Re: Proposal to add a QNX 6.5 port to PostgreSQL

Robert Haas <robertmhaas(at)gmail(dot)com> writes:
> On Tue, Jul 29, 2014 at 7:06 PM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)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