Avoid stack overflow in LoadBalancer with CACHE_DB WAN/server cache

* Add the ability to expose key BagOStuff attributes like whether the
  emulation SQL cache is being used. Several callers use hacks to detect
  this and can be updated later.
* Fallback to a safe empty WAN cache in the CACHE_DB case. This fixes
  a regression from f4bf52e843.
* Also add this protection to the server cache, which could break
  similarly before too.
* Also fix CACHE_ANYTHING by avoid the recursion risk from
  fc1d4d7960 by just checking the disabled service map.

Bug: T123829
Bug: T141804
Change-Id: I17ee26138f69e01ec1aaddb55ab27caa4d542193

includes/Services/ServiceContainer.php

@@ -367,4 +367,12 @@ class ServiceContainer implements DestructibleService {
 		return $service;
 	}
 
+	/**
+	 * @param string $name
+	 * @return bool Whether the service is disabled
+	 * @since 1.28
+	 */
+	public function isServiceDisabled( $name ) {
+		return isset( $this->disabled[$name] );
+	}
 }

includes/db/loadbalancer/LoadBalancer.php

@@ -138,8 +138,20 @@ class LoadBalancer {
 			}
 		}
 
-		$this->srvCache = ObjectCache::getLocalServerInstance();
-		$this->wanCache = ObjectCache::getMainWANInstance();
+		// Use APC/memcached style caching, but avoids loops with CACHE_DB (T141804)
+		// @TODO: inject these in via LBFactory at some point
+		$cache = ObjectCache::getLocalServerInstance();
+		if ( $cache->getQoS( $cache::ATTR_EMULATION ) > $cache::QOS_EMULATION_SQL ) {
+			$this->srvCache = $cache;
+		} else {
+			$this->srvCache = new EmptyBagOStuff();
+		}
+		$wCache = ObjectCache::getMainWANInstance();
+		if ( $wCache->getQoS( $wCache::ATTR_EMULATION ) > $wCache::QOS_EMULATION_SQL ) {
+			$this->wanCache = $wCache;
+		} else {
+			$this->wanCache = WANObjectCache::newEmpty();
+		}
 
 		if ( isset( $params['trxProfiler'] ) ) {
 			$this->trxProfiler = $params['trxProfiler'];

includes/libs/objectcache/BagOStuff.php

@@ -46,7 +46,7 @@ abstract class BagOStuff implements IExpiringStore, LoggerAwareInterface {
 	/** @var array[] Lock tracking */
 	protected $locks = [];
 
-	/** @var integer */
+	/** @var integer ERR_* class constant */
 	protected $lastError = self::ERR_NONE;
 
 	/** @var string */
@@ -70,6 +70,9 @@ abstract class BagOStuff implements IExpiringStore, LoggerAwareInterface {
 	/** @var bool */
 	private $dupeTrackScheduled = false;
 
+	/** @var integer[] Map of (ATTR_* class constant => QOS_* class constant) */
+	protected $attrMap = [];
+
 	/** Possible values for getLastError() */
 	const ERR_NONE = 0; // no error
 	const ERR_NO_RESPONSE = 1; // no response
@@ -734,4 +737,34 @@ abstract class BagOStuff implements IExpiringStore, LoggerAwareInterface {
 	public function makeKey() {
 		return $this->makeKeyInternal( $this->keyspace, func_get_args() );
 	}
+
+	/**
+	 * @param integer $flag ATTR_* class constant
+	 * @return integer QOS_* class constant
+	 * @since 1.28
+	 */
+	public function getQoS( $flag ) {
+		return isset( $this->attrMap[$flag] ) ? $this->attrMap[$flag] : self::QOS_UNKNOWN;
+	}
+
+	/**
+	 * Merge the flag maps of one or more BagOStuff objects into a "lowest common denominator" map
+	 *
+	 * @param BagOStuff[] $bags
+	 * @return integer[] Resulting flag map (class ATTR_* constant => class QOS_* constant)
+	 */
+	protected function mergeFlagMaps( array $bags ) {
+		$map = [];
+		foreach ( $bags as $bag ) {
+			foreach ( $bag->attrMap as $attr => $rank ) {
+				if ( isset( $map[$attr] ) ) {
+					$map[$attr] = min( $map[$attr], $rank );
+				} else {
+					$map[$attr] = $rank;
+				}
+			}
+		}
+
+		return $map;
+	}
 }

