Improve MetaDataStorageHandlerPdo::getMetaData() performance

By using an appropriate WHERE clause in the SQL statement,
we can avoid loading the whole dataset in getMetaData(),
which brings an welcome performance improvement with large
Metadata sets.

lib/SimpleSAML/Metadata/MetaDataStorageHandlerPdo.php

@@ -142,6 +142,48 @@ class SimpleSAML_Metadata_MetaDataStorageHandlerPdo extends SimpleSAML_Metadata_
         return $metadataSet;
     }
 
+    /**      
+     * Retrieve a metadata entry.
+     *      
+     * @param string $entityId The entityId we are looking up.
+     * @param string $set The set we are looking for metadata in.
+     *
+     * @return array An associative array with metadata for the given entity, or NULL if we are unable to
+     *         locate the entity.
+     */
+    public function getMetaData($entityId, $set)      
+    {       
+        assert('is_string($entityId)');
+        assert('is_string($set)');
+
+        $tableName = $this->getTableName($set);
+
+        if (!in_array($set, $this->supportedSets, true)) {
+            return null;
+        }
+
+        $stmt = $this->db->read("SELECT entity_id, entity_data FROM $tableName WHERE entity_id=:entityId", array('entityId' => $entityId));
+        if ($stmt->execute()) {
+            $rowCount = 0;
+
+            while ($d = $stmt->fetch()) {
+                if (++$rowCount > 1) {
+		    SimpleSAML\Logger::warning("Dulicate match for $entityId in set $set");
+		    break;
+                }
+                $data = json_decode($d['entity_data'], true);
+                if ($data === null) {
+                    throw new SimpleSAML_Error_Exception("Cannot decode metadata for entity '${d['entity_id']}'");
+                }
+                if (!array_key_exists('entityid', $data)) {
+                    $data['entityid'] = $d['entity_id'];
+                }
+            }
+            return $data;
+        } else {
+            throw new Exception('PDO metadata handler: Database error: '.var_export($this->db->getLastError(), true));
+        }
+    }
 
     private function generateDynamicHostedEntityID($set)
     {