Add status to the sink interfaces.

6 files changed, 146 insertions, 74 deletions

diff --git a/src/upb_data.c b/src/upb_data.c
index 8ed1646..d5a631d 100644
--- a/src/upb_data.c
+++ b/src/upb_data.c
@@ -297,25 +297,46 @@ void upb_msg_decodestr(upb_msg *msg, upb_msgdef *md, upb_strptr str,
   upb_msgsink_free(s);
 }
 
+#if 0
+void upb_msg_encodestr(upb_msg *msg, upb_msgdef *md, upb_strptr str,
+                       upb_status *status)
+{
+  upb_sizebuilder *sb = upb_sizebuilder_new(md);
+  upb_encoder *e = upb_encoder_new(md);
+  upb_strsink *sink = upb_strsink_new();
+
+  // Get sizes.  We could avoid performing this step in some cases by having a
+  // bool in the msgdef indicating whether it or any of its children have
+  // submessages in the def (groups don't count).
+  upb_sizebuilder_reset(sb);
+  upb_msgsrc_produce(msg, md, upb_sizebuilder_sink(sb), true);
+
+  upb_strsink_reset();
+  upb_encoder_reset(e, sb, sink);
+  upb_msgsrc_produce(msg, md, sink, false);
+}
+#endif
 
 /* upb_msgsrc  ****************************************************************/
 
 static void _upb_msgsrc_produceval(upb_value v, upb_fielddef *f, upb_sink *sink,
-                                   bool reverse)
+                                   bool reverse, upb_status *status)
 {
+  // TODO: We need to check status for failure, but how often?
   if(upb_issubmsg(f)) {
-    upb_sink_onstart(sink, f);
-    upb_msgsrc_produce(v.msg, upb_downcast_msgdef(f->def), sink, reverse);
-    upb_sink_onend(sink, f);
+    upb_msgdef *md = upb_downcast_msgdef(f->def);
+    upb_sink_onstart(sink, f, status);
+    upb_msgsrc_produce(v.msg, md, sink, reverse, status);
+    upb_sink_onend(sink, f, status);
   } else if(upb_isstring(f)) {
-    upb_sink_onstr(sink, f, v.str, 0, upb_strlen(v.str));
+    upb_sink_onstr(sink, f, v.str, 0, upb_strlen(v.str), status);
   } else {
-    upb_sink_onvalue(sink, f, v);
+    upb_sink_onvalue(sink, f, v, status);
   }
 }
 
 void upb_msgsrc_produce(upb_msg *msg, upb_msgdef *md, upb_sink *sink,
-                        bool reverse)
+                        bool reverse, upb_status *status)
 {
   for(int i = 0; i < md->num_fields; i++) {
     upb_fielddef *f = &md->fields[reverse ? md->num_fields - i - 1 : i];
@@ -326,10 +347,10 @@ void upb_msgsrc_produce(upb_msg *msg, upb_msgdef *md, upb_sink *sink,
       upb_arraylen_t len = upb_array_len(arr);
       for(upb_arraylen_t j = 0; j < upb_array_len(arr); j++) {
         upb_value elem = upb_array_get(arr, f, reverse ? len - j - 1 : j);
-        _upb_msgsrc_produceval(elem, f, sink, reverse);
+        _upb_msgsrc_produceval(elem, f, sink, reverse, status);
       }
     } else {
-      _upb_msgsrc_produceval(v, f, sink, reverse);
+      _upb_msgsrc_produceval(v, f, sink, reverse, status);
     }
   }
 }
@@ -345,7 +366,7 @@ typedef struct {
 struct upb_msgsink {
   upb_sink base;
   upb_msgdef *toplevel_msgdef;
-  upb_msgsink_frame stack[UPB_MAX_NESTING], *top;
+  upb_msgsink_frame stack[UPB_MAX_NESTING], *top, *limit;
 };
 
 /* Helper function that returns a pointer to where the next value for field "f"
@@ -377,8 +398,9 @@ static upb_valueptr get_valueptr(upb_msg *msg, upb_fielddef *f)
 // TODO: implement these in terms of public interfaces.
 