includes/libs/objectcache/CachedBagOStuff.php

@@ -42,8 +42,10 @@ class CachedBagOStuff extends HashBagOStuff {
 	 * @param array $params Parameters for HashBagOStuff
 	 */
 	function __construct( BagOStuff $backend, $params = [] ) {
-		$this->backend = $backend;
 		parent::__construct( $params );
+
+		$this->backend = $backend;
+		$this->attrMap = $backend->attrMap;
 	}
 
 	protected function doGet( $key, $flags = 0 ) {

includes/libs/objectcache/IExpiringStore.php

@@ -42,4 +42,11 @@ interface IExpiringStore {
 	const TTL_PROC_LONG = 30; // loose cache time that can survive slow web requests
 
 	const TTL_INDEFINITE = 0;
+
+	// Attribute and QoS constants; higher QOS values with the same prefix rank higher...
+	// Medium attributes constants related to emulation or media type
+	const ATTR_EMULATION = 1;
+	const QOS_EMULATION_SQL = 1;
+	// Generic "unknown" value that is useful for comparisons (e.g. always good enough)
+	const QOS_UNKNOWN = INF;
 }

includes/libs/objectcache/MultiWriteBagOStuff.php

@@ -83,6 +83,7 @@ class MultiWriteBagOStuff extends BagOStuff {
 				$this->caches[] = ObjectFactory::getObjectFromSpec( $cacheInfo );
 			}
 		}
+		$this->mergeFlagMaps( $this->caches );
 
 		$this->asyncWrites = (
 			isset( $params['replication'] ) &&

includes/libs/objectcache/ReplicatedBagOStuff.php

@@ -65,6 +65,7 @@ class ReplicatedBagOStuff extends BagOStuff {
 		$this->readStore = ( $params['readFactory'] instanceof BagOStuff )
 			? $params['readFactory']
 			: ObjectFactory::getObjectFromSpec( $params['readFactory'] );
+		$this->attrMap = $this->mergeFlagMaps( [ $this->readStore, $this->writeStore ] );
 	}
 
 	public function setDebug( $debug ) {

includes/libs/objectcache/WANObjectCache.php

@@ -977,7 +977,7 @@ class WANObjectCache implements IExpiringStore, LoggerAwareInterface {
 
 	/**
 	 * Get the "last error" registered; clearLastError() should be called manually
-	 * @return int ERR_* constant for the "last error" registry
+	 * @return int ERR_* class constant for the "last error" registry
 	 */
 	final public function getLastError() {
 		if ( $this->lastRelayError ) {
@@ -1019,6 +1019,15 @@ class WANObjectCache implements IExpiringStore, LoggerAwareInterface {
 		$this->procCache->clear();
 	}
 
+	/**
+	 * @param integer $flag ATTR_* class constant
+	 * @return integer QOS_* class constant
+	 * @since 1.28
+	 */
+	public function getQoS( $flag ) {
+		return $this->cache->getQoS( $flag );
+	}
+
 	/**
 	 * Do the actual async bus purge of a key
 	 *

includes/objectcache/ObjectCache.php

@@ -228,14 +228,12 @@ class ObjectCache {
 			}
 		}
 
-		try {
-			// Make sure we actually have a DB backend before falling back to CACHE_DB
-			MediaWikiServices::getInstance()->getDBLoadBalancer();
-			$candidate = CACHE_DB;
-		} catch ( ServiceDisabledException $e ) {
+		if ( MediaWikiServices::getInstance()->isServiceDisabled( 'DBLoadBalancer' ) ) {
 			// The LoadBalancer is disabled, probably because
 			// MediaWikiServices::disableStorageBackend was called.
 			$candidate = CACHE_NONE;
+		} else {
+			$candidate = CACHE_DB;
 		}
 
 		return self::getInstance( $candidate );

includes/objectcache/SqlBagOStuff.php

@@ -91,6 +91,9 @@ class SqlBagOStuff extends BagOStuff {
 	 */
 	public function __construct( $params ) {
 		parent::__construct( $params );
+
+		$this->attrMap[self::ATTR_EMULATION] = self::QOS_EMULATION_SQL;
+
 		if ( isset( $params['servers'] ) ) {
 			$this->serverInfos = [];
 			$this->serverTags = [];