On Thu, Aug 6, 2015 at 1:24 PM, Robert Haas <robertmh...@gmail.com> wrote: > On Thu, Aug 6, 2015 at 9:04 AM, Antonin Houska <a...@cybertec.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.
Hmm, actually, maybe it is a live bug anyway, because the if statement tests > rather than >=. If we've read 4 bytes and exactly 4 more bytes are available, we'd set lengthbytes to 0 instead of 4. Oops. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers