Cygwin 3.1.2: Every call of exec(2) in the session starts to create a console window after some TTY outputs from programs compiled with "-mwindows"

[Koichi Murase <myoga.murase@gmail.com>]

Hi, I updated Cygwin from 3.0.7 to 3.1.2 and have two problems.  This
is the report for one of them.  (For the other one, read(2)
occasionally fails on the new cygwin console for some settings, but
I'm still trying to pin down the issue.)

Cygwin Version:

  Cygwin 3.1.2 x86_64 on Windows 10.  I also confirmed that the
  problem is not fixed with the latest commit 85aff2830 of
  newlib-cygwin.  Note that Cygwin 3.0.7 doesn't have this problem.


Brief Description:

  As in the subject, this problem is caused after some TTY outputs from
  programs compiled with "-mwindows".  For example, after running the
  command "mintty.exe --version" in a terminal, every "exec" called in
  the processes in the same session starts to create a console window
  which will be closed instantaneously.  As results, the display is
  always flashing when some scripts are running, the shell scripts
  becomes very slow, some of user inputs to the terminal go to the
  created window and get lost, and CPU usage extremely increases.
  This problem was not present in Cygwin 3.0.7.


Repeat-By:

  The following is one example to reproduce it:

  1. Open Cygwin Terminal (mintty)
  2. Run "mintty --version"
  3. Run some shell scripts, e.g., "for i in {0..99}; do date; done | uniq -c"


Details:

* The terminal

  This phenomena is not reproduced with every terminal.  For example,
  it is reproduced with xterm, lxterminal and mintty, but not with the
  cygwin console (which can be started, e.g., by directly calling
  C:\cygwin64\bin\bash.exe from Windows) and GNU Screen.  It appears
  that it is caused when the terminal process that holds the PTY
  master also has GUI windows.

* The trigger command

  This phenomena is started when a program with
  IMAGE_SUBSYSTEM_WINDOWS_GUI in its PE header (which can be created,
  for example, by passing the option "-mwindows" to "gcc") outputs
  something to TTY.  One example for such a program is "mintty
  --version".  But this is not a specific problem to "mintty".  The
  simplest example "test1.exe" that causes the phenomena can be
  created by the following command:

  $ echo 'main(){puts("A");}' | gcc -xc - -mwindows -o test1.exe

  The above program looks useless, but of course it is a reduced case
  just to reproduce the phenomena.  Actually, in one of my programs, I
  need to output something to TTY in some case, and need to create a
  GUI window (based on Windows API) in the other cases.  Specifically,
  my program is basically a GUI program (terminal emulator) and does
  not want to show a console window when it is launched from, e.g.,
  Explorer, so I need to specify "-mwindows" to tell Windows not to
  create a console.  But when my program is called inside terminal
  with a special option, it runs in a TUI mode, so still I want to
  output something to TTY when some conditions are satisfied.

* exec?

  A new console window appears to be created when a new program is
  loaded with "exec".  A console window is not generated when a
  subshell is created in the shell (which implies "fork" does not create
  a console window) while the command "exec some-command" in the shell
  creates a new console window.


Takashi: Thank you very much for your hard work on recent PTY issues!
Today I checked cygwin-patches mailing list to check possible related
fixes and found your recent patch on "disable_pcon", which is the very
feature I wanted for testing my terminal application with the legacy
console mode.  Thank you!


Best regards,

Koichi

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