Make msgsrc reversable.

src/upb_data.c

@@ -301,11 +301,11 @@ void upb_msg_parsestr(upb_msg *msg, struct upb_msgdef *md, upb_strptr str,
 /* upb_msgsrc  ****************************************************************/
 
 static void _upb_msgsrc_produceval(union upb_value v, struct upb_fielddef *f,
-                                   upb_sink *sink)
+                                   upb_sink *sink, bool reverse)
 {
   if(upb_issubmsg(f)) {
     upb_sink_onstart(sink, f);
-    upb_msgsrc_produce(v.msg, upb_downcast_msgdef(f->def), sink);
+    upb_msgsrc_produce(v.msg, upb_downcast_msgdef(f->def), sink, reverse);
     upb_sink_onend(sink, f);
   } else if(upb_isstring(f)) {
     upb_sink_onstr(sink, f, v.str, 0, upb_strlen(v.str));
@@ -314,20 +314,22 @@ static void _upb_msgsrc_produceval(union upb_value v, struct upb_fielddef *f,
   }
 }
 
-void upb_msgsrc_produce(upb_msg *msg, struct upb_msgdef *md, upb_sink *sink)
+void upb_msgsrc_produce(upb_msg *msg, struct upb_msgdef *md, upb_sink *sink,
+                        bool reverse)
 {
   for(int i = 0; i < md->num_fields; i++) {
-    struct upb_fielddef *f = &md->fields[i];
+    struct upb_fielddef *f = &md->fields[reverse ? md->num_fields - i - 1 : i];
     if(!upb_msg_has(msg, f)) continue;
     union upb_value v = upb_msg_get(msg, f);
     if(upb_isarray(f)) {
       upb_arrayptr arr = v.arr;
+      upb_arraylen_t len = upb_array_len(arr);
       for(upb_arraylen_t j = 0; j < upb_array_len(arr); j++) {
-        union upb_value elem = upb_array_get(arr, f, j);
-        _upb_msgsrc_produceval(elem, f, sink);
+        union upb_value elem = upb_array_get(arr, f, reverse ? len - j - 1 : j);
+        _upb_msgsrc_produceval(elem, f, sink, reverse);
       }
     } else {
-      _upb_msgsrc_produceval(v, f, sink);
+      _upb_msgsrc_produceval(v, f, sink, reverse);
     }
   }
 }

src/upb_data.h

@@ -524,7 +524,8 @@ void upb_msg_parsestr(upb_msg *msg, struct upb_msgdef *md, upb_strptr str,
 
 // A nonresumable, non-interruptable (but simple and fast) source for pushing
 // the data of a upb_msg to a upb_sink.
-void upb_msgsrc_produce(upb_msg *msg, struct upb_msgdef *md, upb_sink *sink);
+void upb_msgsrc_produce(upb_msg *msg, struct upb_msgdef *md, upb_sink *sink,
+                        bool reverse);
 
 
 /* upb_msgsink ****************************************************************/

src/upb_serialize.c

@@ -253,7 +253,7 @@ static upb_sink_status _upb_serializersink_strcb(upb_sink *sink, struct upb_fiel
   // TODO: properly handle partially consumed strings and partially supplied
   // strings.
   _upb_serializer_push_buf(s, buf, ptr - buf);
-  return _upb_serializer_push_buf(s, upb_string_getrobuf(str), end - start);
+  return _upb_serializer_push_buf(s, (uint8_t*)upb_string_getrobuf(str), end - start);
 }
 
 static upb_sink_status _upb_serializersink_startcb(upb_sink *sink,