Add mutate hook to `Write` endpoint (#16958)

agent/grpc-external/services/resource/write.go

@@ -73,6 +73,10 @@ func (s *Server) Write(ctx context.Context, req *pbresource.WriteRequest) (*pbre
 		return nil, status.Error(codes.InvalidArgument, err.Error())
 	}
 
+	if err = reg.Mutate(req.Resource); err != nil {
+		return nil, status.Errorf(codes.Internal, "failed mutate hook: %v", err.Error())
+	}
+
 	// At the storage backend layer, all writes are CAS operations.
 	//
 	// This makes it possible to *safely* do things like keeping the Uid stable

agent/grpc-external/services/resource/write_test.go

@@ -114,6 +114,31 @@ func TestWrite_ACLs(t *testing.T) {
 	}
 }
 
+func TestWrite_Mutate(t *testing.T) {
+	server := testServer(t)
+	client := testClient(t, server)
+	demo.Register(server.Registry)
+
+	artist, err := demo.GenerateV2Artist()
+	require.NoError(t, err)
+
+	artistData := &pbdemov2.Artist{}
+	artist.Data.UnmarshalTo(artistData)
+	require.NoError(t, err)
+
+	// mutate hook sets genre to disco when unspecified
+	artistData.Genre = pbdemov2.Genre_GENRE_UNSPECIFIED
+	artist.Data.MarshalFrom(artistData)
+	require.NoError(t, err)
+
+	rsp, err := client.Write(testContext(t), &pbresource.WriteRequest{Resource: artist})
+	require.NoError(t, err)
+
+	// verify mutate hook set genre to disco
+	require.NoError(t, rsp.Resource.Data.UnmarshalTo(artistData))
+	require.Equal(t, pbdemov2.Genre_GENRE_DISCO, artistData.Genre)
+}
+
 func TestWrite_ResourceCreation_Success(t *testing.T) {
 	server := testServer(t)
 	client := testClient(t, server)

internal/resource/demo/demo.go

@@ -108,6 +108,19 @@ func Register(r resource.Registry) {
 		return nil
 	}
 
+	mutateV2ArtistFn := func(res *pbresource.Resource) error {
+		// Not a realistic use for this hook, but set genre if not specified
+		artist := &pbdemov2.Artist{}
+		if err := anypb.UnmarshalTo(res.Data, artist, proto.UnmarshalOptions{}); err != nil {
+			return err
+		}
+		if artist.Genre == pbdemov2.Genre_GENRE_UNSPECIFIED {
+			artist.Genre = pbdemov2.Genre_GENRE_DISCO
+			return res.Data.MarshalFrom(artist)
+		}
+		return nil
+	}
+
 	r.Register(resource.Registration{
 		Type:  TypeV1Artist,
 		Proto: &pbdemov1.Artist{},
@@ -138,6 +151,7 @@ func Register(r resource.Registry) {
 			List:  makeListACL(TypeV2Artist),
 		},
 		Validate: validateV2ArtistFn,
+		Mutate:   mutateV2ArtistFn,
 	})
 
 	r.Register(resource.Registration{

internal/resource/registry.go

@@ -35,6 +35,9 @@ type Registration struct {
 	// check for required fields).
 	Validate func(*pbresource.Resource) error
 
+	// Mutate is called to fill out any autogenerated fields (e.g. UUIDs).
+	Mutate func(*pbresource.Resource) error
+
 	// In the future, we'll add hooks, the controller etc. here.
 	// TODO: https://github.com/hashicorp/consul/pull/16622#discussion_r1134515909
 }
@@ -109,6 +112,11 @@ func (r *TypeRegistry) Register(registration Registration) {
 		registration.Validate = func(resource *pbresource.Resource) error { return nil }
 	}
 
+	// default mutate to a no-op
+	if registration.Mutate == nil {
+		registration.Mutate = func(resource *pbresource.Resource) error { return nil }
+	}
+
 	r.registrations[key] = registration
 }
 

internal/resource/registry_test.go

@@ -61,10 +61,7 @@ func TestRegister(t *testing.T) {
 
 func TestRegister_Defaults(t *testing.T) {
 	r := resource.NewRegistry()
-	r.Register(resource.Registration{
+	r.Register(resource.Registration{Type: demo.TypeV2Artist})
-		Type: demo.TypeV2Artist,
-		// intentionally don't provide ACLs so defaults kick in
-	})
 	artist, err := demo.GenerateV2Artist()
 	require.NoError(t, err)
 
@@ -85,6 +82,9 @@ func TestRegister_Defaults(t *testing.T) {
 
 	// verify default validate is a no-op
 	require.NoError(t, reg.Validate(nil))
+
+	// verify default mutate is a no-op
+	require.NoError(t, reg.Mutate(nil))
 }
 
 func assertRegisterPanics(t *testing.T, registerFn func(reg resource.Registration), registration resource.Registration, panicString string) {