[v11.2.x] ServerLock: Fix pg concurrency/locking issue (#95934)

ServerLock: Fix pg concurrency/locking issue (#95916)

Fix pg unique constraint validation in serverlock

(cherry picked from commit ab974ddf14)

Co-authored-by: Misi <mgyongyosi@users.noreply.github.com>

pkg/infra/serverlock/serverlock.go

@@ -350,16 +350,26 @@ func (sl *ServerLockService) executeFunc(ctx context.Context, actionName string,
 }
 
 func (sl *ServerLockService) createLock(ctx context.Context,
-	lockRow *serverLock, dbSession *sqlstore.DBSession) (*serverLock, error) {
+	lockRow *serverLock, dbSession *sqlstore.DBSession,
+) (*serverLock, error) {
 	affected := int64(1)
 	rawSQL := `INSERT INTO server_lock (operation_uid, last_execution, version) VALUES (?, ?, ?)`
 	if sl.SQLStore.GetDBType() == migrator.Postgres {
-		rawSQL += ` RETURNING id`
+		rawSQL += ` ON CONFLICT DO NOTHING RETURNING id`
 		var id int64
 		_, err := dbSession.SQL(rawSQL, lockRow.OperationUID, lockRow.LastExecution, 0).Get(&id)
 		if err != nil {
 			return nil, err
 		}
+		if id == 0 {
+			// Considering the default isolation level (READ COMMITTED), an entry could be added to the table
+			// between the SELECT and the INSERT. And inserting a row with the same operation_uid would violate the unique
+			// constraint. In this case, the ON CONFLICT DO NOTHING clause will prevent generating an error.
+			// And the returning id will be 0 which means that there wasn't any row inserted (another server has the lock),
+			// therefore we return the ServerLockExistsError.
+			// https://www.postgresql.org/docs/current/transaction-iso.html#XACT-READ-COMMITTED
+			return nil, &ServerLockExistsError{actionName: lockRow.OperationUID}
+		}
 		lockRow.Id = id
 	} else {
 		res, err := dbSession.Exec(