Re: Rare character glitch in pty?

[Takashi Yano <takashi.yano@nifty.ne.jp>]

On Fri, 4 Aug 2023 17:52:11 +0900
Takashi Yano wrote:
> Hi Thomas,
> 
> On Wed, 2 Aug 2023 08:32:39 +0200
> Thomas Wolff wrote:
> > Am 02.08.2023 um 00:35 schrieb Takashi Yano:
> > > On Tue, 1 Aug 2023 23:05:18 +0200
> > > Thomas Wolff wrote:
> > >> I had reported this to the the main cygwin list:
> > >>
> > >> Am 20.07.2023 um 08:33 schrieb Thomas Wolff via Cygwin:
> > >>> There was a report that on switching terminal windows with the mouse,
> > >>> focus event reports (as enabled with ESC[?1004h) would sometimes get
> > >>> mangled.
> > >>> This is further described in issue reports to mintty/wsltty
> > >>> (https://github.com/mintty/wsltty/issues/335) and also to tmux
> > >>> (https://github.com/tmux/tmux/issues/3601).
> > >>>
> > >>> I could reproduce the issue without either of tmux or WSL. However, it
> > >>> happens only once in a few hundred instances:
> > >>> The escape sequence is changed so that the initial ESC is replaced by
> > >>> one of the two subsequent characters, so instead of ESC[O, either of
> > >>> [[O or sometimes O[O would be sent to the foreground application.
> > >>>
> > >>> I traced the issue down to the pty that connects mintty and its client
> > >>> application, verified that the correct sequence is passed into write()
> > >>> at the pty and the mangled sequence appears at read() in the client
> > >>> process. In addition to a minimised test program that behaves like
> > >>> `cat -v`, I also also put the test output directly into the client
> > >>> process within the mintty code.
> > >>> This leaves the only conclusion that the issue must be happening
> > >>> somehow within pty handling, maybe in the context of some timing
> > >>> condition or Windows messages being passed to mintty.
> > >>> Anyhow, I was about to prepare testing with older mintty versions, in
> > >>> case I overlooked something, when the OP reported that the issue does
> > >>> not happen with cygwin 3.2.0.
> > >>> Is there any recent change that could possibly cause such weird
> > >>> behaviour?
> > >>> Thomas
> > >>>
> > >> I have now automated the test case to chase the issue and associate it
> > >> with a cygwin version. As unlikely as it may appear, results suggest
> > >> that there is a case within the pty handling that may change sequences
> > >> sent through it once in a while, more likely on a system under heavy load.
> > >> I took some of the latest cygwin snapshots (by the way, there are no
> > >> newer ones on the snapshots page):
> > >> I can reproduce the issue with
> > >>       3.3.5-341.x86_64.snap 2022-02-17 11:53 UTC
> > >> while it did not occur with
> > >>       3.4.0-341.x86_64.snap 2022-01-27 14:34 UTC
> > >> Weird enough, the failing newer one is a cygwin 3.3.5 although the older
> > >> one is a 3.4.0 version.
> > > Can you provide the automated test case? I would like to
> > > investigate what is happning.
> > 
> > Manual test case:
> > 1. Run `mintty echokey1` with the attached test program that runs like 
> > `cat -v` and detects the glitch
> > 2. Run a second test window, same setup to increase the hit rate
> > 3. Switch these 2 windows by clicking them in change a few hundred 
> > times, until the test program outputs "FAIL"
> > 
> > Automated test case:
> > 1. Use the tool TinyTask (https://www.tinytask.net/) to save the 
> > clicking. Sorry, I didn't manage to replace this with a self-made click 
> > simulator.
> > 2. Place the 2 test windows in a specific position (e.g. covering the 
> > two screen halves in a top position)
> > 3. Use the Record function of the tool and click to switch windows a 
> > dozen times, stop recording
> > 4. Save the sequence as a TinyTask script, or use mine (2w.rec 
> > attached), better: load the script (Open), then create a standalone test 
> > executable (button "exe") to create tinytask2w.exe (I can send that to 
> > you if you like)
> > 5. Optionally, create some background load on your system
> > 6. Minimize other windows in case the test script does not hit the test 
> > windows
> > 7. Run the test script minptytest (attached) to start the 2 windows and 
> > run the TinyTask standalone test in a loop; the script detects a marker 
> > file created by the test program; it also detect premature termination 
> > of a window (e.g. by hitting ^D in it)
> 
> Thanks for the test case. I could reproduce the issue using
> the automated test case.
> 
> I looked into this problem, and found the cause.
> 
> Previously, though readahead buffer handling in pty master was not
> fully thread-safe, accept_input() was called from peek_pipe() thread
> in select.cc.
> 
> The mechanism of the problem is:
> 1) accept_input() which is called from peek_pipe() thread calls
>    eat_readahead(-1) before reading readahead buffer. This allows
>    writing to the readahead buffer from another (main) thread.
> 2) The main thread calls fhandler_pty_master::write() just after
>    eat_readahead(-1) was called and before reading the readahead
>    buffer by accept_input() called from peek_pipe() thread. This
>    overwrites the readahead buffer.
> 3) The read result from readahead buffer which was overwritten is
>    sent to the slave.
> 
> This issue has existed this decade.
> 
> I'll submit a patch for this issue shortly.

Now, cygwin-3.5.0-0.370.g3b4f6217c371 (Test) is ready.
Please test.

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