cmd/protoc-gen-go: unexport implementation-specific XXX fields

We modify protoc-gen-go to stop generating exported XXX fields.
The unsafe implementation is unaffected by this change since unsafe
can access fields regardless of visibility. However, for the purego
implementation, we need to respect Go visibility rules as enforced
by the reflect package.

We work around this by generating a exporter function that given
a reference to the message and the field to export, returns a reference
to the unexported field value. This exporter function is protected by
a constant such that it is not linked into the final binary in non-purego
build environment.

Updates #276

Change-Id: Idf5c1f158973fa1c61187ff41440acb21c5dac94
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/185141
Reviewed-by: Damien Neil <[email protected]>

cmd/protoc-gen-go/internal_gengo/main.go

@@ -39,6 +39,22 @@ const (
 	// generateOneofWrapperMethods specifies whether to generate
 	// XXX_OneofWrappers methods on messages with oneofs.
 	generateOneofWrapperMethods = false
+
+	// generateNoUnkeyedLiteralFields specifies whether to generate
+	// the XXX_NoUnkeyedLiteral field.
+	generateNoUnkeyedLiteralFields = false
+
+	// generateExportedSizeCacheFields specifies whether to generate an exported
+	// XXX_sizecache field instead of an unexported sizeCache field.
+	generateExportedSizeCacheFields = false
+
+	// generateExportedUnknownFields specifies whether to generate an exported
+	// XXX_unrecognized field instead of an unexported unknownFields field.
+	generateExportedUnknownFields = false
+
+	// generateExportedExtensionFields specifies whether to generate an exported
+	// XXX_InternalExtensions field instead of an unexported extensionFields field.
+	generateExportedExtensionFields = false
 )
 
 const (
@@ -54,11 +70,28 @@ const (
 type fileInfo struct {
 	*protogen.File
 
-	allEnums         []*protogen.Enum
-	allEnumsByPtr    map[*protogen.Enum]int // value is index into allEnums
-	allMessages      []*protogen.Message
-	allMessagesByPtr map[*protogen.Message]int // value is index into allMessages
-	allExtensions    []*protogen.Extension
+	allEnums      []*protogen.Enum
+	allMessages   []*protogen.Message
+	allExtensions []*protogen.Extension
+
+	allEnumsByPtr         map[*protogen.Enum]int    // value is index into allEnums
+	allMessagesByPtr      map[*protogen.Message]int // value is index into allMessages
+	allMessageFieldsByPtr map[*protogen.Message]*structFields
+}
+
+type structFields struct {
+	count      int
+	unexported map[int]string
+}
+
+func (sf *structFields) append(name string) {
+	if r, _ := utf8.DecodeRuneInString(name); !unicode.IsUpper(r) {
+		if sf.unexported == nil {
+			sf.unexported = make(map[int]string)
+		}
+		sf.unexported[sf.count] = name
+	}
+	sf.count++
 }
 
 // GenerateFile generates the contents of a .pb.go file.
@@ -90,8 +123,10 @@ func GenerateFile(gen *protogen.Plugin, file *protogen.File) *protogen.Generated
 	}
 	if len(f.allMessages) > 0 {
 		f.allMessagesByPtr = make(map[*protogen.Message]int)
+		f.allMessageFieldsByPtr = make(map[*protogen.Message]*structFields)
 		for i, m := range f.allMessages {
 			f.allMessagesByPtr[m] = i
+			f.allMessageFieldsByPtr[m] = new(structFields)
 		}
 	}
 
