Re: kern/60792: fifo: poll reader before writers?

["Taylor R Campbell via gnats" <gnats-admin%NetBSD.org@localhost>]

The following reply was made to PR kern/60792; it has been noted by GNATS.

From: Taylor R Campbell <riastradh%NetBSD.org@localhost>
To: gnats-bugs%NetBSD.org@localhost, netbsd-bugs%NetBSD.org@localhost
Cc: 
Subject: Re: kern/60792: fifo: poll reader before writers?
Date: Thu, 1 Oct 2026 22:40:45 +0000

 > Date: Sat, 26 Sep 2026 13:35:00 +0000
 > From: Taylor R Campbell <riastradh%NetBSD.org@localhost>
 > 
 > > => Linux 6.1.0:
 > >    1. read EOF                  poll 0          *bzzt*
 > >    2. read EOF                  poll POLLHUP
 > >    3. rd0: read EOF             poll POLLHUP
 > >       rd1: read EOF             poll 0          *bzzt*
 > > => FreeBSD 13.0:
 > >    1. read *block*              poll 0
 > >    2. read EOF                  poll POLLIN|POLLHUP
 > >    3. rd0: read EOF             poll POLLIN|POLLHUP
 > >       rd1: read EOF             poll 0          *bzzt*
 > > [...]
 > > With my naive patch for PR 60789 (to apply a change that
 > > FreeBSD made back in 1997), I get:
 > > 
 > > => NetBSD patched for PR kern/60789:
 > >    1. read EOF                  poll POLLIN|POLLHUP
 > >    2. read EOF                  poll POLLIN|POLLHUP
 > >    3. rd0: read EOF             poll POLLIN|POLLHUP
 > >       rd1: read EOF             poll POLLIN|POLLHUP
 > 
 > The advantage of the Linux and FreeBSD semantics is that you can open
 > a fifo for reading without blocking to get a file descriptor, and then
 > use select/poll to wait for a writer to open.
 > 
 > But it is internally inconsistent (poll returns 0 events ready but you
 > can read (EOF) without blocking!), and maybe if you want to wait for a
 > connection, you should just use sockets instead of fifos anyway.
 > 
 > And it requires keeping per-open state rather than per-vnode state,
 > which would require restructuring our fifo code, which is annoying!
 
 Also: instead of switching to sockets, you can just keep the fifo open
 for write!  That way, poll will not return ready until a byte of data
 has been written.  And that semantics is guaranteed by POSIX, and
 probably works everywhere already.  For totally unrelated reasons I
 stumbled upon a program I wrote over a decade ago that does exactly
 this, with a comment:
 
 	/*
 	 * Open the fifo for write without blocking.  We don't ever
 	 * write to it, but we need it open to make poll block rather
 	 * than return POLLHUP immediately in a loop.  Must happen
 	 * after we open for read or else this will fail with ENXIO.
 	 */
 
 So I think I'll just go ahead with the naive patch, and if someone
 really wants the semantics requiring per-open/per-vnode state (or, for
 reasons I can't imagine now, longer history), they can file a new PR.