Cygwin-3.1.3: read(2) from /dev/consN returns unexpected EOF after select(2) and redirection

[Koichi Murase <myoga dot murase at gmail dot com>]

Hi, this is another report related to Cygwin console.

Description:

  In a noncanonical mode of console (where tty is /dev/consN), if
  there is some unprocessed inputs from the user side (i.e., terminal
  side), the combination of select(2) (for read) and redirection
  causes the next attempt of read(2) to return EOF [i.e., read(0, buf,
  sz) = 0] even though the TTY is still alive.  Because of this
  behavior, the shell session unexpectedly closes because the shell
  considers the TTY has been closed.

  This only affects the first attempt of read(2) (in each process)
  after the redirection.

Repeat-By:

  This is a reduced test case to reproduce the problem:

  1. Open cygwin console (pseudo console mode) with Bash

    This can be done by, for example, opening Run dialog with "Win +
    R", typing "C:\cygwin64\bin\bash" and finally pressing "Enter".

  2. Run the following command

    $ stty -icanon; printf '\e[6n'; read -t 0; : < /dev/null; stty sane

    As a result, the terminal response CPR (CSI Pn ; Pn R) to DSR(6)
    request (CSI 6 n) is expected to be inserted in the command line.
    It should look like the following:

    $ 7;1R

    However, in the console, the Bash session is closed by the above
    command because Bash receives EOF from the TTY and considers the
    TTY session has been closed.


  Note: Only the first attempt of read(2) after the redirection
  returns EOF.  This can be confirmed by the following command where
  the Bash session does not close.

    $ stty -icanon; printf '\e[6n'; read -t 0; : < /dev/null; read;
    stty sane

  Note: Also the first read(2) fails for each forked process in the
  session:

    $ stty -icanon; printf '\e[6n'; read -t 0; : < /dev/null; wc -c;
    (read||echo fail;read||echo fail2); read; stty sane
    0
    fail

    success
    $

Repeat-By (2):

  This demonstrates the problem more explicitly by system calls.

  1. Open cygwin console (pseudo console mode)

  2. Compile and run the attached program `min1.cpp'.

    $ g++ -o min1.exe min1.cpp
    $ ./min1

  The expected result is something like

    $ ./min1
    CHR CHR CHR CHR CHR CHR CHR
    $

  However, in the console, it prints EOF and exit.  Also the terminal
  response CPR appears to be lost.

    $ ./min1
    EOF
    $


Thank you,

Koichi

Attachment: min1.cpp
Description: Binary data

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