reflect/protodesc: check file imports by name

Weak imports are added to Imports as placeholders even if they can be
found in the Files registry, so we have to look at the name rather than
the actual FileDescriptor.

Change-Id: I28f62e945f233119014e5e8cb1bcbde7dca831a7
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/175897
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

reflect/protodesc/protodesc.go

@@ -112,13 +112,13 @@ func NewFile(fd *descriptorpb.FileDescriptorProto, r *protoregistry.Files) (prot
 	return prototype.NewFile(&f)
 }
 
-type importSet map[protoreflect.FileDescriptor]bool
+type importSet map[string]bool
 
 func importedFiles(imps []protoreflect.FileImport) importSet {
 	ret := make(importSet)
 	for _, imp := range imps {
-		ret[imp.FileDescriptor] = true
-		addPublicImports(imp.FileDescriptor, ret)
+		ret[imp.Path()] = true
+		addPublicImports(imp, ret)
 	}
 	return ret
 }
@@ -128,8 +128,8 @@ func addPublicImports(fd protoreflect.FileDescriptor, out importSet) {
 	for i := 0; i < imps.Len(); i++ {
 		imp := imps.Get(i)
 		if imp.IsPublic {
-			out[imp.FileDescriptor] = true
-			addPublicImports(imp.FileDescriptor, out)
+			out[imp.Path()] = true
+			addPublicImports(imp, out)
 		}
 	}
 }
@@ -381,7 +381,7 @@ func validateFileInImports(d protoreflect.Descriptor, imps importSet) error {
 	if fd == nil {
 		return errors.New("%v has no parent FileDescriptor", d.FullName())
 	}
-	if !imps[fd] {
+	if !imps[fd.Path()] {
 		return errors.New("reference to type %v without import of %v", d.FullName(), fd.Path())
 	}
 	return nil

reflect/protodesc/protodesc_test.go

@@ -553,6 +553,41 @@ func TestNewFile_ValidationOK(t *testing.T) {
 				}},
 			}},
 		},
+	}, {
+		name: "external type from weak import",
+		deps: []*descriptorpb.FileDescriptorProto{{
+			Name:    scalar.String("weak.proto"),
+			Syntax:  scalar.String("proto2"),
+			Package: scalar.String("foo"),
+			MessageType: []*descriptorpb.DescriptorProto{{
+				Name: scalar.String("WeakMessage"),
+			}},
+		}},
+		fd: &descriptorpb.FileDescriptorProto{
+			Name:           scalar.String("external-type-from-weak-import.proto"),
+			Syntax:         scalar.String("proto2"),
+			Package:        scalar.String("bar"),
+			Dependency:     []string{"weak.proto"},
+			WeakDependency: []int32{0},
+			MessageType: []*descriptorpb.DescriptorProto{{
+				Name: scalar.String("Bar"),
+				Field: []*descriptorpb.FieldDescriptorProto{{
+					Name:   scalar.String("id"),
+					Number: scalar.Int32(1),
+					Label:  descriptorpb.FieldDescriptorProto_LABEL_OPTIONAL.Enum(),
+					Type:   descriptorpb.FieldDescriptorProto_TYPE_STRING.Enum(),
+				}, {
+					Name:     scalar.String("weak_message"),
+					Number:   scalar.Int32(2),
+					Label:    descriptorpb.FieldDescriptorProto_LABEL_OPTIONAL.Enum(),
+					Type:     descriptorpb.FieldDescriptorProto_TYPE_MESSAGE.Enum(),
+					TypeName: scalar.String(".foo.WeakMessage"),
+					Options: &descriptorpb.FieldOptions{
+						Weak: scalar.Bool(true),
+					},
+				}},
+			}},
+		},
 	}}
 
 	for _, tc := range testCases {