PR/56429 CVS commit: src/tests/lib/libc/sys

["Taylor R Campbell" <riastradh%netbsd.org@localhost>]

The following reply was made to PR kern/56429; it has been noted by GNATS.

From: "Taylor R Campbell" <riastradh%netbsd.org@localhost>
To: gnats-bugs%gnats.NetBSD.org@localhost
Cc: 
Subject: PR/56429 CVS commit: src/tests/lib/libc/sys
Date: Sun, 9 Feb 2025 17:10:37 +0000

 Module Name:	src
 Committed By:	riastradh
 Date:		Sun Feb  9 17:10:37 UTC 2025
 
 Modified Files:
 	src/tests/lib/libc/sys: Makefile t_poll.c
 
 Log Message:
 t_poll: Expand tests of polling when other endpoint is closed.
 
 This covers fifos, pipes, and sockets; reads and writes; and polling
 when the endpoint is already closed as well as sleeping until the
 endpoint is closed after a delay.
 
 Unfortunately, there is disagreement right now between:
 
 (a) POSIX,
 (b) our poll(2) man page, and
 (c) our implementation,
 
 for many of these cases.  And we recently changed the behaviour of
 poll(2) on fifos between 9 and 10 (PR kern/56429: poll(2) should
 yield POLLHUP when last writer to a fifo close()'s it) but didn't get
 it quite right.
 
 So I've annotated these test cases with xfails for all the cases I
 think we're currently doing wrong -- under a justification from some
 combination of POSIX, our own documentation, and/or a reasonable
 interpretation of what the semantics _should_ be based on how read(2)
 and write(2) will behave.  These are mostly edge cases:
 
 1. [pollclosed_pipe_*_write*] When the read side of a pipe is closed,
    poll(2) on the write side currently returns POLLHUP|POLLOUT, which
    POSIX forbids because it requires POLLHUP to be mutually exclusive
    with POLLOUT.  And our man page concurs with POSIX on this rule,
    and has for a long time (going as far back as NetBSD 1.3 or
    earlier, judging by https://man.NetBSD.org/NetBSD-1.3/poll.2).
 
    => Fixing this won't change whether anything wakes up earlier or
       later -- it will only change which bits are set when the
       wakeups happen.
 
 2. [pollclosed_fifo*_immediate_readsome] When the write side of a
    fifo was previously open with the read side, and has since been
    closed before calling poll(2), poll(2) on the read side returns 0
    instead of POLLHUP|POLLIN as it is expected to, even though
    read(2) will return EOF without blocking.
 
    => Fixing this may lead to more wakeups than before, but only in
       cases where read(2) would actually return immediately anyway.
 
 3. [pollclosed_fifo*_delayed_*_read] When the write side of a fifo is
    closed, poll(2) on the read side returns POLLHUP|POLLIN, as it is
    expected to.  But this state currently isn't persistent, even
    though the behaviour of read(2) in returning EOF without blocking
    is persistent.
 
    => Fixing this may lead to more wakeups than before, but only in
       cases where read(2) would actually return immediately anyway.
 
 That said, it turns out that we are correct, according to POSIX, in
 not setting POLLHUP on a socket whose peer has been closed: POLLHUP
 is only for devices, pipes, and FIFOs.  So one of the issues I
 reported in PR 59056 turns out to be bogus.  (Also POLLHUP is only
 for the reader side anyway, not for the writer side.)
 
 https://pubs.opengroup.org/onlinepubs/9799919799/functions/poll.html
 
 PR kern/59056: poll POLLHUP bugs
 
 
 To generate a diff of this commit:
 cvs rdiff -u -r1.77 -r1.78 src/tests/lib/libc/sys/Makefile
 cvs rdiff -u -r1.10 -r1.11 src/tests/lib/libc/sys/t_poll.c
 
 Please note that diffs are not public domain; they are subject to the
 copyright notices on the relevant files.