llb: ensure meta resolver uses platform form constraints

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

client/llb/resolver_test.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"testing"
 
+	"github.com/containerd/containerd/platforms"
 	"github.com/moby/buildkit/solver/pb"
 	digest "github.com/opencontainers/go-digest"
 	"github.com/pkg/errors"
@@ -21,10 +22,11 @@ func TestImageMetaResolver(t *testing.T) {
 
 	require.Equal(t, false, tr.called)
 
-	def, err := st.Marshal(context.TODO())
+	def, err := st.Marshal(context.TODO(), LinuxPpc64le)
 	require.NoError(t, err)
 
 	require.Equal(t, true, tr.called)
+	require.Equal(t, "linux/ppc64le", tr.platform)
 
 	m, arr := parseDef(t, def.Def)
 	require.Equal(t, 2, len(arr))
@@ -69,6 +71,7 @@ type testResolver struct {
 	digest   digest.Digest
 	dir      string
 	called   bool
+	platform string
 }
 
 func (r *testResolver) ResolveImageConfig(ctx context.Context, ref string, opt ResolveImageConfigOpt) (digest.Digest, []byte, error) {
@@ -83,6 +86,10 @@ func (r *testResolver) ResolveImageConfig(ctx context.Context, ref string, opt R
 
 	img.Config.WorkingDir = r.dir
 
+	if opt.Platform != nil {
+		r.platform = platforms.Format(*opt.Platform)
+	}
+
 	dt, err := json.Marshal(img)
 	if err != nil {
 		return "", nil, errors.WithStack(err)

client/llb/source.go

@@ -122,8 +122,12 @@ func Image(ref string, opts ...ImageOption) State {
 	} else if info.metaResolver != nil {
 		if _, ok := r.(reference.Digested); ok || !info.resolveDigest {
 			return NewState(src.Output()).Async(func(ctx context.Context, st State, c *Constraints) (State, error) {
+				p := info.Constraints.Platform
+				if p == nil {
+					p = c.Platform
+				}
 				_, dt, err := info.metaResolver.ResolveImageConfig(ctx, ref, ResolveImageConfigOpt{
-					Platform:    info.Constraints.Platform,
+					Platform:    p,
 					ResolveMode: info.resolveMode.String(),
 				})
 				if err != nil {
@@ -133,8 +137,12 @@ func Image(ref string, opts ...ImageOption) State {
 			})
 		}
 		return Scratch().Async(func(ctx context.Context, _ State, c *Constraints) (State, error) {
+			p := info.Constraints.Platform
+			if p == nil {
+				p = c.Platform
+			}
 			dgst, dt, err := info.metaResolver.ResolveImageConfig(context.TODO(), ref, ResolveImageConfigOpt{
-				Platform:    info.Constraints.Platform,
+				Platform:    p,
 				ResolveMode: info.resolveMode.String(),
 			})
 			if err != nil {