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diff --git a/www/wiki/includes/externalstore/ExternalStoreDB.php b/www/wiki/includes/externalstore/ExternalStoreDB.php
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+<?php
+/**
+ * External storage in SQL database.
+ *
+ * This program is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with this program; if not, write to the Free Software Foundation, Inc.,
+ * 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA.
+ * http://www.gnu.org/copyleft/gpl.html
+ *
+ * @file
+ */
+
+use MediaWiki\MediaWikiServices;
+use Wikimedia\Rdbms\ILoadBalancer;
+use Wikimedia\Rdbms\IDatabase;
+use Wikimedia\Rdbms\DBConnRef;
+use Wikimedia\Rdbms\MaintainableDBConnRef;
+use Wikimedia\Rdbms\DatabaseDomain;
+
+/**
+ * DB accessible external objects.
+ *
+ * In this system, each store "location" maps to a database "cluster".
+ * The clusters must be defined in the normal LBFactory configuration.
+ *
+ * @ingroup ExternalStorage
+ */
+class ExternalStoreDB extends ExternalStoreMedium {
+	/**
+	 * The provided URL is in the form of DB://cluster/id
+	 * or DB://cluster/id/itemid for concatened storage.
+	 *
+	 * @param string $url
+	 * @return string|bool False if missing
+	 * @see ExternalStoreMedium::fetchFromURL()
+	 */
+	public function fetchFromURL( $url ) {
+		list( $cluster, $id, $itemID ) = $this->parseURL( $url );
+		$ret = $this->fetchBlob( $cluster, $id, $itemID );
+
+		if ( $itemID !== false && $ret !== false ) {
+			return $ret->getItem( $itemID );
+		}
+
+		return $ret;
+	}
+
+	/**
+	 * Fetch data from given external store URLs.
+	 * The provided URLs are in the form of DB://cluster/id
+	 * or DB://cluster/id/itemid for concatened storage.
+	 *
+	 * @param array $urls An array of external store URLs
+	 * @return array A map from url to stored content. Failed results
+	 *     are not represented.
+	 */
+	public function batchFetchFromURLs( array $urls ) {
+		$batched = $inverseUrlMap = [];
+		foreach ( $urls as $url ) {
+			list( $cluster, $id, $itemID ) = $this->parseURL( $url );
+			$batched[$cluster][$id][] = $itemID;
+			// false $itemID gets cast to int, but should be ok
+			// since we do === from the $itemID in $batched
+			$inverseUrlMap[$cluster][$id][$itemID] = $url;
+		}
+		$ret = [];
+		foreach ( $batched as $cluster => $batchByCluster ) {
+			$res = $this->batchFetchBlobs( $cluster, $batchByCluster );
+			/** @var HistoryBlob $blob */
+			foreach ( $res as $id => $blob ) {
+				foreach ( $batchByCluster[$id] as $itemID ) {
+					$url = $inverseUrlMap[$cluster][$id][$itemID];
+					if ( $itemID === false ) {
+						$ret[$url] = $blob;
+					} else {
+						$ret[$url] = $blob->getItem( $itemID );
+					}
+				}
+			}
+		}
+
+		return $ret;
+	}
+
+	public function store( $location, $data ) {
+		$dbw = $this->getMaster( $location );
+		$dbw->insert( $this->getTable( $dbw ),
+			[ 'blob_text' => $data ],
+			__METHOD__ );
+		$id = $dbw->insertId();
+		if ( !$id ) {
+			throw new MWException( __METHOD__ . ': no insert ID' );
+		}
+
+		return "DB://$location/$id";
+	}
+
+	public function isReadOnly( $location ) {
+		return ( $this->getLoadBalancer( $location )->getReadOnlyReason() !== false );
+	}
+
+	/**
+	 * Get a LoadBalancer for the specified cluster
+	 *
+	 * @param string $cluster Cluster name
