Mail Archives: cygwin/2012/10/19/11:01:00
On Fri, Oct 19, 2012 at 02:17:29PM +0200, Corinna Vinschen wrote:
>On Oct 19 12:26, Andy Koppe wrote:
>> On 13 October 2012 16:38, Christopher Faylor wrote:
>> > On Sat, Oct 06, 2012 at 05:48:44AM +0100, Andy Koppe wrote:
>> >>The issue isn't specific to any of mintty, cat or Ctrl+S; for example,
>> >>I've managed to reproduce it with xterm, hexdump, and just hitting
>> >>Enter. Any other key that sends a keycode will do too. Ctrl+Q isn't
>> >>needed for the freeze to happen. In xterm I've even managed it with
>> >>find, by hitting Enter repeatedly.
>> >>
>> >>If you then look at the situation in ps, you'll see something like this:
>> >>
>> >>O 3396 1 3396 1472 ? 1004 05:11:07 /usr/bin/xterm
>> >>O 3528 4460 3528 528 pty3 1004 05:25:01 /usr/bin/cat
>> >>
>> >>The interesting bit there is the two 'O's in the first column, which
>> >>means both processes are waiting to output. I think what's happening
>> >>is that both of them are trying to write to their side of the
>> >>underlying pty device, but that those writes are blocking until data
>> >>is read from the other side of the pty. Result: deadlock. If the cat
>> >>is killed (possibly with -9, because of its nine lives), the terminal
>> >>happily continues on its way.
>> >>
>> >>So why doesn't this happen more often? Not sure. The speed difference
>> >>between the client process output and the terminal seems to play a
>> >>role here. I can only guess that the issue occurs if a buffer in the
>> >>pty's slave->master pipe overflows and something is written to the
>> >>master->slave pipe at the same time (which is unbuffered?).
>> >>
>> >>I don't understand the pty implementation enough to verify any of
>> >>that, so cgf would need to comment further. Note besides: I couldn't
>> >>make this deadlock happen on Ubuntu.
>> >
>> > This should work in the latest snapshot. I added a polling kludge for
>> > 1.7.17 while I mull over the best way to handle this.
>>
>> Using snapshot 2012-10-16, I confirmed that Ctrl+S during a long cat
>> no longer freezes the terminal and that Ctrl+Q
>>
>> However, I still see the deadlock described above when hitting any
>> other key that sends something, e.g. just Enter.
>
>Too bad. Are you sure? I tried really hard to get a deadlock and could
>not reproduce it anymore under W7. My Enter key is still on paracetamol.
I can't duplicate it either. I do notice that Cygwin doesn't work like
UNIX in that characters are still echoed after CTRL-S is hit. That's
wrong but it looks like making it right will require major surgery.
Must. Resist. YA. Pty. Rewrite.
cgf
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