 static upb_sink_status _upb_msgsink_valuecb(upb_sink *s, upb_fielddef *f,
-                                            upb_value val)
+                                            upb_value val, upb_status *status)
 {
+  (void)status;  // No detectable errors can occur.
   upb_msgsink *ms = (upb_msgsink*)s;
   upb_msg *msg = ms->top->msg;
   upb_valueptr p = get_valueptr(msg, f);
@@ -389,8 +411,10 @@ static upb_sink_status _upb_msgsink_valuecb(upb_sink *s, upb_fielddef *f,
 
 static upb_sink_status _upb_msgsink_strcb(upb_sink *s, upb_fielddef *f,
                                           upb_strptr str,
-                                          int32_t start, uint32_t end)
+                                          int32_t start, uint32_t end,
+                                          upb_status *status)
 {
+  (void)status;  // No detectable errors can occur.
   upb_msgsink *ms = (upb_msgsink*)s;
   upb_msg *msg = ms->top->msg;
   upb_valueptr p = get_valueptr(msg, f);
@@ -405,11 +429,19 @@ static upb_sink_status _upb_msgsink_strcb(upb_sink *s, upb_fielddef *f,
   return UPB_SINK_CONTINUE;
 }
 
-static upb_sink_status _upb_msgsink_startcb(upb_sink *s, upb_fielddef *f)
+static upb_sink_status _upb_msgsink_startcb(upb_sink *s, upb_fielddef *f,
+                                            upb_status *status)
 {
   upb_msgsink *ms = (upb_msgsink*)s;
   upb_msg *oldmsg = ms->top->msg;
   upb_valueptr p = get_valueptr(oldmsg, f);
+  ms->top++;
+  if(ms->top == ms->limit) {
+    upb_seterr(status, UPB_ERROR_MAX_NESTING_EXCEEDED,
+               "Nesting exceeded maximum (%d levels)\n",
+               UPB_MAX_NESTING);
+    return UPB_SINK_STOP;
+  }
 
   if(upb_isarray(f) || !upb_msg_has(oldmsg, f)) {
     upb_msgdef *md = upb_downcast_msgdef(f->def);
@@ -422,13 +454,14 @@ static upb_sink_status _upb_msgsink_startcb(upb_sink *s, upb_fielddef *f)
     upb_msg_sethas(oldmsg, f);
   }
 
-  ms->top++;
   ms->top->msg = *p.msg;
   return UPB_SINK_CONTINUE;
 }
 
-static upb_sink_status _upb_msgsink_endcb(upb_sink *s, upb_fielddef *f)
+static upb_sink_status _upb_msgsink_endcb(upb_sink *s, upb_fielddef *f,
+                                          upb_status *status)
 {
+  (void)status;  // No detectable errors can occur.
   (void)f;  // Unused.
   upb_msgsink *ms = (upb_msgsink*)s;
   ms->top--;
@@ -467,6 +500,7 @@ upb_sink *upb_msgsink_sink(upb_msgsink *sink)
 void upb_msgsink_reset(upb_msgsink *ms, upb_msg *msg)
 {
   ms->top = ms->stack;
+  ms->limit = ms->stack + UPB_MAX_NESTING;
   ms->top->msg = msg;
   ms->top->md = ms->toplevel_msgdef;
 }
diff --git a/src/upb_data.h b/src/upb_data.h
index 2d2f402..cdb7af2 100644
--- a/src/upb_data.h
+++ b/src/upb_data.h
@@ -509,18 +509,24 @@ INLINE void upb_msg_clear(upb_msg *msg, upb_msgdef *md) {
   memset(msg->data, 0, md->set_flags_bytes);
 }
 
-// A convenience function for parsing an entire protobuf all at once, without
+// A convenience function for decoding an entire protobuf all at once, without
 // having to worry about setting up the appropriate objects.
 void upb_msg_decodestr(upb_msg *msg, upb_msgdef *md, upb_strptr str,
                        upb_status *status);
 
+// A convenience function for encoding an entire protobuf all at once.  If an
+// error occurs, the null string is returned and the status object contains
+// the error.
+void upb_msg_encodestr(upb_msg *msg, upb_msgdef *md, upb_strptr str,
+                       upb_status *status);
+
 
 /* upb_msgsrc *****************************************************************/
 
 // 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, upb_msgdef *md, upb_sink *sink,
-                        bool reverse);
+                        bool reverse, upb_status *status);
 
 
 /* upb_msgsink ****************************************************************/
diff --git a/src/upb_decoder.c b/src/upb_decoder.c
index 440eaef..209db56 100644
--- a/src/upb_decoder.c
+++ b/src/upb_decoder.c
@@ -383,16 +383,16 @@ static const uint8_t *push(upb_decoder *d, const uint8_t *start,
   frame->end_offset = d->completed_offset + submsg_len;
   frame->msgdef = upb_downcast_msgdef(f->def);
 
