Add startseq/endseq handlers.

Startseq/endseq handlers are called at the beginning
and end of a sequence of repeated values.  Protobuf
does not really have direct support for this (repeated
primitive fields do not delimit "begin" and "end" of
the sequence) but we can infer them from the bytestream.

The benefit of supporting them explicitly is that they
get their own stack frame and closure, so we can avoid
having to find the array's address over and over and
deciding if we need to initialize it.

This will also pave the way for better support of JSON,
which does have explicit "startseq/endseq" markers: [].

1 files changed, 4 insertions, 3 deletions

diff --git a/src/upb_msg.h b/src/upb_msg.h
index a0b2ad2..ac113a8 100644
--- a/src/upb_msg.h
+++ b/src/upb_msg.h
@@ -144,14 +144,15 @@ typedef uint32_t upb_arraylen_t;
 struct _upb_array {
   upb_atomic_refcount_t refcount;
   // "len" and "size" are measured in elements, not bytes.
-  upb_arraylen_t len;
-  upb_arraylen_t size;
+  int32_t len;
+  int32_t size;
   char *ptr;
 };
 
 void _upb_array_free(upb_array *a, upb_fielddef *f);
 INLINE upb_valueptr _upb_array_getptrforsize(upb_array *a, size_t type_size,
-                                             uint32_t elem) {
+                                             int32_t elem) {
+  assert(elem >= 0);
   upb_valueptr p;
   p._void = &a->ptr[elem * type_size];
   return p;