableDatabase, LoggerAware
                        $this->open( $this->server, $this->user, $this->password, $this->dbName );
                        $this->lastPing = microtime( true );
                        $ok = true;
+
+                       $this->connLogger->warning(
+                               $fname . ': lost connection to {dbserver}; reconnected',
+                               [
+                                       'dbserver' => $this->getServer(),
+                                       'trace' => ( new RuntimeException() )->getTraceAsString()
+                               ]
+                       );
                } catch ( DBConnectionError $e ) {
                        $ok = false;
+
+                       $this->connLogger->error(
+                               $fname . ': lost connection to {dbserver} permanently',
+                               [ 'dbserver' => $this->getServer() ]
+                       );
                }
 
+               $this->handleSessionLoss();
+
                return $ok;
        }
 
        public function getSessionLagStatus() {
-               return $this->getTransactionLagStatus() ?: $this->getApproximateLagStatus();
+               return $this->getRecordedTransactionLagStatus() ?: $this->getApproximateLagStatus();
        }
 
        /**
@@ -3677,11 +3899,13 @@ abstract class Database implements IDatabase, IMaintainableDatabase, LoggerAware
         * is this lag plus transaction duration. If they don't, it is still
         * safe to be pessimistic. This returns null if there is no transaction.
         *
+        * This returns null if the lag status for this transaction was not yet recorded.
+        *
         * @return array|null ('lag': seconds or false on error, 'since': UNIX timestamp of BEGIN)
         * @since 1.27
         */
-       final protected function getTransactionLagStatus() {
-               return $this->trxLevel
+       final protected function getRecordedTransactionLagStatus() {
+               return ( $this->trxLevel && $this->trxReplicaLag !== null )
                        ? [ 'lag' => $this->trxReplicaLag, 'since' => $this->trxTimestamp() ]
                        : null;
        }