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Date: Fri, 28 Jun 2024 21:17:26 +0900
To: cygwin@cygwin.com
Subject: Re: cygwin application hangs on closing console
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From: Takashi Yano via Cygwin <cygwin@cygwin.com>
Reply-To: Takashi Yano <takashi.yano@nifty.ne.jp>
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Sorry for very late replay.

On Mon, 3 Jun 2024 15:20:32 +0200
Johannes Khoshnazar-Thoma wrote:
> We did more testing and it looks like the name of the event
> that signals console master thread start and end is shared between
> unrelated processes (it uses the console minor which is always (?)
> 0 when running from a powershell).
> 
> So since it is a two-state event (as opposed to a semaphore)
> in theory the following can happen:
> 
> Process A	Process B
> SetEvent(e)
> 		SetEvent(e)
> Waitforevent(e)
> 		Waitforevent(e)

This should not happen. Master thread is unique to console.
get_minor() number is always 0 for the first opened console.
If you open another powershell window and start cygwin process
while the first cygwin process is still active, the get_minor()
returns 1.

Waiting for thread_sync_event is executed only
  if (shared_console_info[unit] && con.owner == myself->pid)
con.owner is in the shared memory which is shared among all
processes started in the same console. Therefore, only the
one process start to wait the event.

> The second SetEvent does nothing. As a result the
> later Waitforevent is stuck (which is what we observe).
> 
> So the question is: why should this event be used in
> unrelated cygwin processes? Is there a technical reason
> we don't understand (yet) for doing that (sharing the event).
> 
> We patched cygwin to use pseudo random event names (the
> tm_usec field of gettimeofday()) and the stuckness vanished.
> So unless there is a reason for sharing the event between
> cygwin processes this patch should work:

Do you really confirm that your patch resolves the issue?
If so, the cause might be some kind of race issue.

-- 
Takashi Yano <takashi.yano@nifty.ne.jp>

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