Detect support for protobuf array ref on server on frontend return call

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

frontend/gateway/grpcclient/client.go

@@ -105,6 +105,7 @@ func (c *grpcClient) Run(ctx context.Context, f client.BuildFunc) (retError erro
 					Metadata: res.Metadata,
 				}
 				if res.Refs != nil {
+					if c.caps.Supports(pb.CapProtoRefArray) == nil {
 						m := map[string]*pb.Ref{}
 						for k, r := range res.Refs {
 							id, err := convertRef(r)
@@ -115,12 +116,28 @@ func (c *grpcClient) Run(ctx context.Context, f client.BuildFunc) (retError erro
 							m[k] = pb.NewRef(id)
 						}
 						pbRes.Result = &pb.Result_Refs{Refs: &pb.RefMap{Refs: m}}
+					} else {
+						m := map[string]string{}
+						for k, r := range res.Refs {
+							id, err := convertRef(r)
+							if err != nil {
+								retError = err
+								continue
+							}
+							m[k] = id
+						}
+						pbRes.Result = &pb.Result_RefsDeprecated{RefsDeprecated: &pb.RefMapDeprecated{Refs: m}}
+					}
 				} else {
 					id, err := convertRef(res.Ref)
 					if err != nil {
 						retError = err
 					} else {
+						if c.caps.Supports(pb.CapProtoRefArray) == nil {
 							pbRes.Result = &pb.Result_Ref{Ref: pb.NewRef(id)}
+						} else {
+							pbRes.Result = &pb.Result_RefDeprecated{RefDeprecated: id}
+						}
 					}
 				}
 				if retError == nil {

frontend/gateway/pb/caps.go

@@ -19,6 +19,11 @@ const (
 	CapReadDir                 apicaps.CapID = "readdir"
 	CapStatFile                apicaps.CapID = "statfile"
 	CapImportCaches            apicaps.CapID = "importcaches"
+
+	// CapProtoRefArray is a capability to return arrays of refs instead of single
+	// refs. This capability is only for the wire format change and shouldn't be
+	// used in frontends for feature detection.
+	CapProtoRefArray apicaps.CapID = "proto.refarray"
 )
 
 func init() {
@@ -92,4 +97,11 @@ func init() {
 		Enabled: true,
 		Status:  apicaps.CapStatusExperimental,
 	})
+
+	Caps.Init(apicaps.Cap{
+		ID:      CapProtoRefArray,
+		Name:    "wire format ref arrays",
+		Enabled: true,
+		Status:  apicaps.CapStatusExperimental,
+	})
 }

solver/pb/caps.go

@@ -288,5 +288,4 @@ func init() {
 		Enabled: true,
 		Status:  apicaps.CapStatusExperimental,
 	})
-
 }