objectcache: Deprecate BagOStuff::addBusyCallback() and reduce to a stub

It's not called by anything since it was removed from
ChronologyProtector, and it is not needed for production.

Pass the timeout directly to LoadBalancer::waitForAll() instead of
using a WaitConditionLoop, as it was before e99bb1b7dc

Change-Id: I5a1903e6e9ac145c06f77c690a0f6a487b4aa43f

RELEASE-NOTES-1.39

@@ -273,6 +273,7 @@ because of Phabricator reports.
   - ::getUserObj() → ::getUser()
   - ::setUserObj() → ::setUsername()
   - ::setUserIP() → ::setUsername()
+* ObjectCache::addBusyCallback() is deprecated and non-functional.
 * …
 
 === Other changes in 1.39 ===

includes/libs/objectcache/BagOStuff.php

@@ -520,22 +520,12 @@ abstract class BagOStuff implements
 	/**
 	 * Let a callback be run to avoid wasting time on special blocking calls
 	 *
-	 * The callbacks may or may not be called ever, in any particular order.
-	 * They are likely to be invoked when something WRITE_SYNC is used used.
-	 * They should follow a caching pattern as shown below, so that any code
-	 * using the work will get it's result no matter what happens.
-	 * @code
-	 *     $result = null;
-	 *     $workCallback = function () use ( &$result ) {
-	 *         if ( !$result ) {
-	 *             $result = ....
-	 *         }
-	 *         return $result;
-	 *     }
-	 * @endcode
+	 * This is hard-deprecated and non-functional since 1.39. The callback
+	 * will not be called.
 	 *
 	 * @param callable $workCallback
 	 * @since 1.28
+	 * @deprecated since 1.39
 	 */
 	abstract public function addBusyCallback( callable $workCallback );
 

includes/libs/objectcache/MediumSpecificBagOStuff.php

@@ -48,9 +48,6 @@ abstract class MediumSpecificBagOStuff extends BagOStuff {
 	/** @var bool */
 	private $dupeTrackScheduled = false;
 
-	/** @var callable[] */
-	protected $busyCallbacks = [];
-
 	/** @var array[] Map of (key => (PHP variable value, serialized value)) */
 	protected $preparedValues = [];
 
@@ -805,7 +802,7 @@ abstract class MediumSpecificBagOStuff extends BagOStuff {
 	}
 
 	final public function addBusyCallback( callable $workCallback ) {
-		$this->busyCallbacks[] = $workCallback;
+		wfDeprecated( __METHOD__, '1.39' );
 	}
 
 	/**

includes/objectcache/SqlBagOStuff.php

@@ -32,7 +32,6 @@ use Wikimedia\Rdbms\ILoadBalancer;
 use Wikimedia\Rdbms\IMaintainableDatabase;
 use Wikimedia\ScopedCallback;
 use Wikimedia\Timestamp\ConvertibleTimestamp;
-use Wikimedia\WaitConditionLoop;
 
 /**
  * RDBMS-based caching module
@@ -1835,15 +1834,7 @@ class SqlBagOStuff extends MediumSpecificBagOStuff {
 				return true; // not applicable
 			}
 
-			$loop = new WaitConditionLoop(
-				static function () use ( $lb, $primaryPos ) {
-					return $lb->waitForAll( $primaryPos, 1 );
-				},
-				$this->syncTimeout,
-				$this->busyCallbacks
-			);
-
-			return ( $loop->invoke() === $loop::CONDITION_REACHED );
+			return $lb->waitForAll( $primaryPos, $this->syncTimeout );
 		} catch ( DBError $e ) {
 			$this->setAndLogDBError( $e );