+ if (stream->bytes_written + size > stream->max_size)
+ return false;
+
+ /* Use a substream to verify that a callback doesn't write more than
+ * what it did the first time. */
+ substream.callback = stream->callback;
+ substream.state = stream->state;
+ substream.max_size = size;
+ substream.bytes_written = 0;
+
+ status = pb_encode(&substream, (pb_field_t*)field->ptr, src);
+
+ stream->bytes_written += substream.bytes_written;
+
+ if (substream.bytes_written != size)
+ return false;
+
+ return status;