raftpb: atomic access alignment

The Entry struct has misaligned fields that are accessed atomically.  The
misalignment is caused by the EntryType enum which the Protocol Buffers
spec forces to be a 32bit int.

Moving the order of the fields without renumbering them in the .proto file
seems to align the go structure without changing the wire format.

raft/raftpb/raft.pb.go

@@ -189,9 +189,9 @@ func (x *ConfChangeType) UnmarshalJSON(data []byte) error {
 func (ConfChangeType) EnumDescriptor() ([]byte, []int) { return fileDescriptorRaft, []int{2} }
 
 type Entry struct {
-	Type             EntryType `protobuf:"varint,1,opt,name=Type,json=type,enum=raftpb.EntryType" json:"Type"`
 	Term             uint64    `protobuf:"varint,2,opt,name=Term,json=term" json:"Term"`
 	Index            uint64    `protobuf:"varint,3,opt,name=Index,json=index" json:"Index"`
+	Type             EntryType `protobuf:"varint,1,opt,name=Type,json=type,enum=raftpb.EntryType" json:"Type"`
 	Data             []byte    `protobuf:"bytes,4,opt,name=Data,json=data" json:"Data,omitempty"`
 	XXX_unrecognized []byte    `json:"-"`
 }

raft/raftpb/raft.proto

@@ -15,9 +15,9 @@ enum EntryType {
 }
 
 message Entry {
+	optional uint64     Term  = 2 [(gogoproto.nullable) = false]; // must be 64-bit aligned for atomic operations
+	optional uint64     Index = 3 [(gogoproto.nullable) = false]; // must be 64-bit aligned for atomic operations
 	optional EntryType  Type  = 1 [(gogoproto.nullable) = false];
-	optional uint64     Term  = 2 [(gogoproto.nullable) = false];
-	optional uint64     Index = 3 [(gogoproto.nullable) = false];
 	optional bytes      Data  = 4;
 }