protobuf: fix delimited fields under editions in go

This brings go into conformance with other implementations.  Group-like message fields with delimited encoding will continue to use the type name for text-format, but everything else will use the field name.

Change-Id: Ib6d07f19ccfa853ce0370392c89fd24fb7148793
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/575896
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Stapelberg <stapelberg@google.com>
Commit-Queue: Michael Stapelberg <stapelberg@google.com>

encoding/prototext/decode_test.go

@@ -622,10 +622,12 @@ OptGroup: {}
 		inputText: `
 opt_nested: {}
 OptGroup: {}
+delimited_field: {}
 `,
 		wantMessage: &pbeditions.Nests{
-			OptNested: &pbeditions.Nested{},
-			Optgroup:  &pbeditions.Nests_OptGroup{},
+			OptNested:      &pbeditions.Nested{},
+			Optgroup:       &pbeditions.Nests_OptGroup{},
+			DelimitedField: &pbeditions.Nests_OptGroup{},
 		},
 	}, {
 		desc:         "message fields with no field separator",
@@ -644,21 +646,28 @@ OptGroup {}
 		inputText: `
 opt_nested {}
 OptGroup {}
+delimited_field {}
 `,
 		wantMessage: &pbeditions.Nests{
-			OptNested: &pbeditions.Nested{},
-			Optgroup:  &pbeditions.Nests_OptGroup{},
+			OptNested:      &pbeditions.Nested{},
+			Optgroup:       &pbeditions.Nests_OptGroup{},
+			DelimitedField: &pbeditions.Nests_OptGroup{},
 		},
 	}, {
 		desc:         "group field name",
 		inputMessage: &pb2.Nests{},
 		inputText:    `optgroup: {}`,
 		wantErr:      "unknown field: optgroup",
 	}, {
-		desc:         "delimited encoded message field name",
+		desc:         "delimited encoded group-line message field name",
 		inputMessage: &pbeditions.Nests{},
 		inputText:    `optgroup: {}`,
 		wantErr:      "unknown field: optgroup",
+	}, {
+		desc:         "delimited encoded message field name",
+		inputMessage: &pbeditions.Nests{},
+		inputText:    `Delimited_Field: {}`,
+		wantErr:      "unknown field: Delimited_Field",
 	}, {
 		desc:         "proto2 nested messages",
 		inputMessage: &pb2.Nests{},

encoding/prototext/encode_test.go

@@ -527,7 +527,7 @@ OptGroup: {}
 }
 `,
 	}, {
-		desc: "protoeditions delimited encoded meesage field",
+		desc: "protoeditions group-like delimited encoded message field",
 		input: &pbeditions.Nests{
 			Optgroup: &pbeditions.Nests_OptGroup{
 				OptString: proto.String("inside a group"),
@@ -548,6 +548,29 @@ OptGroup: {}
     opt_fixed32: 47
   }
 }
+`,
+	}, {
+		desc: "protoeditions delimited encoded message field",
+		input: &pbeditions.Nests{
+			DelimitedField: &pbeditions.Nests_OptGroup{
+				OptString: proto.String("second group"),
+				OptNested: &pbeditions.Nested{
+					OptString: proto.String("second nested message inside a group"),
+				},
+				NestedDelimitedField: &pbeditions.Nests_OptGroup_OptNestedGroup{
+					OptFixed32: proto.Uint32(47),
+				},
+			},
+		},
+		want: `delimited_field: {
+  opt_string: "second group"
+  opt_nested: {
+    opt_string: "second nested message inside a group"
+  }
+  nested_delimited_field: {
+    opt_fixed32: 47
+  }
+}
 `,
 	}, {
 		desc:  "proto3 nested message not set",

internal/filedesc/desc.go

@@ -7,6 +7,7 @@ package filedesc
 import (
 	"bytes"
 	"fmt"
+	"strings"
 	"sync"
 	"sync/atomic"
 
@@ -580,6 +581,34 @@ func (s *stringName) InitJSON(name string) {
 	s.nameJSON = name
 }
 
+// Returns true if this field is structured like the synthetic field of a proto2
+// group. This allows us to expand our treatment of delimited fields without
+// breaking proto2 files that have been upgraded to editions.
+func isGroupLike(fd protoreflect.FieldDescriptor) bool {
+	// Groups are always group types.
+	if fd.Kind() != protoreflect.GroupKind {
+		return false
+	}
+
+	// Group fields are always the lowercase type name.
+	if strings.ToLower(string(fd.Message().Name())) != string(fd.Name()) {
+		return false
+	}
+
+	// Groups could only be defined in the same file they're used.
+	if fd.Message().ParentFile() != fd.ParentFile() {
+		return false
+	}
+
+	// Group messages are always defined in the same scope as the field.  File
+	// level extensions will compare NULL == NULL here, which is why the file
+	// comparison above is necessary to ensure both come from the same file.
+	if fd.IsExtension() {
+		return fd.Parent() == fd.Message().Parent()
+	}
+	return fd.ContainingMessage() == fd.Message().Parent()
+}
+
 func (s *stringName) lazyInit(fd protoreflect.FieldDescriptor) *stringName {
 	s.once.Do(func() {
 		if fd.IsExtension() {
@@ -600,7 +629,7 @@ func (s *stringName) lazyInit(fd protoreflect.FieldDescriptor) *stringName {
 
 			// Format the text name.
 			s.nameText = string(fd.Name())
-			if fd.Kind() == protoreflect.GroupKind {
+			if isGroupLike(fd) {
 				s.nameText = string(fd.Message().Name())
 			}
 		}

internal/testprotos/testeditions/test.proto

@@ -58,6 +58,8 @@ message TestAllTypes {
     int32 same_field_number = 16;
   }
   OptionalGroup optionalgroup = 16 [features.message_encoding = DELIMITED];
+  OptionalGroup not_group_like_delimited = 17
+      [features.message_encoding = DELIMITED];
   NestedMessage optional_nested_message = 18;
   ForeignMessage optional_foreign_message = 19;
   NestedEnum optional_nested_enum = 21;