+	 * @return ILoadBalancer
+	 */
+	private function getLoadBalancer( $cluster ) {
+		$lbFactory = MediaWikiServices::getInstance()->getDBLoadBalancerFactory();
+		return $lbFactory->getExternalLB( $cluster );
+	}
+
+	/**
+	 * Get a replica DB connection for the specified cluster
+	 *
+	 * @param string $cluster Cluster name
+	 * @return DBConnRef
+	 */
+	public function getSlave( $cluster ) {
+		global $wgDefaultExternalStore;
+
+		$lb = $this->getLoadBalancer( $cluster );
+		$domainId = $this->getDomainId( $lb->getServerInfo( $lb->getWriterIndex() ) );
+
+		if ( !in_array( "DB://" . $cluster, (array)$wgDefaultExternalStore ) ) {
+			wfDebug( "read only external store\n" );
+			$lb->allowLagged( true );
+		} else {
+			wfDebug( "writable external store\n" );
+		}
+
+		$db = $lb->getConnectionRef( DB_REPLICA, [], $domainId );
+		$db->clearFlag( DBO_TRX ); // sanity
+
+		return $db;
+	}
+
+	/**
+	 * Get a master database connection for the specified cluster
+	 *
+	 * @param string $cluster Cluster name
+	 * @return MaintainableDBConnRef
+	 */
+	public function getMaster( $cluster ) {
+		$lb = $this->getLoadBalancer( $cluster );
+		$domainId = $this->getDomainId( $lb->getServerInfo( $lb->getWriterIndex() ) );
+
+		$db = $lb->getMaintenanceConnectionRef( DB_MASTER, [], $domainId );
+		$db->clearFlag( DBO_TRX ); // sanity
+
+		return $db;
+	}
+
+	/**
+	 * @param array $server Master DB server configuration array for LoadBalancer
+	 * @return string|bool Database domain ID or false
+	 */
+	private function getDomainId( array $server ) {
+		if ( isset( $this->params['wiki'] ) ) {
+			return $this->params['wiki']; // explicit domain
+		}
+
+		if ( isset( $server['dbname'] ) ) {
+			// T200471: for b/c, treat any "dbname" field as forcing which database to use.
+			// MediaWiki/LoadBalancer previously did not enforce any concept of a local DB
+			// domain, but rather assumed that the LB server configuration matched $wgDBname.
+			// This check is useful when the external storage DB for this cluster does not use
+			// the same name as the corresponding "main" DB(s) for wikis.
+			$domain = new DatabaseDomain(
+				$server['dbname'],
+				$server['schema'] ?? null,
+				$server['tablePrefix'] ?? ''
+			);
+
+			return $domain->getId();
+		}
+
+		return false; // local LB domain
+	}
+
+	/**
+	 * Get the 'blobs' table name for this database
+	 *
+	 * @param IDatabase $db
+	 * @return string Table name ('blobs' by default)
+	 */
+	public function getTable( $db ) {
+		$table = $db->getLBInfo( 'blobs table' );
+		if ( is_null( $table ) ) {
+			$table = 'blobs';
+		}
+
+		return $table;
+	}
+
+	/**
+	 * Fetch a blob item out of the database; a cache of the last-loaded
+	 * blob will be kept so that multiple loads out of a multi-item blob
+	 * can avoid redundant database access and decompression.
+	 * @param string $cluster
+	 * @param string $id
+	 * @param string $itemID
+	 * @return HistoryBlob|bool Returns false if missing
+	 */
+	private function fetchBlob( $cluster, $id, $itemID ) {
+		/**
+		 * One-step cache variable to hold base blobs; operations that
+		 * pull multiple revisions may often pull multiple times from
+		 * the same blob. By keeping the last-used one open, we avoid
+		 * redundant unserialization and decompression overhead.
