Re: Thinko in processing of SHM message size info?

On Thu, Aug 6, 2015 at 9:04 AM, Antonin Houska <ah(at)cybertec(dot)at> wrote:
> 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.

Hmm, I think you are correct. This would matter in the case where the
message length word was read in more than two chunks. I don't *think*
that's possible right now because I believe the only systems where
MAXIMUM_ALIGNOF < sizeof(Size) are those with MAXIMUM_ALIGNOF == 4 and
sizeof(Size) == 8. However, if we had systems where MAXIMUM_ALIGNOF
== 4 and sizeof(Size) == 16, or systems where MAXIMUM_ALIGNOF == 2 and
sizeof(Size) == 8, this would be a live bug.

Thanks for reviewing; I'll go push this change.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
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