Tom and Robert, I tried a combination of PIPE lock and file lock (fcntl) as Tom had suggested. Attached experimental patch has this logic...
Postmaster : - get exclusive fcntl lock (to guard against race condition in PIPE-based lock) - check PIPE for any existing readers - open PIPE for read All other backends: - get shared fcnlt lock - open PIPE for read Your feedback is appreciated. Thanks. -Keith Baker > -----Original Message----- > From: pgsql-hackers-ow...@postgresql.org [mailto:pgsql-hackers- > ow...@postgresql.org] On Behalf Of Tom Lane > Sent: Wednesday, July 30, 2014 11:02 AM > To: Robert Haas > Cc: Baker, Keith [OCDUS Non-J&J]; pgsql-hackers@postgresql.org > Subject: Re: [HACKERS] Proposal to add a QNX 6.5 port to PostgreSQL > > Robert Haas <robertmh...@gmail.com> writes: > > On Tue, Jul 29, 2014 at 7:06 PM, Tom Lane <t...@sss.pgh.pa.us> wrote: > >> Hm. That particular protocol is broken: two postmasters doing it at > >> the same time would both pass (because neither has it open for read > >> at the instant where they try to write). But we could possibly frob > >> the idea until it works. Bigger question is how portable is this behavior? > >> I see named pipes (fifos) in SUS v2, which is our usual baseline > >> assumption about what's portable across Unixen, so maybe it would > work. > >> But does NFS support named pipes? > > > Looks iffy, on a quick search. Sigh. > > I poked around, and it seems like a lot of the people who think it's flaky are > imagining that they should be able to use a named pipe on an NFS server to > pass data between two different machines. That doesn't work, but it's not > what we need, either. For communication between processes on the same > server, all that's needed is that the filesystem entry looks like a pipe to > the > local kernel --- and that's been required NFS functionality since RFC1813 (v3, > in 1995). > > So it seems like we could possibly go this route, assuming we can think of a > variant of your proposal that's race-condition-free. A disadvantage > compared to a true file lock is that it would not protect against people > trying > to start postmasters from two different NFS client machines --- but we don't > have protection against that now. (Maybe we could do this *and* do a > regular file lock to offer some protection against that case, even if it's not > bulletproof?) > > regards, tom lane > > > -- > Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make > changes to your subscription: > http://www.postgresql.org/mailpref/pgsql-hackers
locking_20140814.patch
Description: locking_20140814.patch
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