fix(preview): Fix possible data race in oc_preview_location insert

If two process try to create at the same time the same row in
oc_preview_location, catch the error in the second process and fetch the
location id again.

Signed-off-by: Carl Schwan <carl.schwan@nextcloud.com>

Author: CarlSchwan

core/Migrations/Version33000Date20250819110529.php

@@ -34,6 +34,7 @@ public function changeSchema(IOutput $output, Closure $schemaClosure, array $opt
 			$table->addColumn('id', Types::BIGINT, ['notnull' => true, 'length' => 20, 'unsigned' => true]);
 			$table->addColumn('bucket_name', Types::STRING, ['notnull' => true, 'length' => 40]);
 			$table->addColumn('object_store_name', Types::STRING, ['notnull' => true, 'length' => 40]);
+			$table->addUniqueIndex(['bucket_name', 'object_store_name']);
 			$table->setPrimaryKey(['id']);
 		}
 

lib/private/Preview/Db/PreviewMapper.php

@@ -152,6 +152,12 @@ protected function joinLocation(IQueryBuilder $qb): IQueryBuilder {
 			));
 	}
 
+	/**
+	 * Get the location id corresponding to the $bucket and $objectStore. Create one
+	 * if not existing yet.
+	 *
+	 * @throws Exception
+	 */
 	public function getLocationId(string $bucket, string $objectStore): string {
 		$qb = $this->db->getQueryBuilder();
 		$result = $qb->select('id')
@@ -161,16 +167,33 @@ public function getLocationId(string $bucket, string $objectStore): string {
 			->executeQuery();
 		$data = $result->fetchOne();
 		if ($data) {
-			return $data;
+			return (string)$data;
 		} else {
-			$id = $this->snowflake->id();
-			$qb->insert(self::LOCATION_TABLE_NAME)
-				->values([
-					'id' => $qb->createNamedParameter($id),
-					'bucket_name' => $qb->createNamedParameter($bucket),
-					'object_store_name' => $qb->createNamedParameter($objectStore),
-				])->executeStatement();
-			return $id;
+			try {
+				$id = $this->snowflake->id();
+				$qb->insert(self::LOCATION_TABLE_NAME)
+					->values([
+						'id' => $qb->createNamedParameter($id),
+						'bucket_name' => $qb->createNamedParameter($bucket),
+						'object_store_name' => $qb->createNamedParameter($objectStore),
+					])->executeStatement();
+				return $id;
+			} catch (Exception $e) {
+				if ($e->getReason() === Exception::REASON_UNIQUE_CONSTRAINT_VIOLATION) {
+					// Fetch again as there seems to be another entry added meanwhile
+					$result = $qb->select('id')
+						->from(self::LOCATION_TABLE_NAME)
+						->where($qb->expr()->eq('bucket_name', $qb->createNamedParameter($bucket)))
+						->andWhere($qb->expr()->eq('object_store_name', $qb->createNamedParameter($objectStore)))
+						->executeQuery();
+					$data = $result->fetchOne();
+					if ($data) {
+						return $data;
+					}
+				}
+
+				throw $e;
+			}
 		}
 	}