Make MultiWriteBagOStuff use the native merge() of each backend

This lets backends that support CAS take advantage of it.

Also fix race condition with merge() in asyncWrites mode where two
threads might add to a list/set but the first request finishes after
the second, which would erase an item during the set().

Bug: T198239
Change-Id: Ibd2539429746f12b4de74d0f031089cb80aa8b4b

diff --git a/includes/libs/objectcache/MultiWriteBagOStuff.php b/includes/libs/objectcache/MultiWriteBagOStuff.php
index 91f4167..043f8cb 100644 (file)
--- a/includes/libs/objectcache/MultiWriteBagOStuff.php
+++ b/includes/libs/objectcache/MultiWriteBagOStuff.php
@@ -122,7 +122,7 @@ class MultiWriteBagOStuff extends BagOStuff {
                        && $missIndexes
                        && ( $flags & self::READ_VERIFIED ) == self::READ_VERIFIED
                ) {
-                       // Backfill the value to the lower (and often larger) cache tiers
+                       // Backfill the value to the higher (and often faster/smaller) cache tiers
                        $this->doWrite(
                                $missIndexes, $this->asyncWrites, 'set', $key, $value, self::UPGRADE_TTL
                        );
@@ -171,6 +171,23 @@ class MultiWriteBagOStuff extends BagOStuff {
                return $this->doWrite( $this->cacheIndexes, $this->asyncWrites, 'decr', $key, $value );
        }
 
+       public function merge( $key, callable $callback, $exptime = 0, $attempts = 10, $flags = 0 ) {
+               $asyncWrites = ( ( $flags & self::WRITE_SYNC ) == self::WRITE_SYNC )
+                       ? false
+                       : $this->asyncWrites;
+
+               return $this->doWrite(
+                       $this->cacheIndexes,
+                       $asyncWrites,
+                       'merge',
+                       $key,
+                       $callback,
+                       $exptime,
+                       $attempts,
+                       $flags
+               );
+       }
+
        public function lock( $key, $timeout = 6, $expiry = 6, $rclass = '' ) {
                // Only need to lock the first cache; also avoids deadlocks
                return $this->caches[0]->lock( $key, $timeout, $expiry, $rclass );
@@ -202,7 +219,7 @@ class MultiWriteBagOStuff extends BagOStuff {
                $ret = true;
                $args = array_slice( func_get_args(), 3 );
 
-               if ( array_diff( $indexes, [ 0 ] ) && $asyncWrites ) {
+               if ( array_diff( $indexes, [ 0 ] ) && $asyncWrites && $method !== 'merge' ) {
                        // Deep-clone $args to prevent misbehavior when something writes an
                        // object to the BagOStuff then modifies it afterwards, e.g. T168040.
                        $args = unserialize( serialize( $args ) );