fix: retry connection once in pgproxy to support credential rotations…

… in the DSN function

internal/pgproxy/pgproxy.go

@@ -93,16 +93,16 @@ func HandleConnection(ctx context.Context, conn net.Conn, connectionFn DSNConstr
 	logger.Debugf("startup message: %+v", startup)
 	logger.Debugf("backend connected: %s", conn.RemoteAddr())
 
-	dsn, err := connectionFn(ctx, startup.Parameters)
+	frontend, err := connectFrontend(ctx, connectionFn, startup)
 	if err != nil {
-		handleBackendError(ctx, backend, err)
-		return
-	}
+		// try again, in case there was a credential rotation
+		logger.Warnf("failed to connect frontend: %s, trying again", err)
 
-	frontend, err := connectFrontend(ctx, dsn)
-	if err != nil {
-		handleBackendError(ctx, backend, err)
-		return
+		frontend, err = connectFrontend(ctx, connectionFn, startup)
+		if err != nil {
+			handleBackendError(ctx, backend, err)
+			return
+		}
 	}
 	logger.Debugf("frontend connected")
 
@@ -171,7 +171,12 @@ func connectBackend(ctx context.Context, conn net.Conn) (*pgproto3.Backend, *pgp
 	}
 }
 
-func connectFrontend(ctx context.Context, dsn string) (*pgproto3.Frontend, error) {
+func connectFrontend(ctx context.Context, connectionFn DSNConstructor, startup *pgproto3.StartupMessage) (*pgproto3.Frontend, error) {
+	dsn, err := connectionFn(ctx, startup.Parameters)
+	if err != nil {
+		return nil, fmt.Errorf("failed to construct dsn: %w", err)
+	}
+
 	conn, err := pgconn.Connect(ctx, dsn)
 	if err != nil {
 		return nil, fmt.Errorf("failed to connect to backend: %w", err)