@@ -347,15 +382,28 @@ func genMessage(gen *protogen.Plugin, g *protogen.GeneratedFile, f *fileInfo, me
 	}
 	g.Annotate(message.GoIdent.GoName, message.Location)
 	g.P("type ", message.GoIdent, " struct {")
+	sf := f.allMessageFieldsByPtr[message]
 	for _, field := range message.Fields {
 		if field.Oneof != nil {
 			// It would be a bit simpler to iterate over the oneofs below,
 			// but generating the field here keeps the contents of the Go
 			// struct in the same order as the contents of the source
 			// .proto file.
-			if field == field.Oneof.Fields[0] {
-				genOneofField(gen, g, f, message, field.Oneof)
+			oneof := field.Oneof
+			if field != oneof.Fields[0] {
+				continue // already generated oneof field for first entry
+			}
+			if g.PrintLeadingComments(oneof.Location) {
+				g.P("//")
+			}
+			g.P("// Types that are valid to be assigned to ", oneofFieldName(oneof), ":")
+			for _, field := range oneof.Fields {
+				g.PrintLeadingComments(field.Location)
+				g.P("//\t*", fieldOneofType(field))
 			}
+			g.Annotate(message.GoIdent.GoName+"."+oneofFieldName(oneof), oneof.Location)
+			g.P(oneofFieldName(oneof), " ", oneofInterfaceName(oneof), " `protobuf_oneof:\"", oneof.Desc.Name(), "\"`")
+			sf.append(oneofFieldName(oneof))
 			continue
 		}
 		g.PrintLeadingComments(field.Location)
@@ -378,19 +426,42 @@ func genMessage(gen *protogen.Plugin, g *protogen.GeneratedFile, f *fileInfo, me
 		g.Annotate(message.GoIdent.GoName+"."+field.GoName, field.Location)
 		g.P(field.GoName, " ", goType, " `", strings.Join(tags, " "), "`",
 			deprecationComment(field.Desc.Options().(*descriptorpb.FieldOptions).GetDeprecated()))
+		sf.append(field.GoName)
 	}
-	g.P("XXX_NoUnkeyedLiteral struct{} `json:\"-\"`")
 
+	if generateNoUnkeyedLiteralFields {
+		g.P("XXX_NoUnkeyedLiteral", " struct{} `json:\"-\"`")
+		sf.append("XXX_NoUnkeyedLiteral")
+	}
+	if generateExportedSizeCacheFields {
+		g.P("XXX_sizecache", " ", protoimplPackage.Ident("SizeCache"), " `json:\"-\"`")
+		sf.append("XXX_sizecache")
+	} else {
+		g.P("sizeCache", " ", protoimplPackage.Ident("SizeCache"))
+		sf.append("sizeCache")
+	}
+	if generateExportedUnknownFields {
+		g.P("XXX_unrecognized", " ", protoimplPackage.Ident("UnknownFields"), " `json:\"-\"`")
+		sf.append("XXX_unrecognized")
+	} else {
+		g.P("unknownFields", " ", protoimplPackage.Ident("UnknownFields"))
+		sf.append("unknownFields")
+	}
 	if message.Desc.ExtensionRanges().Len() > 0 {
+		// TODO: Remove this tag when we drop v1 support.
 		var tags []string
 		if message.Desc.Options().(*descriptorpb.MessageOptions).GetMessageSetWireFormat() {
 			tags = append(tags, `protobuf_messageset:"1"`)
 		}
-		tags = append(tags, `json:"-"`)
-		g.P("XXX_InternalExtensions ", protoimplPackage.Ident("ExtensionFields"), " `", strings.Join(tags, " "), "`")
+		if generateExportedExtensionFields {
+			tags = append(tags, `json:"-"`)
+			g.P("XXX_InternalExtensions", " ", protoimplPackage.Ident("ExtensionFields"), " `", strings.Join(tags, " "), "`")
+			sf.append("XXX_InternalExtensions")
+		} else {
+			g.P("extensionFields", " ", protoimplPackage.Ident("ExtensionFields"), " `", strings.Join(tags, " "), "`")
+			sf.append("extensionFields")
+		}
 	}
-	g.P("XXX_unrecognized ", protoimplPackage.Ident("UnknownFields"), " `json:\"-\"`")
-	g.P("XXX_sizecache ", protoimplPackage.Ident("SizeCache"), " `json:\"-\"`")
 	g.P("}")
 	g.P()
 
