dockerfile2llb: set default llb.Local when client is not available

Previously, we would panic on marshalling when attempting to use
./examples/dockerfile2llb to print a target. This was because no client
was available to call MainContext.

Signed-off-by: Justin Chadwell <me@jedevc.com>

Author: jedevc

frontend/dockerfile/dockerfile2llb/convert.go

@@ -573,6 +573,9 @@ func toDispatchState(ctx context.Context, dt []byte, opt ConvertOpt) (*dispatchS
 			return nil, err
 		}
 		buildContext.Output = bctx.Output()
+	} else {
+		bctx := dockerui.DefaultMainContext(opts...)
+		buildContext.Output = bctx.Output()
 	}
 
 	defaults := []llb.ConstraintsOpt{

frontend/dockerfile/dockerfile2llb/convert_test.go

@@ -1,6 +1,7 @@
 package dockerfile2llb
 
 import (
+	"context"
 	"testing"
 
 	"github.com/moby/buildkit/frontend/dockerfile/instructions"
@@ -85,6 +86,20 @@ COPY --from=0 f2 /
 	assert.Error(t, err)
 }
 
+func TestDockerfileParsingMarshal(t *testing.T) {
+	t.Parallel()
+	df := `FROM scratch
+ENV FOO bar
+COPY f1 f2 /sub/
+RUN ls -l
+`
+	state, _, _, err := Dockerfile2LLB(appcontext.Context(), []byte(df), ConvertOpt{})
+	assert.NoError(t, err)
+
+	_, err = state.Marshal(context.TODO())
+	assert.NoError(t, err)
+}
+
 func TestAddEnv(t *testing.T) {
 	// k exists in env as key
 	// override = true

frontend/dockerui/config.go

@@ -494,6 +494,15 @@ func (bc *Client) IsNoCache(name string) bool {
 	return false
 }
 
+func DefaultMainContext(opts ...llb.LocalOption) *llb.State {
+	opts = append([]llb.LocalOption{
+		llb.SharedKeyHint(DefaultLocalNameContext),
+		WithInternalName("load build context"),
+	}, opts...)
+	st := llb.Local(DefaultLocalNameContext, opts...)
+	return &st
+}
+
 func WithInternalName(name string) llb.ConstraintsOpt {
 	return llb.WithCustomName("[internal] " + name)
 }