-  upb_sink_onstart(d->sink, f);
+  upb_sink_onstart(d->sink, f, status);
   return get_msgend(d, start);
 }
 
 // Pops a stack frame, returning a pointer for where the next submsg should
 // end (or a pointer that is out of range for a group).
-static const void *pop(upb_decoder *d, const uint8_t *start)
+static const void *pop(upb_decoder *d, const uint8_t *start, upb_status *status)
 {
   d->top--;
-  upb_sink_onend(d->sink, d->top->field);
+  upb_sink_onend(d->sink, d->top->field, status);
   return get_msgend(d, start);
 }
 
@@ -430,7 +430,7 @@ size_t upb_decoder_decode(upb_decoder *d, upb_strptr str, upb_status *status)
                    d->completed_offset + (completed - start));
         goto err;
       }
-      submsg_end = pop(d, start);
+      submsg_end = pop(d, start, status);
       msgdef = d->top->msgdef;
       completed = buf;
       continue;
@@ -451,8 +451,8 @@ size_t upb_decoder_decode(upb_decoder *d, upb_strptr str, upb_status *status)
       } else {
         if(f && upb_isstringtype(f->type)) {
           int32_t str_start = buf - start;
-          sink_status =
-              upb_sink_onstr(d->sink, f, str, str_start, str_start + delim_len);
+          uint32_t len = str_start + delim_len;
+          sink_status = upb_sink_onstr(d->sink, f, str, str_start, len, status);
         } // else { TODO: packed arrays }
         // If field was not found, it is skipped silently.
         buf = delim_end;  // Could be >end.
@@ -468,7 +468,7 @@ size_t upb_decoder_decode(upb_decoder *d, upb_strptr str, upb_status *status)
         buf = upb_decode_value(buf, end, f->type, upb_value_addrof(&val),
                               status);
         CHECK_STATUS();  // Checking upb_decode_value().
-        sink_status = upb_sink_onvalue(d->sink, f, val);
+        sink_status = upb_sink_onvalue(d->sink, f, val, status);
       }
     }
     CHECK_STATUS();
@@ -479,7 +479,7 @@ size_t upb_decoder_decode(upb_decoder *d, upb_strptr str, upb_status *status)
                    "did not lie on a tag/value boundary.");
         goto err;
       }
-      submsg_end = pop(d, start);
+      submsg_end = pop(d, start, status);
       msgdef = d->top->msgdef;
     }
     // while(buf < d->packed_end) { TODO: packed arrays }
diff --git a/src/upb_encoder.c b/src/upb_encoder.c
index 6d57acc..048b2b7 100644
--- a/src/upb_encoder.c
+++ b/src/upb_encoder.c
@@ -248,8 +248,10 @@ struct upb_sizebuilder {
 
 // upb_sink callbacks.
 static upb_sink_status _upb_sizebuilder_valuecb(upb_sink *sink, upb_fielddef *f,
-                                                upb_value val)
+                                                upb_value val,
+                                                upb_status *status)
 {
+  (void)status;
   upb_sizebuilder *sb = (upb_sizebuilder*)sink;
   uint32_t size = 0;
   size += _upb_get_tag_size(f->number);
@@ -260,8 +262,10 @@ static upb_sink_status _upb_sizebuilder_valuecb(upb_sink *sink, upb_fielddef *f,
 
 static upb_sink_status _upb_sizebuilder_strcb(upb_sink *sink, upb_fielddef *f,
                                               upb_strptr str,
-                                              int32_t start, uint32_t end)
+                                              int32_t start, uint32_t end,
+                                              upb_status *status)
 {
+  (void)status;
   (void)str;   // String data itself is not used.
   upb_sizebuilder *sb = (upb_sizebuilder*)sink;
   if(start >= 0) {
@@ -273,37 +277,54 @@ static upb_sink_status _upb_sizebuilder_strcb(upb_sink *sink, upb_fielddef *f,
   return UPB_SINK_CONTINUE;
 }
 
