grpcclient: return proper nil reference from grpcclient

The grpcclient would take an empty reference and convert it into a
`(*reference)(nil)` and then store that in a `client.Reference`. This
caused the `nil` check in `convertRef` to return false because it wasn't
`nil` but then it panicked because it was `nil`.

`newReference` has been minorly refactored to not return an error. The
method is not exported, the error value is not used, and it obscured the
functionality which made it harder to use correctly.

Signed-off-by: Jonathan A. Sternberg <jonathan.sternberg@docker.com>

frontend/dockerfile/dockerfile_test.go

@@ -196,6 +196,7 @@ var allTests = integration.TestFuncs(
 	testCommandSourceMapping,
 	testSBOMScannerArgs,
 	testNilContextInSolveGateway,
+	testMultiNilRefsInSolveGateway,
 	testCopyUnicodePath,
 	testFrontendDeduplicateSources,
 	testDuplicateLayersProvenance,
@@ -8297,6 +8298,41 @@ func testNilContextInSolveGateway(t *testing.T, sb integration.Sandbox) {
 	require.ErrorContains(t, err, "invalid nil input definition to definition op")
 }
 
+func testMultiNilRefsInSolveGateway(t *testing.T, sb integration.Sandbox) {
+	workers.CheckFeatureCompat(t, sb, workers.FeatureMultiPlatform)
+	ctx := sb.Context()
+
+	c, err := client.New(ctx, sb.Address())
+	require.NoError(t, err)
+	defer c.Close()
+
+	f := getFrontend(t, sb)
+
+	_, err = c.Build(sb.Context(), client.SolveOpt{}, "", func(ctx context.Context, c gateway.Client) (*gateway.Result, error) {
+		localDockerfile, err := llb.Scratch().
+			File(llb.Mkfile("Dockerfile", 0644, []byte(`FROM scratch`))).
+			Marshal(ctx)
+		if err != nil {
+			return nil, err
+		}
+
+		res, err := f.SolveGateway(ctx, c, gateway.SolveRequest{
+			Frontend: "dockerfile.v0",
+			FrontendOpt: map[string]string{
+				"platform": "linux/amd64,linux/arm64",
+			},
+			FrontendInputs: map[string]*pb.Definition{
+				dockerui.DefaultLocalNameDockerfile: localDockerfile.ToPB(),
+			},
+		})
+		if err != nil {
+			return nil, err
+		}
+		return res, nil
+	}, nil)
+	require.NoError(t, err)
+}
+
 func testCopyUnicodePath(t *testing.T, sb integration.Sandbox) {
 	f := getFrontend(t, sb)
 	c, err := client.New(sb.Context(), sb.Address())

frontend/gateway/grpcclient/client.go

@@ -429,28 +429,21 @@ func (c *grpcClient) Solve(ctx context.Context, creq client.SolveRequest) (res *
 			}
 		case *pb.Result_RefsDeprecated:
 			for k, v := range pbRes.RefsDeprecated.Refs {
-				ref := &reference{id: v, c: c}
-				if v == "" {
-					ref = nil
+				var ref client.Reference
+				if v != "" {
+					ref = &reference{id: v, c: c}
 				}
 				res.AddRef(k, ref)
 			}
 		case *pb.Result_Ref:
 			if pbRes.Ref.Id != "" {
-				ref, err := newReference(c, pbRes.Ref)
-				if err != nil {
-					return nil, err
-				}
-				res.SetRef(ref)
+				res.SetRef(newReference(c, pbRes.Ref))
 			}
 		case *pb.Result_Refs:
 			for k, v := range pbRes.Refs.Refs {
-				var ref *reference
+				var ref client.Reference
 				if v.Id != "" {
-					ref, err = newReference(c, v)
-					if err != nil {
-						return nil, err
-					}
+					ref = newReference(c, v)
 				}
 				res.AddRef(k, ref)
 			}
@@ -464,11 +457,7 @@ func (c *grpcClient) Solve(ctx context.Context, creq client.SolveRequest) (res *
 						return nil, err
 					}
 					if a.Ref.Id != "" {
-						ref, err := newReference(c, a.Ref)
-						if err != nil {
-							return nil, err
-						}
-						att.Ref = ref
+						att.Ref = newReference(c, a.Ref)
 					}
 					res.AddAttestation(p, *att)
 				}
@@ -1168,8 +1157,8 @@ type reference struct {
 	def *opspb.Definition
 }
 
-func newReference(c *grpcClient, ref *pb.Ref) (*reference, error) {
-	return &reference{c: c, id: ref.Id, def: ref.Def}, nil
+func newReference(c *grpcClient, ref *pb.Ref) *reference {
+	return &reference{c: c, id: ref.Id, def: ref.Def}
 }
 
 func (r *reference) ToState() (st llb.State, err error) {