@@ -744,20 +815,6 @@ var wellKnownTypes = map[protoreflect.FullName]bool{
 	"google.protobuf.Value":       true,
 }
 
-// genOneofField generates the struct field for a oneof.
-func genOneofField(gen *protogen.Plugin, g *protogen.GeneratedFile, f *fileInfo, message *protogen.Message, oneof *protogen.Oneof) {
-	if g.PrintLeadingComments(oneof.Location) {
-		g.P("//")
-	}
-	g.P("// Types that are valid to be assigned to ", oneofFieldName(oneof), ":")
-	for _, field := range oneof.Fields {
-		g.PrintLeadingComments(field.Location)
-		g.P("//\t*", fieldOneofType(field))
-	}
-	g.Annotate(message.GoIdent.GoName+"."+oneofFieldName(oneof), oneof.Location)
-	g.P(oneofFieldName(oneof), " ", oneofInterfaceName(oneof), " `protobuf_oneof:\"", oneof.Desc.Name(), "\"`")
-}
-
 // genOneofGetter generate a Get method for a oneof.
 func genOneofGetter(gen *protogen.Plugin, g *protogen.GeneratedFile, f *fileInfo, message *protogen.Message, oneof *protogen.Oneof) {
 	g.Annotate(message.GoIdent.GoName+".Get"+oneof.GoName, oneof.Location)

cmd/protoc-gen-go/internal_gengo/reflect.go

@@ -154,6 +154,27 @@ func genReflectFileDescriptor(gen *protogen.Plugin, g *protogen.GeneratedFile, f
 	}
 
 	if len(f.allMessages) > 0 {
+		// Populate MessageInfo.Exporters.
+		g.P("if !", protoimplPackage.Ident("UnsafeEnabled"), " {")
+		for _, message := range f.allMessages {
+			if sf := f.allMessageFieldsByPtr[message]; len(sf.unexported) > 0 {
+				idx := f.allMessagesByPtr[message]
+				typesVar := messageTypesVarName(f)
+
+				g.P(typesVar, "[", idx, "].Exporter = func(v interface{}, i int) interface{} {")
+				g.P("switch v := v.(*", message.GoIdent, "); i {")
+				for i := 0; i < sf.count; i++ {
+					if name := sf.unexported[i]; name != "" {
+						g.P("case ", i, ": return &v.", name)
+					}
+				}
+				g.P("default: return nil")
+				g.P("}")
+				g.P("}")
+			}
+		}
+		g.P("}")
+
 		// Populate MessageInfo.OneofWrappers.
 		for _, message := range f.allMessages {
 			if len(message.Oneofs) > 0 {

cmd/protoc-gen-go/testdata/annotations/annotations.pb.go.meta

@@ -1 +1 @@
-annotation:{path:5 path:0 source_file:"annotations/annotations.proto" begin:631 end:650} annotation:{path:5 path:0 path:2 path:0 source_file:"annotations/annotations.proto" begin:667 end:714} annotation:{path:4 path:0 source_file:"annotations/annotations.proto" begin:1821 end:1843} annotation:{path:4 path:0 path:2 path:0 source_file:"annotations/annotations.proto" begin:1854 end:1874} annotation:{path:4 path:0 path:2 path:0 source_file:"annotations/annotations.proto" begin:2895 end:2918}
+annotation:{path:5 path:0 source_file:"annotations/annotations.proto" begin:631 end:650} annotation:{path:5 path:0 path:2 path:0 source_file:"annotations/annotations.proto" begin:667 end:714} annotation:{path:4 path:0 source_file:"annotations/annotations.proto" begin:1821 end:1843} annotation:{path:4 path:0 path:2 path:0 source_file:"annotations/annotations.proto" begin:1854 end:1874} annotation:{path:4 path:0 path:2 path:0 source_file:"annotations/annotations.proto" begin:2796 end:2819}