reflect/protoregistry: restore conflicting file names check

There are inconsistencies between implementations on what happens when
a single program contains generated code for multiple files with the
same source path. At least one canonical implementation (C++) will fail
at link time. Others print warnings. Some silently resolve the registry
conflict in favor of one file or the other. The protobuf maintainers
agree, however, that the desired behavior is for this condition to be an
error.

Updates golang/protobuf#1122

Change-Id: I716708f16ef90210bdfceb0888691e47783df172
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/322729
Trust: Damien Neil <dneil@google.com>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

reflect/protoregistry/registry.go

@@ -121,8 +121,13 @@ func (r *Files) RegisterFile(file protoreflect.FileDescriptor) error {
 		r.filesByPath = make(map[string][]protoreflect.FileDescriptor)
 	}
 	path := file.Path()
-	if len(r.filesByPath[path]) > 0 {
+	if prev := r.filesByPath[path]; len(prev) > 0 {
 		r.checkGenProtoConflict(path)
+		err := errors.New("file %q is already registered", file.Path())
+		err = amendErrorWithCaller(err, prev[0], file)
+		if !(r == GlobalFiles && ignoreConflict(file, err)) {
+			return err
+		}
 	}
 
 	for name := file.Package(); name != ""; name = name.Parent() {

reflect/protoregistry/registry_test.go

@@ -69,7 +69,7 @@ func TestFiles(t *testing.T) {
 		files: []testFile{
 			{inFile: mustMakeFile(`syntax:"proto2" name:"test1.proto" package:"foo.bar"`)},
 			{inFile: mustMakeFile(`syntax:"proto2" name:"foo/bar/test.proto" package:"my.test"`)},
-			{inFile: mustMakeFile(`syntax:"proto2" name:"foo/bar/test.proto" package:"foo.bar.baz"`)},
+			{inFile: mustMakeFile(`syntax:"proto2" name:"foo/bar/test.proto" package:"foo.bar.baz"`), wantErr: "already registered"},
 			{inFile: mustMakeFile(`syntax:"proto2" name:"test2.proto" package:"my.test.package"`)},
 			{inFile: mustMakeFile(`syntax:"proto2" name:"weird" package:"foo.bar"`)},
 			{inFile: mustMakeFile(`syntax:"proto2" name:"foo/bar/baz/../test.proto" package:"my.test"`)},
@@ -112,8 +112,10 @@ func TestFiles(t *testing.T) {
 				{"weird", "foo.bar"},
 			},
 		}, {
-			inPath:  "foo/bar/test.proto",
-			wantErr: `multiple files named "foo/bar/test.proto"`,
+			inPath: "foo/bar/test.proto",
+			wantFiles: []file{
+				{"foo/bar/test.proto", "my.test"},
+			},
 		}},
 	}, {
 		// Test when new enum conflicts with existing package.