Merge pull request #5 from getkalido/fix/lock

Change table lock to SHARE ROW EXCLUSIVE

Author: HeinKalido

migrations.go

@@ -391,7 +391,10 @@ func lockTable(tx *pg.Tx) error {
 	if err != nil {
 		return err
 	}
-	_, err = tx.Exec("LOCK ? ", pg.Ident(migrationTableName))
+	// https://www.postgresql.org/docs/current/explicit-locking.html
+	// This mode protects a table against concurrent data changes, and is self-exclusive so that only one session can hold it at a time.
+	// This means only one migration can run at a time, but pg_dump can still COPY from the table (since it acquires a ACCESS SHARE lock, or so I am told)
+	_, err = tx.Exec("LOCK ? in SHARE ROW EXCLUSIVE MODE", pg.Ident(migrationTableName))
 
 	return err
 }