-static upb_sink_status _upb_sizebuilder_startcb(upb_sink *sink, upb_fielddef *f)
+static upb_sink_status _upb_sizebuilder_startcb(upb_sink *sink, upb_fielddef *f,
+                                                upb_status *status)
 {
   (void)f;  // Unused (we calculate tag size and delimiter in endcb).
   upb_sizebuilder *sb = (upb_sizebuilder*)sink;
-  *sb->top = sb->size;
-  sb->top++;
-  sb->size = 0;
-  if(sb->top == sb->limit) {
-    upb_seterr(&sb->status, UPB_ERROR_MAX_NESTING_EXCEEDED,
-               "Nesting exceeded maximum (%d levels)\n",
-               UPB_MAX_NESTING);
-    return UPB_SINK_STOP;
+  if(f->type == UPB_TYPE(MESSAGE)) {
+    *sb->top = sb->size;
+    sb->top++;
+    sb->size = 0;
+    if(sb->top == sb->limit) {
+      upb_seterr(status, UPB_ERROR_MAX_NESTING_EXCEEDED,
+                 "Nesting exceeded maximum (%d levels)\n",
+                 UPB_MAX_NESTING);
+      return UPB_SINK_STOP;
+    }
+  } else {
+    assert(f->type == UPB_TYPE(GROUP));
+    sb->size += _upb_get_tag_size(f->number);
   }
   return UPB_SINK_CONTINUE;
 }
 
-static upb_sink_status _upb_sizebuilder_endcb(upb_sink *sink, upb_fielddef *f)
+static upb_sink_status _upb_sizebuilder_endcb(upb_sink *sink, upb_fielddef *f,
+                                              upb_status *status)
 {
+  (void)status;
   upb_sizebuilder *sb = (upb_sizebuilder*)sink;
-  if(sb->sizes_len == sb->sizes_size) {
-    sb->sizes_size *= 2;
-    sb->sizes = realloc(sb->sizes, sb->sizes_size * sizeof(*sb->sizes));
+  if(f->type == UPB_TYPE(MESSAGE)) {
+    sb->top--;
+    if(sb->sizes_len == sb->sizes_size) {
+      sb->sizes_size *= 2;
+      sb->sizes = realloc(sb->sizes, sb->sizes_size * sizeof(*sb->sizes));
+    }
+    uint32_t child_size = sb->size;
+    uint32_t parent_size = *sb->top;
+    sb->sizes[sb->sizes_len++] = child_size;
+    // The size according to the parent includes the tag size and delimiter of
+    // the submessage.
+    parent_size += upb_get_UINT32_size(child_size);
+    parent_size += _upb_get_tag_size(f->number);
+    // Include size accumulated in parent before child began.
+    sb->size = child_size + parent_size;
+  } else {
+    assert(f->type == UPB_TYPE(GROUP));
+    // As an optimization, we could just add this number twice in startcb, to
+    // avoid having to recalculate it.
+    sb->size += _upb_get_tag_size(f->number);
   }
-  sb->sizes[sb->sizes_len++] = sb->size;
-  sb->top--;
-  // The size according to the parent includes the tag size and delimiter of
-  // the submessage.
-  sb->size += upb_get_UINT32_size(sb->size);
-  sb->size += _upb_get_tag_size(f->number);
-  // Include size accumulated in parent before child began.
-  sb->size += *sb->top;
   return UPB_SINK_CONTINUE;
 }
 
@@ -329,12 +350,13 @@ struct upb_encoder {
 #define UPB_ENCODER_BUFSIZE (UPB_MAX_ENCODED_SIZE * 2)
 
 static upb_sink_status _upb_encoder_push_buf(upb_encoder *s, const uint8_t *buf,
-                                             size_t len)
+                                             size_t len, upb_status *status)
 {
   // TODO: conjure a upb_strptr that points to buf.
   //upb_strptr ptr;
   (void)s;
   (void)buf;
+  (void)status;
   size_t written = 5;// = upb_bytesink_onbytes(s->bytesink, ptr);
   if(written < len) {
     // TODO: mark to skip "written" bytes next time.
@@ -345,7 +367,7 @@ static upb_sink_status _upb_encoder_push_buf(upb_encoder *s, const uint8_t *buf,
 }
 
 static upb_sink_status _upb_encoder_valuecb(upb_sink *sink, upb_fielddef *f,
-                                            upb_value val)
+                                            upb_value val, upb_status *status)
 {
   upb_encoder *s = (upb_encoder*)sink;
   uint8_t buf[UPB_ENCODER_BUFSIZE], *ptr = buf;
@@ -353,12 +375,13 @@ static upb_sink_status _upb_encoder_valuecb(upb_sink *sink, upb_fielddef *f,
   // TODO: handle packed encoding.
   ptr = _upb_put_tag(ptr, f->number, wt);
   ptr = upb_encode_value(ptr, f->type, val);
-  return _upb_encoder_push_buf(s, buf, ptr - buf);
+  return _upb_encoder_push_buf(s, buf, ptr - buf, status);
 }
 
