* Skip COMMIT query when no write queries have been done.

* Improvement to ChronologyProtector: only record the master position if a write query was done. This should help to avoid the worst of the ChronologyProtector side-effects, such as having action=raw CSS requests block for a time equivalent to the slave lag on every page view, while maintaining the benefits, like preventing a 404 from being displayed after page creation.

includes/db/Database.php

@@ -26,6 +26,7 @@ class Database {
 #------------------------------------------------------------------------------
 
 	protected $mLastQuery = '';
+	protected $mDoneWrites = false;
 	protected $mPHPError = false;
 
 	protected $mServer, $mUser, $mPassword, $mConn = null, $mDBname;
@@ -210,7 +211,14 @@ class Database {
 	 * @return String
 	 */
 	function lastQuery() { return $this->mLastQuery; }
-	
+
+
+	/**
+	 * Returns true if the connection may have been used for write queries.
+	 * Should return true if unsure.
+	 */
+	function doneWrites() { return $this->mDoneWrites; }
+
 	/**
 	 * Is a connection to the database open?
 	 * @return Boolean
@@ -492,6 +500,14 @@ class Database {
 		}
 	}
 
+	/**
+	 * Determine whether a query writes to the DB.
+	 * Should return true if unsure.
+	 */
+	function isWriteQuery( $sql ) {
+		return !preg_match( '/^(?:SELECT|BEGIN|COMMIT|SET|SHOW)\b/i', $sql );
+	}
+
 	/**
 	 * Usually aborts on failure.  If errors are explicitly ignored, returns success.
 	 *
@@ -527,6 +543,11 @@ class Database {
 		}
 
 		$this->mLastQuery = $sql;
+		if ( !$this->mDoneWrites && $this->isWriteQuery( $sql ) ) {
+			// Set a flag indicating that writes have been done
+			wfDebug( __METHOD__.": Writes done: $sql\n" );
+			$this->mDoneWrites = true;
+		}
 
 		# Add a comment for easy SHOW PROCESSLIST interpretation
 		#if ( $fname ) {

includes/db/LBFactory.php

@@ -236,15 +236,25 @@ class ChronologyProtector {
 	 * @param LoadBalancer $lb
 	 */
 	function shutdownLB( $lb ) {
-		if ( session_id() != '' && $lb->getServerCount() > 1 ) {
+		// Don't start a session, don't bother with non-replicated setups
-			$masterName = $lb->getServerName( 0 );
+		if ( strval( session_id() ) == '' || $lb->getServerCount() <= 1 ) {
-			if ( !isset( $this->shutdownPos[$masterName] ) ) {
+			return;
-				$pos = $lb->getMasterPos();
-				$info = $lb->parentInfo();
-				wfDebug( __METHOD__.": LB " . $info['id'] . " has master pos $pos\n" );
-				$this->shutdownPos[$masterName] = $pos;
-			}
 		}
+		$masterName = $lb->getServerName( 0 );
+		if ( isset( $this->shutdownPos[$masterName] ) ) {
+			// Already done
+			return;
+		}
+		// Only save the position if writes have been done on the connection
+		$db = $lb->getAnyOpenConnection( 0 );
+		$info = $lb->parentInfo();
+		if ( !$db || !$db->doneWrites() ) {
+			wfDebug( __METHOD__.": LB {$info['id']}, no writes done\n" );
+			return;
+		}
+		$pos = $db->getMasterPos();
+		wfDebug( __METHOD__.": LB {$info['id']} has master pos $pos\n" );
+		$this->shutdownPos[$masterName] = $pos;
 	}
 
 	/**

includes/db/LoadBalancer.php

@@ -824,7 +824,7 @@ class LoadBalancer {
 				continue;
 			}
 			foreach ( $conns2[$masterIndex] as $conn ) {
-				if ( $conn->lastQuery() != '' ) {
+				if ( $conn->doneWrites() ) {
 					$conn->commit();
 				}
 			}