+		 */
+		static $externalBlobCache = [];
+
+		$cacheID = ( $itemID === false ) ? "$cluster/$id" : "$cluster/$id/";
+		if ( isset( $externalBlobCache[$cacheID] ) ) {
+			wfDebugLog( 'ExternalStoreDB-cache',
+				"ExternalStoreDB::fetchBlob cache hit on $cacheID" );
+
+			return $externalBlobCache[$cacheID];
+		}
+
+		wfDebugLog( 'ExternalStoreDB-cache',
+			"ExternalStoreDB::fetchBlob cache miss on $cacheID" );
+
+		$dbr = $this->getSlave( $cluster );
+		$ret = $dbr->selectField( $this->getTable( $dbr ),
+			'blob_text', [ 'blob_id' => $id ], __METHOD__ );
+		if ( $ret === false ) {
+			wfDebugLog( 'ExternalStoreDB',
+				"ExternalStoreDB::fetchBlob master fallback on $cacheID" );
+			// Try the master
+			$dbw = $this->getMaster( $cluster );
+			$ret = $dbw->selectField( $this->getTable( $dbw ),
+				'blob_text', [ 'blob_id' => $id ], __METHOD__ );
+			if ( $ret === false ) {
+				wfDebugLog( 'ExternalStoreDB',
+					"ExternalStoreDB::fetchBlob master failed to find $cacheID" );
+			}
+		}
+		if ( $itemID !== false && $ret !== false ) {
+			// Unserialise object; caller extracts item
+			$ret = unserialize( $ret );
+		}
+
+		$externalBlobCache = [ $cacheID => $ret ];
+
+		return $ret;
+	}
+
+	/**
+	 * Fetch multiple blob items out of the database
+	 *
+	 * @param string $cluster A cluster name valid for use with LBFactory
+	 * @param array $ids A map from the blob_id's to look for to the requested itemIDs in the blobs
+	 * @return array A map from the blob_id's requested to their content.
+	 *   Unlocated ids are not represented
+	 */
+	private function batchFetchBlobs( $cluster, array $ids ) {
+		$dbr = $this->getSlave( $cluster );
+		$res = $dbr->select( $this->getTable( $dbr ),
+			[ 'blob_id', 'blob_text' ], [ 'blob_id' => array_keys( $ids ) ], __METHOD__ );
+		$ret = [];
+		if ( $res !== false ) {
+			$this->mergeBatchResult( $ret, $ids, $res );
+		}
+		if ( $ids ) {
+			wfDebugLog( __CLASS__, __METHOD__ .
+				" master fallback on '$cluster' for: " .
+				implode( ',', array_keys( $ids ) ) );
+			// Try the master
+			$dbw = $this->getMaster( $cluster );
+			$res = $dbw->select( $this->getTable( $dbr ),
+				[ 'blob_id', 'blob_text' ],
+				[ 'blob_id' => array_keys( $ids ) ],
+				__METHOD__ );
+			if ( $res === false ) {
+				wfDebugLog( __CLASS__, __METHOD__ . " master failed on '$cluster'" );
+			} else {
+				$this->mergeBatchResult( $ret, $ids, $res );
+			}
+		}
+		if ( $ids ) {
+			wfDebugLog( __CLASS__, __METHOD__ .
+				" master on '$cluster' failed locating items: " .
+				implode( ',', array_keys( $ids ) ) );
+		}
+
+		return $ret;
+	}
+
+	/**
+	 * Helper function for self::batchFetchBlobs for merging master/replica DB results
+	 * @param array &$ret Current self::batchFetchBlobs return value
+	 * @param array &$ids Map from blob_id to requested itemIDs
+	 * @param mixed $res DB result from Database::select
+	 */
+	private function mergeBatchResult( array &$ret, array &$ids, $res ) {
+		foreach ( $res as $row ) {
+			$id = $row->blob_id;
+			$itemIDs = $ids[$id];
+			unset( $ids[$id] ); // to track if everything is found
+			if ( count( $itemIDs ) === 1 && reset( $itemIDs ) === false ) {
+				// single result stored per blob
+				$ret[$id] = $row->blob_text;
+			} else {
+				// multi result stored per blob
+				$ret[$id] = unserialize( $row->blob_text );
+			}
+		}
+	}
+
+	/**
+	 * @param string $url
+	 * @return array
+	 */
+	protected function parseURL( $url ) {
+		$path = explode( '/', $url );
+
+		return [
+			$path[2], // cluster
+			$path[3], // id
+			isset( $path[4] ) ? $path[4] : false // itemID
+		];
+	}
+}