Fix nil panic on scratch ref release.

client/build_test.go

@@ -60,6 +60,7 @@ func TestClientGatewayIntegration(t *testing.T) {
 		testWarnings,
 		testClientGatewayFrontendAttrs,
 		testClientGatewayNilResult,
+		testClientGatewayEmptyImageExec,
 	), integration.WithMirroredImages(integration.OfficialImages("busybox:latest")))
 
 	integration.Run(t, integration.TestFuncs(
@@ -2086,6 +2087,54 @@ func testClientGatewayNilResult(t *testing.T, sb integration.Sandbox) {
 	require.NoError(t, err)
 }
 
+func testClientGatewayEmptyImageExec(t *testing.T, sb integration.Sandbox) {
+	integration.SkipIfDockerd(t, sb, "direct push")
+	c, err := New(sb.Context(), sb.Address())
+	require.NoError(t, err)
+	defer c.Close()
+
+	registry, err := sb.NewRegistry()
+	if errors.Is(err, integration.ErrRequirements) {
+		t.Skip(err.Error())
+	}
+	require.NoError(t, err)
+	target := registry + "/buildkit/testemptyimage:latest"
+
+	// push an empty image
+	_, err = c.Build(sb.Context(), SolveOpt{
+		Exports: []ExportEntry{
+			{
+				Type: ExporterImage,
+				Attrs: map[string]string{
+					"name": target,
+					"push": "true",
+				},
+			},
+		},
+	}, "", func(ctx context.Context, c client.Client) (*client.Result, error) {
+		return client.NewResult(), nil
+	}, nil)
+	require.NoError(t, err)
+
+	_, err = c.Build(sb.Context(), SolveOpt{}, "", func(ctx context.Context, gw client.Client) (*client.Result, error) {
+		// create an exec on that empty image (expected to fail, but not to panic)
+		st := llb.Image(target).Run(
+			llb.Args([]string{"echo", "hello"}),
+		).Root()
+		def, err := st.Marshal(sb.Context())
+		if err != nil {
+			return nil, err
+		}
+		_, err = gw.Solve(ctx, client.SolveRequest{
+			Definition: def.ToPB(),
+			Evaluate:   true,
+		})
+		require.ErrorContains(t, err, `process "echo hello" did not complete successfully`)
+		return nil, nil
+	}, nil)
+	require.NoError(t, err)
+}
+
 type nopCloser struct {
 	io.Writer
 }

frontend/gateway/gateway.go

@@ -353,7 +353,7 @@ func (lbf *llbBridgeForwarder) Discard() {
 	}
 
 	for id, workerRef := range lbf.workerRefByID {
-		workerRef.ImmutableRef.Release(context.TODO())
+		workerRef.Release(context.TODO())
 		delete(lbf.workerRefByID, id)
 	}
 	if lbf.err != nil && lbf.result != nil {

worker/result.go

@@ -26,6 +26,13 @@ func (wr *WorkerRef) ID() string {
 	return wr.Worker.ID() + "::" + refID
 }
 
+func (wr *WorkerRef) Release(ctx context.Context) error {
+	if wr.ImmutableRef == nil {
+		return nil
+	}
+	return wr.ImmutableRef.Release(ctx)
+}
+
 // GetRemotes method abstracts ImmutableRef's GetRemotes to allow a Worker to override.
 // This is needed for moby integration.
 // Use this method instead of calling ImmutableRef.GetRemotes() directly.