Prevent the default message from getting mutated.

If a Lua program references the default message, it will
be copied into a mutable object.

benchmarks/google_messages.proto

@@ -51,7 +51,7 @@ message SpeedMessage1SubMessage {
   optional int32 field1 = 1 [default=0];
   optional int32 field2 = 2 [default=0];
   optional int32 field3 = 3 [default=0];
-  optional string field15 = 15;
+  optional string field15 = 15 [default="FOOBAR!"];
   optional bool field12 = 12 [default=true];
   optional int64 field13 = 13;
   optional int64 field14 = 14;

lang_ext/lua/test.lua

@@ -23,15 +23,17 @@ SpeedMessage1 = symtab:lookup("benchmarks.SpeedMessage1")
 print(SpeedMessage1:name())
 
 msg = SpeedMessage1()
--- print(msg.field1)
+--print(msg.field1)
 -- print(msg.field129)
 -- print(msg.field271)
--- print(msg.field15.field15)
+print(msg.field15.field15)
+msg.field15.field15 = "my override"
+print(msg.field15.field15)
 -- print(msg.field1)
 -- print(msg.field1)
 -- msg.field1 = "YEAH BABY!"
 -- print(msg.field1)
-print(msg.field129)
-msg.field129 = 5
-print(msg.field129)
+-- print(msg.field129)
+-- msg.field129 = 5
+-- print(msg.field129)
 

lang_ext/lua/upb.c

@@ -170,7 +170,7 @@ static void lupb_pushvalue(lua_State *L, upb_value val, upb_fielddef *f) {
 
 static upb_value lupb_getvalue(lua_State *L, int narg, upb_fielddef *f) {
   upb_value val;
-  lua_Number num;
+  lua_Number num = 0;
   if (!upb_issubmsg(f) && !upb_isstring(f) && f->type != UPB_TYPE(BOOL)) {
     num = luaL_checknumber(L, narg);
     if (f->type != UPB_TYPE(DOUBLE) && f->type != UPB_TYPE(FLOAT) &&

src/upb_msg.c

@@ -161,6 +161,26 @@ void upb_msg_set(upb_msg *msg, upb_fielddef *f, upb_value val) {
   return upb_value_write(ptr, val, upb_field_valuetype(f));
 }
 
+upb_value upb_msg_get(upb_msg *msg, upb_fielddef *f) {
+  if (!upb_msg_has(msg, f)) {
+    upb_value val = f->default_value;
+    if (upb_issubmsg(f)) {
+      // TODO: handle arrays also, which must be treated similarly.
+      upb_msgdef *md = upb_downcast_msgdef(f->def);
+      upb_msg *m = upb_msg_new(md);
+      // Copy all set bits and values, except the refcount.
+      memcpy(m , upb_value_getmsg(val), md->size);
+      upb_atomic_refcount_init(&m->refcount, 0); // The msg will take a ref.
+      upb_value_setmsg(&val, m);
+    }
+    upb_msg_set(msg, f, val);
+    return val;
+  } else {
+    return upb_value_read(_upb_msg_getptr(msg, f), upb_field_valuetype(f));
+  }
+}
+
+
 static upb_valueptr upb_msg_getappendptr(upb_msg *msg, upb_fielddef *f) {
   upb_valueptr p = _upb_msg_getptr(msg, f);
   if (upb_isarray(f)) {

src/upb_msg.h

@@ -10,6 +10,8 @@
  *
  * upb's parsers and serializers could also be used to populate and serialize
  * other kinds of message objects (even one generated by Google's protobuf).
+ *
+ * TODO: consider properly supporting const instances.
  */
 
 #ifndef UPB_MSG_H
@@ -204,12 +206,16 @@ upb_msg *upb_msg_new(upb_msgdef *md);
 INLINE void upb_msg_unref(upb_msg *msg, upb_msgdef *md) {
   if (msg && upb_atomic_unref(&msg->refcount)) _upb_msg_free(msg, md);
 }
+
 INLINE upb_msg *upb_msg_getref(upb_msg *msg) {
   assert(msg);
   upb_atomic_ref(&msg->refcount);
   return msg;
 }
 
+// Modifies *msg to point to a newly initialized msg instance.  If the msg had
+// no other referents, reuses the same msg, otherwise allocates a new one.
+// The caller *must* own a ref on the msg prior to calling this method!
 void upb_msg_recycle(upb_msg **msg, upb_msgdef *msgdef);
 
 // Tests whether the given field is explicitly set, or whether it will return a
@@ -240,9 +246,17 @@ INLINE bool upb_msg_has(upb_msg *msg, upb_fielddef *f) {
 //
 // For the moment we go with (2).  Google's protobuf does (3), which is likely
 // part of the reason we beat it in some benchmarks.
+//
+// If the branchiness of (2) is too great, this could be mitigated with cmov
+// (both values and the conditional are cheap to calculate, much cheaper than
+// the cost of a misprediction).
 
 // For submessages and strings, the returned value is not owned.
-INLINE upb_value upb_msg_get(upb_msg *msg, upb_fielddef *f) {
+upb_value upb_msg_get(upb_msg *msg, upb_fielddef *f);
+
+// A specialized version of the previous that is cheaper because it doesn't
+// support submessages or arrays.
+INLINE upb_value upb_msg_getscalar(upb_msg *msg, upb_fielddef *f) {
   if (upb_msg_has(msg, f)) {
     return upb_value_read(_upb_msg_getptr(msg, f), upb_field_valuetype(f));
   } else {
@@ -250,6 +264,9 @@ INLINE upb_value upb_msg_get(upb_msg *msg, upb_fielddef *f) {
   }
 }
 
+// Sets the given field to the given value.  If the field is a string, array,
+// or submessage, releases the ref on any object we may have been referencing
+// and takes a ref on the new object (if any).
 void upb_msg_set(upb_msg *msg, upb_fielddef *f, upb_value val);
 
 // Unsets all field values back to their defaults.