reflect/protodesc: add NewFiles

Add a function that takes a protoreflect.FileDescriptorSet and returns
a protoregistry.Files.

Updates golang/protobuf##1065.

Change-Id: I2715d042053ef7d3f1bfcee1866c20cac423e327
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/226237
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

reflect/protodesc/desc.go

@@ -67,6 +67,12 @@ func NewFile(fd *descriptorpb.FileDescriptorProto, r Resolver) (protoreflect.Fil
 	return FileOptions{}.New(fd, r)
 }
 
+// NewFiles creates a new protoregistry.Files from the provided
+// FileDescriptorSet message. See FileOptions.NewFiles for more information.
+func NewFiles(fd *descriptorpb.FileDescriptorSet) (*protoregistry.Files, error) {
+	return FileOptions{}.NewFiles(fd)
+}
+
 // New creates a new protoreflect.FileDescriptor from the provided
 // file descriptor message. The file must represent a valid proto file according
 // to protobuf semantics. The returned descriptor is a deep copy of the input.
@@ -223,3 +229,47 @@ func (is importSet) importPublic(imps protoreflect.FileImports) {
 		}
 	}
 }
+
+// NewFiles creates a new protoregistry.Files from the provided
+// FileDescriptorSet message. The descriptor set must include only
+// valid files according to protobuf semantics. The returned descriptors
+// are a deep copy of the input.
+func (o FileOptions) NewFiles(fds *descriptorpb.FileDescriptorSet) (*protoregistry.Files, error) {
+	files := make(map[string]*descriptorpb.FileDescriptorProto)
+	for _, fd := range fds.File {
+		if _, ok := files[fd.GetName()]; ok {
+			return nil, errors.New("file appears multiple times: %q", fd.GetName())
+		}
+		files[fd.GetName()] = fd
+	}
+	r := &protoregistry.Files{}
+	for _, fd := range files {
+		if err := o.addFileDeps(r, fd, files); err != nil {
+			return nil, err
+		}
+	}
+	return r, nil
+}
+func (o FileOptions) addFileDeps(r *protoregistry.Files, fd *descriptorpb.FileDescriptorProto, files map[string]*descriptorpb.FileDescriptorProto) error {
+	// Set the entry to nil while descending into a file's dependencies to detect cycles.
+	files[fd.GetName()] = nil
+	for _, dep := range fd.Dependency {
+		depfd, ok := files[dep]
+		if depfd == nil {
+			if ok {
+				return errors.New("import cycle in file: %q", dep)
+			}
+			continue
+		}
+		if err := o.addFileDeps(r, depfd, files); err != nil {
+			return err
+		}
+	}
+	// Delete the entry once dependencies are processed.
+	delete(files, fd.GetName())
+	f, err := o.New(fd, r)
+	if err != nil {
+		return err
+	}
+	return r.RegisterFile(f)
+}

reflect/protodesc/file_test.go

@@ -12,6 +12,7 @@ import (
 	"google.golang.org/protobuf/encoding/prototext"
 	"google.golang.org/protobuf/internal/flags"
 	"google.golang.org/protobuf/proto"
+	"google.golang.org/protobuf/reflect/protoreflect"
 	"google.golang.org/protobuf/reflect/protoregistry"
 
 	"google.golang.org/protobuf/types/descriptorpb"
@@ -935,3 +936,61 @@ func TestNewFile(t *testing.T) {
 		})
 	}
 }
+
+func TestNewFiles(t *testing.T) {
+	fdset := &descriptorpb.FileDescriptorSet{
+		File: []*descriptorpb.FileDescriptorProto{
+			mustParseFile(`
+				name: "test.proto"
+				package: "fizz"
+				dependency: "dep.proto"
+				message_type: [{
+					name: "M2"
+					field: [{name:"F" number:1 label:LABEL_OPTIONAL type:TYPE_MESSAGE type_name:"M1"}]
+				}]
+			`),
+			// Inputs deliberately out of order.
+			mustParseFile(`
+				name: "dep.proto"
+				package: "fizz"
+				message_type: [{name:"M1"}]
+			`),
+		},
+	}
+	f, err := NewFiles(fdset)
+	if err != nil {
+		t.Fatal(err)
+	}
+	m1, err := f.FindDescriptorByName("fizz.M1")
+	if err != nil {
+		t.Fatalf(`f.FindDescriptorByName("fizz.M1") = %v`, err)
+	}
+	m2, err := f.FindDescriptorByName("fizz.M2")
+	if err != nil {
+		t.Fatalf(`f.FindDescriptorByName("fizz.M2") = %v`, err)
+	}
+	if m2.(protoreflect.MessageDescriptor).Fields().ByName("F").Message() != m1 {
+		t.Fatalf(`m1.Fields().ByName("F").Message() != m2`)
+	}
+}
+
+func TestNewFilesImportCycle(t *testing.T) {
+	fdset := &descriptorpb.FileDescriptorSet{
+		File: []*descriptorpb.FileDescriptorProto{
+			mustParseFile(`
+				name: "test.proto"
+				package: "fizz"
+				dependency: "dep.proto"
+			`),
+			mustParseFile(`
+				name: "dep.proto"
+				package: "fizz"
+				dependency: "test.proto"
+			`),
+		},
+	}
+	_, err := NewFiles(fdset)
+	if err == nil {
+		t.Fatal("NewFiles with import cycle: success, want error")
+	}
+}