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[bug #22861] bogus answer from pflocal to io_select SELECT_URG

From: Kalle Olavi Niemitalo
Subject: [bug #22861] bogus answer from pflocal to io_select SELECT_URG
Date: Sat, 16 Jul 2016 13:46:38 +0000 (UTC)
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:38.0) Gecko/20100101 Firefox/38.0 Iceweasel/38.8.0

Follow-up Comment #4, bug #22861 (project hurd):

The t/io_select_timeout patches in glibc complicate the fix.  Now if
glibc/hurd/hurdselect.c (_hurd_select) is called with a timeout and at least
one file descriptor, it passes the current time + the timeout to the servers,
instead of letting __mach_msg time out.  And if each server returns
MIG_NO_REPLY to its message loop and so never sends a reply, then _hurd_select
sleeps until interrupted by a signal, regardless of what the timeout was.

I guess the io_select_timeout implementation in each server could just wait
until the timeout occurs (at which time it would send an reply with
select_type=0) or the reply port dies (in which case no reply can be sent, and
any related resources should be freed).  As an optimization, the server could
use a timer queue so that the wait does not tie up a thread.

io_select could be implemented in a similar way except with an infinite
timeout, or it could be done with MIG_NO_REPLY as originally planned, I

I removed the workaround from ELinks and tried to reproduce the original
problem but couldn't; it never called select in such a way that the pipe was
only in exceptfds.  To test any fix here, one should construct a dedicated
test program instead of relying on ELinks.


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