db: Allow some internal queries to fail

The first ever query to check if the version DB exists may fail. We allow
this, but we need to restart the DB transaction since postgres fails the
current transaction and rolls back any changes.

This just commits (and fails) and starts a new transaction so the rest of the
migration can continue.

Signed-off-by: Christian Decker <decker.christian@gmail.com>

wallet/db.c

@@ -697,13 +697,23 @@ static int db_get_version(struct db *db)
 {
 	int res = -1;
 	struct db_stmt *stmt = db_prepare_v2(db, SQL("SELECT version FROM version LIMIT 1"));
+
+	/*
+	 * Tentatively execute a query, but allow failures. Some databases
+	 * like postgres will terminate the DB transaction if there is an
+	 * error during the execution of a query, e.g., trying to access a
+	 * table that doesn't exist yet, so we need to terminate and restart
+	 * the DB transaction.
+	 */
 	if (!db_query_prepared(stmt)) {
+		db_commit_transaction(stmt->db);
+		db_begin_transaction(stmt->db);
 		tal_free(stmt);
 		return res;
 	}
 
 	if (db_step(stmt))
-		res = db_column_u64(stmt, 0);
+		res = db_column_int(stmt, 0);
 
 	tal_free(stmt);
 	return res;