initialize empty diff in apply

While the schema Diff fucntion returns a nil diff when creating an empty
(except for id) resource, the Apply function expects the diff to be
initialized and ampty.

helper/plugin/grpc_provider.go

@@ -502,7 +502,14 @@ func (s *GRPCProviderServer) ApplyResourceChange(_ context.Context, req *proto.A
 		return resp, nil
 	}
 
+	if diff == nil {
+		diff = &terraform.InstanceDiff{
+			Attributes: make(map[string]*terraform.ResourceAttrDiff),
+			Meta:       make(map[string]interface{}),
+		}
+	}
 	diff.Meta = private
+
 	newInstanceState, err := s.provider.Apply(info, priorState, diff)
 	if err != nil {
 		resp.Diagnostics = convert.AppendProtoDiag(resp.Diagnostics, err)

helper/plugin/grpc_provider_test.go

@@ -357,3 +357,71 @@ func TestPlanResourceChange(t *testing.T) {
 		t.Fatal(cmp.Diff(proposedVal, plannedStateVal, valueComparer))
 	}
 }
+
+func TestApplyResourceChange(t *testing.T) {
+	r := &schema.Resource{
+		SchemaVersion: 4,
+		Schema: map[string]*schema.Schema{
+			"foo": {
+				Type:     schema.TypeInt,
+				Optional: true,
+			},
+		},
+		Create: func(rd *schema.ResourceData, _ interface{}) error {
+			rd.SetId("bar")
+			return nil
+		},
+	}
+
+	server := &GRPCProviderServer{
+		provider: &schema.Provider{
+			ResourcesMap: map[string]*schema.Resource{
+				"test": r,
+			},
+		},
+	}
+
+	schema := r.CoreConfigSchema()
+	priorState, err := msgpack.Marshal(cty.NullVal(schema.ImpliedType()), schema.ImpliedType())
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// A propsed state with only the ID unknown will produce a nil diff, and
+	// should return the propsed state value.
+	plannedVal, err := schema.CoerceValue(cty.ObjectVal(map[string]cty.Value{
+		"id": cty.UnknownVal(cty.String),
+	}))
+	if err != nil {
+		t.Fatal(err)
+	}
+	plannedState, err := msgpack.Marshal(plannedVal, schema.ImpliedType())
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	testReq := &proto.ApplyResourceChange_Request{
+		TypeName: "test",
+		PriorState: &proto.DynamicValue{
+			Msgpack: priorState,
+		},
+		PlannedState: &proto.DynamicValue{
+			Msgpack: plannedState,
+		},
+	}
+
+	resp, err := server.ApplyResourceChange(context.Background(), testReq)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	newStateVal, err := msgpack.Unmarshal(resp.NewState.Msgpack, schema.ImpliedType())
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	id := newStateVal.GetAttr("id").AsString()
+	if id != "bar" {
+		t.Fatalf("incorrect final state: %#v\n", newStateVal)
+	}
+}