gateway: fix returning nil references

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

frontend/dockerfile/dockerfile.go

@@ -24,7 +24,7 @@ func (f *dfFrontend) Solve(ctx context.Context, llbBridge frontend.FrontendLLBBr
 
 	defer func() {
 		for _, r := range c.refs {
-			if c.final != r || retErr != nil {
+			if r != nil && (c.final != r || retErr != nil) {
 				r.Release(context.TODO())
 			}
 		}
@@ -38,5 +38,5 @@ func (f *dfFrontend) Solve(ctx context.Context, llbBridge frontend.FrontendLLBBr
 		return nil, nil, errors.Errorf("invalid empty return") // shouldn't happen
 	}
 
-	return c.final, c.exporterAttr, nil
+	return c.final.ImmutableRef, c.exporterAttr, nil
 }

frontend/dockerfile/forward.go

@@ -1,11 +1,14 @@
 package dockerfile
 
 import (
+	"os"
+
 	"github.com/moby/buildkit/cache"
 	"github.com/moby/buildkit/frontend"
 	"github.com/moby/buildkit/frontend/gateway/client"
 	"github.com/moby/buildkit/session"
 	"github.com/moby/buildkit/solver/pb"
+	"github.com/pkg/errors"
 	"golang.org/x/net/context"
 )
 
@@ -16,7 +19,7 @@ func llbBridgeToGatewayClient(ctx context.Context, llbBridge frontend.FrontendLL
 type bridgeClient struct {
 	frontend.FrontendLLBBridge
 	opts         map[string]string
-	final        cache.ImmutableRef
+	final        *ref
 	sid          string
 	exporterAttr map[string][]byte
 	refs         []*ref
@@ -56,5 +59,8 @@ type ref struct {
 }
 
 func (r *ref) ReadFile(ctx context.Context, fp string) ([]byte, error) {
+	if r.ImmutableRef == nil {
+		return nil, errors.Wrapf(os.ErrNotExist, "%s no found", fp)
+	}
 	return cache.ReadFile(ctx, r.ImmutableRef, fp)
 }

frontend/gateway/gateway.go

@@ -145,7 +145,7 @@ func (gf *gatewayFrontend) Solve(ctx context.Context, llbBridge frontend.Fronten
 
 	defer func() {
 		for _, r := range lbf.refs {
-			if lbf.lastRef != r || retErr != nil {
+			if r != nil && (lbf.lastRef != r || retErr != nil) {
 				r.Release(context.TODO())
 			}
 		}
@@ -278,6 +278,9 @@ func (lbf *llbBrideForwarder) Solve(ctx context.Context, req *pb.SolveRequest) (
 		lbf.lastRef = ref
 		lbf.exporterAttr = exp
 	}
+	if ref == nil {
+		id = ""
+	}
 	return &pb.SolveResponse{Ref: id}, nil
 }
 func (lbf *llbBrideForwarder) ReadFile(ctx context.Context, req *pb.ReadFileRequest) (*pb.ReadFileResponse, error) {
@@ -285,7 +288,9 @@ func (lbf *llbBrideForwarder) ReadFile(ctx context.Context, req *pb.ReadFileRequ
 	if !ok {
 		return nil, errors.Errorf("no such ref: %v", req.Ref)
 	}
-
+	if ref == nil {
+		return nil, errors.Wrapf(os.ErrNotExist, "%s no found", req.FilePath)
+	}
 	dt, err := cache.ReadFile(ctx, ref, req.FilePath)
 	if err != nil {
 		return nil, err

frontend/gateway/grpcclient/client.go

@@ -51,6 +51,9 @@ func (c *grpcClient) Solve(ctx context.Context, def *opspb.Definition, frontend
 	if err != nil {
 		return nil, err
 	}
+	if resp.Ref == "" {
+		return nil, nil
+	}
 	return &reference{id: resp.Ref, c: c}, nil
 }