Thinko in processing of SHM message size info?

Can anyone please explain why the following patch shouldn't be applied?

diff --git a/src/backend/storage/ipc/shm_mq.c b/src/backend/storage/ipc/shm_mq.c
index 126cb07..4cd52ac 100644
--- a/src/backend/storage/ipc/shm_mq.c
+++ b/src/backend/storage/ipc/shm_mq.c
@@ -584,7 +584,7 @@ shm_mq_receive(shm_mq_handle *mqh, Size *nbytesp, void **datap, bool nowait)
if (mqh->mqh_partial_bytes + rb > sizeof(Size))
lengthbytes = sizeof(Size) - mqh->mqh_partial_bytes;
else
- lengthbytes = rb - mqh->mqh_partial_bytes;
+ lengthbytes = rb;
memcpy(&mqh->mqh_buffer[mqh->mqh_partial_bytes], rawdata,
lengthbytes);
mqh->mqh_partial_bytes += lengthbytes;

I'm failing to understand why anything should be subtracted. Note that the
previous iteration must have called shm_mq_inc_bytes_read(), so "rb" should
not include anything of mqh->mqh_partial_bytes. Thanks.

--
Antonin Houska
Cybertec Schönig & Schönig GmbH
Gröhrmühlgasse 26
A-2700 Wiener Neustadt
Web: http://www.postgresql-support.de, http://www.cybertec.at