 static upb_sink_status _upb_encoder_strcb(upb_sink *sink, upb_fielddef *f,
                                           upb_strptr str,
-                                          int32_t start, uint32_t end)
+                                          int32_t start, uint32_t end,
+                                          upb_status *status)
 {
   upb_encoder *s = (upb_encoder*)sink;
   uint8_t buf[UPB_ENCODER_BUFSIZE], *ptr = buf;
@@ -368,11 +391,12 @@ static upb_sink_status _upb_encoder_strcb(upb_sink *sink, upb_fielddef *f,
   }
   // TODO: properly handle partially consumed strings and partially supplied
   // strings.
-  _upb_encoder_push_buf(s, buf, ptr - buf);
-  return _upb_encoder_push_buf(s, (uint8_t*)upb_string_getrobuf(str), end - start);
+  _upb_encoder_push_buf(s, buf, ptr - buf, status);
+  return _upb_encoder_push_buf(s, (uint8_t*)upb_string_getrobuf(str), end - start, status);
 }
 
-static upb_sink_status _upb_encoder_startcb(upb_sink *sink, upb_fielddef *f)
+static upb_sink_status _upb_encoder_startcb(upb_sink *sink, upb_fielddef *f,
+                                            upb_status *status)
 {
   upb_encoder *s = (upb_encoder*)sink;
   uint8_t buf[UPB_ENCODER_BUFSIZE], *ptr = buf;
@@ -382,16 +406,17 @@ static upb_sink_status _upb_encoder_startcb(upb_sink *sink, upb_fielddef *f)
     ptr = _upb_put_tag(ptr, f->number, UPB_WIRE_TYPE_DELIMITED);
     ptr = upb_put_UINT32(ptr, s->sizes[--s->size_offset]);
   }
-  return _upb_encoder_push_buf(s, buf, ptr - buf);
+  return _upb_encoder_push_buf(s, buf, ptr - buf, status);
 }
 
-static upb_sink_status _upb_encoder_endcb(upb_sink *sink, upb_fielddef *f)
+static upb_sink_status _upb_encoder_endcb(upb_sink *sink, upb_fielddef *f,
+                                          upb_status *status)
 {
   upb_encoder *s = (upb_encoder*)sink;
   uint8_t buf[UPB_ENCODER_BUFSIZE], *ptr = buf;
   if(f->type != UPB_TYPE(GROUP)) return UPB_SINK_CONTINUE;
   ptr = _upb_put_tag(ptr, f->number, UPB_WIRE_TYPE_END_GROUP);
-  return _upb_encoder_push_buf(s, buf, ptr - buf);
+  return _upb_encoder_push_buf(s, buf, ptr - buf, status);
 }
 
 upb_sink_callbacks _upb_encoder_sink_vtbl = {
diff --git a/src/upb_encoder.h b/src/upb_encoder.h
index 1137c7f..b4d0c98 100644
--- a/src/upb_encoder.h
+++ b/src/upb_encoder.h
@@ -29,9 +29,11 @@ extern "C" {
 struct upb_sizebuilder;
 typedef struct upb_sizebuilder upb_sizebuilder;
 
-upb_sizebuilder *upb_sizebuilder_new();
+upb_sizebuilder *upb_sizebuilder_new(upb_msgdef *md);
 void upb_sizebuilder_free(upb_sizebuilder *sb);
 
+void upb_sizebuilder_reset(upb_sizebuilder *sb);
+
 // Returns a sink that must be used to perform the pre-pass.  Note that the
 // pre-pass *must* occur in the opposite order from the actual encode that
 // follows, and the data *must* be identical both times (except for the
@@ -46,8 +48,8 @@ upb_sink *upb_sizebuilder_sink(upb_sizebuilder *sb);
 struct upb_encoder;
 typedef struct upb_encoder upb_encoder;
 
-upb_encoder *upb_encoder_new();
-void upb_encoder_free(upb_encoder *s);
+upb_encoder *upb_encoder_new(upb_msgdef *md);
+void upb_encoder_free(upb_encoder *e);
 
