From db170ebe78a9888b4966cc934f9571ee22ad2821 Mon Sep 17 00:00:00 2001
From: Aaron Schulz <aschulz@wikimedia.org>
Date: Sun, 4 Oct 2015 22:15:33 -0700
Subject: [PATCH] Enforce lagged-slave read-only mode on the DB layer

* Most callers gracefully check wfReadOnly(),
  but fail in case they dont. This also catches
  foreign DBs which might slip through the cracks.
* Also remove useless wfDebug() call around
  mDoneWrites check as write queries show in
  the logs anyway.

Change-Id: I560ebd19c4eb2b3a040d4331702346440617cfaa
---
 autoload.php                              | 1 +
 includes/db/Database.php                  | 6 +++---
 includes/db/DatabaseError.php             | 6 ++++++
 includes/db/loadbalancer/LoadBalancer.php | 8 ++++++++
 4 files changed, 18 insertions(+), 3 deletions(-)

diff --git a/autoload.php b/autoload.php
index 9c859b7d18..46f592aca3 100644
--- a/autoload.php
+++ b/autoload.php
@@ -287,6 +287,7 @@ $wgAutoloadLocalClasses = array(
 	'DBLockManager' => __DIR__ . '/includes/filebackend/lockmanager/DBLockManager.php',
 	'DBMasterPos' => __DIR__ . '/includes/db/DatabaseUtility.php',
 	'DBQueryError' => __DIR__ . '/includes/db/DatabaseError.php',
+	'DBReadOnlyError' => __DIR__ . '/includes/db/DatabaseError.php',
 	'DBSiteStore' => __DIR__ . '/includes/site/DBSiteStore.php',
 	'DBUnexpectedError' => __DIR__ . '/includes/db/DatabaseError.php',
 	'DataUpdate' => __DIR__ . '/includes/deferred/DataUpdate.php',
diff --git a/includes/db/Database.php b/includes/db/Database.php
index 05d1934797..2c2d37f29b 100644
--- a/includes/db/Database.php
+++ b/includes/db/Database.php
@@ -932,9 +932,9 @@ abstract class DatabaseBase implements IDatabase {
 
 		$isWriteQuery = $this->isWriteQuery( $sql );
 		if ( $isWriteQuery ) {
-			if ( !$this->mDoneWrites ) {
-				wfDebug( __METHOD__ . ': Writes done: ' .
-					DatabaseBase::generalizeSQL( $sql ) . "\n" );
+			$reason = $this->getLBInfo( 'readOnlyReason' );
+			if ( is_string( $reason ) ) {
+				throw new DBReadOnlyError( $this, "Database is read-only: $reason" );
 			}
 			# Set a flag indicating that writes have been done
 			$this->mDoneWrites = microtime( true );
diff --git a/includes/db/DatabaseError.php b/includes/db/DatabaseError.php
index 928de61612..6453854a7b 100644
--- a/includes/db/DatabaseError.php
+++ b/includes/db/DatabaseError.php
@@ -451,3 +451,9 @@ This may indicate a bug in the software.',
  */
 class DBUnexpectedError extends DBError {
 }
+
+/**
+ * @ingroup Database
+ */
+class DBReadOnlyError extends DBError {
+}
diff --git a/includes/db/loadbalancer/LoadBalancer.php b/includes/db/loadbalancer/LoadBalancer.php
index a0ef753b42..cd3b006528 100644
--- a/includes/db/loadbalancer/LoadBalancer.php
+++ b/includes/db/loadbalancer/LoadBalancer.php
@@ -546,6 +546,14 @@ class LoadBalancer {
 			$trxProf->recordConnection( $host, $dbname, $masterOnly );
 		}
 
+		# Make master connections read only if in lagged slave mode
+		if ( $masterOnly && $this->getServerCount() > 1 && $this->getLaggedSlaveMode() ) {
+			$conn->setLBInfo( 'readOnlyReason',
+				'The database has been automatically locked ' .
+				'while the slave database servers catch up to the master'
+			);
+		}
+
 		return $conn;
 	}
 
-- 
2.20.1