protoc-gen-go: fix oneof name mangling regression (#782)

The generator currently uses an unintuitive and stateful algorithm
for name generation where it "fixes" name conflicts by appending "_"
to the end of the new name.

PR#657 refactored the generator code and noticed that the above
algorithm was not properly taking into account that a Get method is
generated for parent oneofs, fixing it in the same PR. While this is
more correct, this breaks users (see #780) since it means that the
generation of names can change.

This PR changes the name mangling logic to be as it was previously.
This does mean that some new proto files may be unbuildable,
but that is arguably better than breaking existing proto files.

Fixes #780

protoc-gen-go/generator/generator.go

@@ -2246,8 +2246,7 @@ func (g *Generator) generateMessage(message *Descriptor) {
 		if oneof && oFields[*field.OneofIndex] == nil {
 			odp := message.OneofDecl[int(*field.OneofIndex)]
 			base := CamelCase(odp.GetName())
-			names := allocNames(base, "Get"+base)
-			fname, gname := names[0], names[1]
+			fname := allocNames(base)[0]
 
 			// This is the first field of a oneof we haven't seen before.
 			// Generate the union field.
@@ -2265,7 +2264,7 @@ func (g *Generator) generateMessage(message *Descriptor) {
 			of := oneofField{
 				fieldCommon: fieldCommon{
 					goName:     fname,
-					getterName: gname,
+					getterName: "Get"+fname,
 					goType:     dname,
 					tags:       tag,
 					protoName:  odp.GetName(),

protoc-gen-go/testdata/issue780_oneof_conflict/test.pb.go

@@ -0,0 +1,103 @@
+// Code generated by protoc-gen-go. DO NOT EDIT.
+// source: issue780_oneof_conflict/test.proto
+
+package oneoftest
+
+import (
+	fmt "fmt"
+	proto "github.com/golang/protobuf/proto"
+	math "math"
+)
+
+// Reference imports to suppress errors if they are not otherwise used.
+var _ = proto.Marshal
+var _ = fmt.Errorf
+var _ = math.Inf
+
+// This is a compile-time assertion to ensure that this generated file
+// is compatible with the proto package it is being compiled against.
+// A compilation error at this line likely means your copy of the
+// proto package needs to be updated.
+const _ = proto.ProtoPackageIsVersion3 // please upgrade the proto package
+
+type Foo struct {
+	// Types that are valid to be assigned to Bar:
+	//	*Foo_GetBar
+	Bar                  isFoo_Bar `protobuf_oneof:"bar"`
+	XXX_NoUnkeyedLiteral struct{}  `json:"-"`
+	XXX_unrecognized     []byte    `json:"-"`
+	XXX_sizecache        int32     `json:"-"`
+}
+
+func (m *Foo) Reset()         { *m = Foo{} }
+func (m *Foo) String() string { return proto.CompactTextString(m) }
+func (*Foo) ProtoMessage()    {}
+func (*Foo) Descriptor() ([]byte, []int) {
+	return fileDescriptor_48462cafc802a68e, []int{0}
+}
+
+func (m *Foo) XXX_Unmarshal(b []byte) error {
+	return xxx_messageInfo_Foo.Unmarshal(m, b)
+}
+func (m *Foo) XXX_Marshal(b []byte, deterministic bool) ([]byte, error) {
+	return xxx_messageInfo_Foo.Marshal(b, m, deterministic)
+}
+func (m *Foo) XXX_Merge(src proto.Message) {
+	xxx_messageInfo_Foo.Merge(m, src)
+}
+func (m *Foo) XXX_Size() int {
+	return xxx_messageInfo_Foo.Size(m)
+}
+func (m *Foo) XXX_DiscardUnknown() {
+	xxx_messageInfo_Foo.DiscardUnknown(m)
+}
+
+var xxx_messageInfo_Foo proto.InternalMessageInfo
+
+type isFoo_Bar interface {
+	isFoo_Bar()
+}
+
+type Foo_GetBar struct {
+	GetBar string `protobuf:"bytes,1,opt,name=get_bar,json=getBar,oneof"`
+}
+
+func (*Foo_GetBar) isFoo_Bar() {}
+
+func (m *Foo) GetBar() isFoo_Bar {
+	if m != nil {
+		return m.Bar
+	}
+	return nil
+}
+
+func (m *Foo) GetGetBar() string {
+	if x, ok := m.GetBar().(*Foo_GetBar); ok {
+		return x.GetBar
+	}
+	return ""
+}
+
+// XXX_OneofWrappers is for the internal use of the proto package.
+func (*Foo) XXX_OneofWrappers() []interface{} {
+	return []interface{}{
+		(*Foo_GetBar)(nil),
+	}
+}
+
+func init() {
+	proto.RegisterType((*Foo)(nil), "oneoftest.Foo")
+}
+
+func init() { proto.RegisterFile("issue780_oneof_conflict/test.proto", fileDescriptor_48462cafc802a68e) }
+
+var fileDescriptor_48462cafc802a68e = []byte{
+	// 107 bytes of a gzipped FileDescriptorProto
+	0x1f, 0x8b, 0x08, 0x00, 0x00, 0x00, 0x00, 0x00, 0x02, 0xff, 0xe2, 0x52, 0xca, 0x2c, 0x2e, 0x2e,
+	0x4d, 0x35, 0xb7, 0x30, 0x88, 0xcf, 0xcf, 0x4b, 0xcd, 0x4f, 0x8b, 0x4f, 0xce, 0xcf, 0x4b, 0xcb,
+	0xc9, 0x4c, 0x2e, 0xd1, 0x2f, 0x49, 0x2d, 0x2e, 0xd1, 0x2b, 0x28, 0xca, 0x2f, 0xc9, 0x17, 0xe2,
+	0x04, 0x4b, 0x81, 0x04, 0x94, 0xd4, 0xb9, 0x98, 0xdd, 0xf2, 0xf3, 0x85, 0x24, 0xb9, 0xd8, 0xd3,
+	0x53, 0x4b, 0xe2, 0x93, 0x12, 0x8b, 0x24, 0x18, 0x15, 0x18, 0x35, 0x38, 0x3d, 0x18, 0x82, 0xd8,
+	0xd2, 0x53, 0x4b, 0x9c, 0x12, 0x8b, 0x9c, 0x58, 0xb9, 0x98, 0x93, 0x12, 0x8b, 0x00, 0x01, 0x00,
+	0x00, 0xff, 0xff, 0x12, 0x66, 0x0c, 0x02, 0x58, 0x00, 0x00, 0x00,
+}

protoc-gen-go/testdata/issue780_oneof_conflict/test.proto

@@ -0,0 +1,9 @@
+syntax = "proto2";
+
+package oneoftest;
+
+message Foo {
+	oneof bar {
+		string get_bar = 1;
+	}
+}