Verify platform is stored in state directly in defop test

Signed-off-by: Edgar Lee <edgarl@netflix.com>

client/llb/definition_test.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"testing"
 
+	"github.com/containerd/containerd/platforms"
 	"github.com/stretchr/testify/require"
 )
 
@@ -49,6 +50,16 @@ func TestDefinitionEquivalence(t *testing.T) {
 			for dgst := range def.Metadata {
 				require.Equal(t, def.Metadata[dgst], def2.Metadata[dgst])
 			}
+
+			expectedPlatform := tc.state.GetPlatform()
+			actualPlatform := st2.GetPlatform()
+
+			if expectedPlatform == nil && actualPlatform != nil {
+				defaultPlatform := platforms.Normalize(platforms.DefaultSpec())
+				expectedPlatform = &defaultPlatform
+			}
+
+			require.Equal(t, expectedPlatform, actualPlatform)
 		})
 	}
 }

frontend/gateway/grpcclient/client.go

@@ -118,13 +118,20 @@ func (c *grpcClient) Run(ctx context.Context, f client.BuildFunc) (retError erro
 						}
 						pbRes.Result = &pb.Result_Refs{Refs: &pb.RefMap{Refs: m}}
 					} else {
+						// Server doesn't support the new wire format for refs, so we construct
+						// a deprecated result ref map.
 						m := map[string]string{}
 						for k, r := range res.Refs {
-							id, err := convertRef(r)
+							pbRef, err := convertRef(r)
 							if err != nil {
 								retError = err
 								continue
 							}
+
+							var id string
+							if len(pbRef.Ids) > 0 {
+								id = pbRef.Ids[0]
+							}
 							m[k] = id
 						}
 						pbRes.Result = &pb.Result_RefsDeprecated{RefsDeprecated: &pb.RefMapDeprecated{Refs: m}}
@@ -137,6 +144,8 @@ func (c *grpcClient) Run(ctx context.Context, f client.BuildFunc) (retError erro
 						if c.caps.Supports(pb.CapProtoRefArray) == nil {
 							pbRes.Result = &pb.Result_Ref{Ref: pbRef}
 						} else {
+							// Server doesn't support the new wire format for refs, so we construct
+							// a deprecated result ref.
 							var id string
 							if len(pbRef.Ids) > 0 {
 								id = pbRef.Ids[0]