Merge "Enforce lagged-slave read-only mode on the DB layer"

[lhc/web/wiklou.git] / includes / db / Database.php

diff --git a/includes/db/Database.php b/includes/db/Database.php
index ebcf540..b9d344f 100644 (file)
--- a/includes/db/Database.php
+++ b/includes/db/Database.php
@@ -45,6 +45,9 @@ abstract class DatabaseBase implements IDatabase {
 
        protected $mServer, $mUser, $mPassword, $mDBname;
 
+       /** @var BagOStuff APC cache */
+       protected $srvCache;
+
        /** @var resource Database connection */
        protected $mConn = null;
        protected $mOpened = false;
@@ -96,6 +99,9 @@ abstract class DatabaseBase implements IDatabase {
         */
        private $mTrxTimestamp = null;
 
+       /** @var float Lag estimate at the time of BEGIN */
+       private $mTrxSlaveLag = null;
+
        /**
         * Remembers the function name given for starting the most recent transaction via begin().
         * Used to provide additional context for error reporting.
@@ -601,6 +607,9 @@ abstract class DatabaseBase implements IDatabase {
        function __construct( array $params ) {
                global $wgDBprefix, $wgDBmwschema, $wgCommandLineMode;
 
+               $this->mTrxAtomicLevels = new SplStack;
+               $this->srvCache = ObjectCache::newAccelerator( 'hash' );
+
                $server = $params['host'];
                $user = $params['user'];
                $password = $params['password'];
@@ -918,9 +927,9 @@ 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 );
@@ -3466,6 +3475,11 @@ abstract class DatabaseBase implements IDatabase {
                $this->mTrxPreCommitCallbacks = array();
                $this->mTrxShortId = wfRandomString( 12 );
                $this->mTrxWriteDuration = 0.0;
+               // First SELECT after BEGIN will establish the snapshot in REPEATABLE-READ.
+               // Get an estimate of the slave lag before then, treating estimate staleness
+               // as lag itself just to be safe
+               $status = $this->getApproximateLagStatus();
+               $this->mTrxSlaveLag = $status['lag'] + ( microtime( true ) - $status['since'] );
        }
 
        /**
@@ -3742,6 +3756,80 @@ abstract class DatabaseBase implements IDatabase {
                return true;
        }
 
+       /**
+        * Get the slave lag when the current transaction started
+        * or a general lag estimate if not transaction is active
+        *
+        * This is useful when transactions might use snapshot isolation
+        * (e.g. REPEATABLE-READ in innodb), so the "real" lag of that data
+        * is this lag plus transaction duration. If they don't, it is still
+        * safe to be pessimistic. In AUTO-COMMIT mode, this still gives an
+        * indication of the staleness of subsequent reads.
+        *
+        * @return array ('lag': seconds, 'since': UNIX timestamp of BEGIN)
+        * @since 1.27
+        */
+       public function getSessionLagStatus() {
+               return $this->getTransactionLagStatus() ?: $this->getApproximateLagStatus();
+       }
+
+       /**
+        * Get the slave lag when the current transaction started
+        *
+        * This is useful when transactions might use snapshot isolation
+        * (e.g. REPEATABLE-READ in innodb), so the "real" lag of that data
+        * 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.
+        *
+        * @return array|null ('lag': seconds, 'since': UNIX timestamp of BEGIN)
+        * @since 1.27
+        */
+       public function getTransactionLagStatus() {
+               return $this->mTrxLevel
+                       ? array( 'lag' => $this->mTrxSlaveLag, 'since' => $this->trxTimestamp() )
+                       : null;
+       }
+
+       /**
+        * Get a slave lag estimate for this server
+        *
+        * @return array ('lag': seconds, 'since': UNIX timestamp of estimate)
+        * @since 1.27
+        */
+       public function getApproximateLagStatus() {
+               return array(
+                       'lag'   => $this->getLBInfo( 'slave' ) ? $this->getLag() : 0,
+                       'since' => microtime( true )
+               );
+       }
+
+       /**
+        * Merge the result of getSessionLagStatus() for several DBs
+        * using the most pessimistic values to estimate the lag of
+        * any data derived from them in combination
+        *
+        * This is information is useful for caching modules
+        *
+        * @see WANObjectCache::set()
+        * @see WANObjectCache::getWithSetCallback()
+        *
+        * @param IDatabase $db1
+        * @param IDatabase ...
+        * @return array ('lag': highest lag, 'since': lowest estimate UNIX timestamp)
+        * @since 1.27
+        */
+       public static function getCacheSetOptions( IDatabase $db1 ) {
+               $res = array( 'lag' => 0, 'since' => INF );
+               foreach ( func_get_args() as $db ) {
+                       /** @var IDatabase $db */
+                       $status = $db->getSessionLagStatus();
+                       $res['lag'] = max( $res['lag'], $status['lag'] );
+                       $res['since'] = min( $res['since'], $status['since'] );
+               }
+
+               return $res;
+       }
+
        /**
         * Get slave lag. Currently supported only by MySQL.
         *
@@ -4219,6 +4307,9 @@ abstract class DatabaseBase implements IDatabase {
        }
 }
 
+/**
+ * @since 1.27
+ */
 abstract class Database extends DatabaseBase {
        // B/C until nothing type hints for DatabaseBase
        // @TODO: finish renaming DatabaseBase => Database