 // Resets the given upb_encoder such that is is ready to begin encoding.  The
 // upb_sizebuilder "sb" is used to determine submessage sizes; it must have
@@ -57,12 +59,12 @@ void upb_encoder_free(upb_encoder *s);
 // "out" is where the encoded output data will be sent.
 //
 // Both "sb" and "out" must live until the encoder is either reset or freed.
-void upb_encoder_reset(upb_encoder *s, upb_sizebuilder *sb, upb_bytesink *out);
+void upb_encoder_reset(upb_encoder *e, upb_sizebuilder *sb, upb_bytesink *out);
 
 // The upb_sink to which data can be sent to be encoded.  Note that this data
 // must be identical to the data that was previously given to the sizebuilder
 // (if any).
-upb_sink *upb_encoder_sink(upb_encoder *s);
+upb_sink *upb_encoder_sink(upb_encoder *e);
 
 #ifdef __cplusplus
 }  /* extern "C" */
diff --git a/src/upb_sink.h b/src/upb_sink.h
index 41e5275..2b8369e 100644
--- a/src/upb_sink.h
+++ b/src/upb_sink.h
@@ -66,7 +66,7 @@ typedef struct {
 // The value callback is called for a regular value (ie. not a string or
 // submessage).
 typedef upb_sink_status (*upb_value_cb)(upb_sink *s, upb_fielddef *f,
-                                        upb_value val);
+                                        upb_value val, upb_status *status);
 
 // The string callback is called for string data.  "str" is the string in which
 // the data lives, but it may contain more data than the effective string.
@@ -81,12 +81,15 @@ typedef upb_sink_status (*upb_value_cb)(upb_sink *s, upb_fielddef *f,
 // copying if it is unavoidable.
 typedef upb_sink_status (*upb_str_cb)(upb_sink *s, upb_fielddef *f,
                                       upb_strptr str,
-                                      int32_t start, uint32_t end);
+                                      int32_t start, uint32_t end,
+                                      upb_status *status);
 
 // The start and end callbacks are called when a submessage begins and ends,
 // respectively.
-typedef upb_sink_status (*upb_start_cb)(upb_sink *s, upb_fielddef *f);
-typedef upb_sink_status (*upb_end_cb)(upb_sink *s, upb_fielddef *f);
+typedef upb_sink_status (*upb_start_cb)(upb_sink *s, upb_fielddef *f,
+                                        upb_status *status);
+typedef upb_sink_status (*upb_end_cb)(upb_sink *s, upb_fielddef *f,
+                                      upb_status *status);
 
 
 /* upb_sink implementation ****************************************************/
@@ -109,10 +112,10 @@ typedef struct upb_sink_callbacks {
 // possible to write C++ sinks in a more natural style without loss of
 // efficiency.  We could have a flag in upb_sink defining whether it is a C
 // sink or a C++ one.
-#define upb_sink_onvalue(s, f, val) s->vtbl->value_cb(s, f, val)
-#define upb_sink_onstr(s, f, str, start, end) s->vtbl->str_cb(s, f, str, start, end)
-#define upb_sink_onstart(s, f) s->vtbl->start_cb(s, f)
-#define upb_sink_onend(s, f) s->vtbl->end_cb(s, f)
+#define upb_sink_onvalue(s, f, val, status) s->vtbl->value_cb(s, f, val, status)
+#define upb_sink_onstr(s, f, str, start, end, status) s->vtbl->str_cb(s, f, str, start, end, status)
+#define upb_sink_onstart(s, f, status) s->vtbl->start_cb(s, f, status)
+#define upb_sink_onend(s, f, status) s->vtbl->end_cb(s, f, status)
 
 // Initializes a plain C visitor with the given vtbl.  The sink must have been
 // allocated separately.
@@ -134,9 +137,11 @@ INLINE void upb_sink_init(upb_sink *s, upb_sink_callbacks *vtbl) {
 struct _upb_bytesink;
 
 // The single bytesink callback; it takes the bytes to be written and returns
-// how many were successfully written.  If zero is returned, it indicates that
-// no more bytes can be accepted right now.
-typedef size_t (*upb_byte_cb)(struct _upb_bytesink *s, upb_strptr str);
+// how many were successfully written.  If the return value is <0, the caller
+// should stop processing.
+typedef int32_t (*upb_byte_cb)(struct _upb_bytesink *s, upb_strptr str,
+                               uint32_t start, uint32_t end,
+                               upb_status *status);
 
 typedef struct _upb_bytesink {
   upb_byte_cb *cb;