From: Aaron Schulz <aschulz@wikimedia.org>
Date: Thu, 29 Mar 2018 23:30:38 +0000 (-0700)
Subject: rdbms: avoid lag estimates in getLagFromPtHeartbeat ruined by snapshots
X-Git-Tag: 1.31.0-rc.0~237^2
X-Git-Url: https://git.heureux-cyclage.org/?p=lhc%2Fweb%2Fwiklou.git;a=commitdiff_plain;h=24353a60d2c860cd24593d721b45291782a8489f

rdbms: avoid lag estimates in getLagFromPtHeartbeat ruined by snapshots

Bug: T190960
Change-Id: I57dd8d3d0ca96d6fb2f9e83f062f29b1d53224dd
---

diff --git a/includes/libs/rdbms/database/Database.php b/includes/libs/rdbms/database/Database.php
index 00d9b0b33d..d736477396 100644
--- a/includes/libs/rdbms/database/Database.php
+++ b/includes/libs/rdbms/database/Database.php
@@ -3682,7 +3682,7 @@ abstract class Database implements IDatabase, IMaintainableDatabase, LoggerAware
 	 * @return array|null ('lag': seconds or false on error, 'since': UNIX timestamp of BEGIN)
 	 * @since 1.27
 	 */
-	protected function getTransactionLagStatus() {
+	final protected function getTransactionLagStatus() {
 		return $this->trxLevel
 			? [ 'lag' => $this->trxReplicaLag, 'since' => $this->trxTimestamp() ]
 			: null;
diff --git a/includes/libs/rdbms/database/DatabaseMysqlBase.php b/includes/libs/rdbms/database/DatabaseMysqlBase.php
index 315c9eb0ec..bc4beba29a 100644
--- a/includes/libs/rdbms/database/DatabaseMysqlBase.php
+++ b/includes/libs/rdbms/database/DatabaseMysqlBase.php
@@ -763,6 +763,17 @@ abstract class DatabaseMysqlBase extends Database {
 	protected function getLagFromPtHeartbeat() {
 		$options = $this->lagDetectionOptions;
 
+		if ( $this->trxLevel ) {
+			// Avoid returning higher and higher lag value due to snapshot age
+			// given that the isolation level will typically be REPEATABLE-READ
+			$this->queryLogger->warning(
+				"Using cached lag value for {db_server} due to active transaction",
+				$this->getLogContext( [ 'method' => __METHOD__ ] )
+			);
+
+			return $this->getTransactionLagStatus()['lag'];
+		}
+
 		if ( isset( $options['conds'] ) ) {
 			// Best method for multi-DC setups: use logical channel names
 			$data = $this->getHeartbeatData( $options['conds'] );
diff --git a/includes/libs/rdbms/loadmonitor/LoadMonitor.php b/includes/libs/rdbms/loadmonitor/LoadMonitor.php
index 50c878d06b..3372839648 100644
--- a/includes/libs/rdbms/loadmonitor/LoadMonitor.php
+++ b/includes/libs/rdbms/loadmonitor/LoadMonitor.php
@@ -157,7 +157,9 @@ class LoadMonitor implements ILoadMonitor {
 			}
 
 			$conn = $this->parent->getAnyOpenConnection( $i );
-			if ( $conn ) {
+			if ( $conn && !$conn->trxLevel() ) {
+				# Handles with open transactions are avoided since they might be subject
+				# to REPEATABLE-READ snapshots, which could affect the lag estimate query.
 				$close = false; // already open
 			} else {
 				$conn = $this->parent->openConnection( $i, '' );