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This happens from time to time on my desktop computer:
[151/354/1] test_ioctl
test test_ioctl failed -- Traceback (most recent call last):
 File "/home/antoine/cpython/32/Lib/test/test_ioctl.py", line 36, in test_ioctl
 self.assertIn(rpgrp, ids)
AssertionError: 25242 not found in (23865, 23615)

Can you still reproduce this issue? Do you use any specific flag to run the tests? What OS is this?
I tried on linux, solaris, and aix with ``./python -m test -F test_ioctl`` for about 1k loops, and couldn't reproduce the issue.

Third argument of ioctl should be not " ", but a bytes object with platform dependent length sizeof(pid_t).
Beside this, the code at the top of the module checks the same condition and skip the tests if it is wrong. Race condition happened -- the process has been put into the background between executing these two codes. What command line you run the test?

> Beside this, the code at the top of the module checks the same
> condition and skip the tests if it is wrong. Race condition happened
> -- the process has been put into the background between executing
> these two codes. What command line you run the test?
It happens (or happened) quite often when using the -j option to regrtest.

Do you use -r option?

> Serhiy Storchaka added the comment:
> 
> Do you use -r option?
No. Usually just -j<something>

What is <something>? Can you please expose several lines of regrtest output before and after this error? When "/1" appeared?
I suspect test_gdb:
$ ./python -m test.regrtest -j 2 test_gdb test_gdb
[1]+ Stopped ./python -m test.regrtest -j 2 test_gdb test_gdb

$ ./python -m test.regrtest -j 2 test_gdb test_ioctl test_ioctl test_ioctl test_ioctl test_ioctl test_ioctl test_ioctl test_ioctl test_ioctl
[ 1/10] test_ioctl
[ 2/10] test_ioctl
[ 3/10] test_ioctl
[ 4/10] test_ioctl
[ 5/10] test_ioctl
[ 6/10] test_ioctl
[ 7/10] test_ioctl
[ 8/10] test_ioctl
test_ioctl skipped -- Neither the process group nor the session are attached to /dev/tty
[ 9/10] test_ioctl
test_ioctl skipped -- Neither the process group nor the session are attached to /dev/tty
[10/10] test_gdb
8 tests OK.
2 tests skipped:
 test_ioctl test_ioctl

$ ./python -m test.regrtest -j 4 test_gdb test_ioctl test_ioctl test_ioctl test_ioctl test_ioctl test_ioctl test_ioctl test_ioctl test_ioctl test_ioctl test_ioctl test_ioctl test_ioctl test_ioctl test_ioctl test_ioctl test_ioctl test_ioctl test_ioctl
[ 1/20] test_ioctl
[ 2/20] test_ioctl
[ 3/20] test_ioctl
[ 4/20] test_ioctl
[ 5/20] test_ioctl
[ 6/20] test_ioctl
[ 7/20] test_ioctl
[ 8/20] test_ioctl
[ 9/20] test_ioctl
[10/20] test_ioctl
[11/20] test_ioctl
[12/20] test_ioctl
[13/20] test_ioctl
[14/20] test_ioctl
[15/20] test_ioctl
[16/20] test_ioctl
[17/20] test_ioctl
[18/20] test_ioctl
test_ioctl skipped -- Neither the process group nor the session are attached to /dev/tty
[19/20] test_ioctl
[20/20/1] test_gdb
test test_gdb failed -- Traceback (most recent call last):
 File "/home/serhiy/py/cpython/Lib/test/test_gdb.py", line 716, in test_threads
 cmds_after_breakpoint=['thread apply all py-bt'])
 File "/home/serhiy/py/cpython/Lib/test/test_gdb.py", line 170, in get_stack_trace
 self.assertEqual(err, '')
AssertionError: 'Error occurred in Python command.\n' != ''
- Error occurred in Python command.
18 tests OK.
1 test failed:
 test_gdb
1 test skipped:
 test_ioctl

Bingo!
$ ./python -m test.regrtest -j 4 test_gdb test_ioctl test_ioctl test_ioctl test_ioctl test_ioctl test_ioctl test_ioctl test_ioctl test_ioctl test_ioctl test_ioctl test_ioctl test_ioctl test_ioctl test_ioctl test_ioctl test_ioctl test_ioctl test_ioctl
[ 1/20] test_ioctl
[ 2/20] test_ioctl
[ 3/20] test_ioctl
[ 4/20] test_ioctl
[ 5/20] test_ioctl
[ 6/20] test_ioctl
[ 7/20] test_ioctl
[ 8/20] test_ioctl
[ 9/20] test_ioctl
test_ioctl skipped -- Neither the process group nor the session are attached to /dev/tty
[10/20] test_ioctl
[11/20/1] test_ioctl
test test_ioctl failed -- Traceback (most recent call last):
 File "/home/serhiy/py/cpython/Lib/test/test_ioctl.py", line 66, in test_ioctl_mutate_2048
 self._check_ioctl_mutate_len(2048)
 File "/home/serhiy/py/cpython/Lib/test/test_ioctl.py", line 54, in _check_ioctl_mutate_len
 self.assertIn(rpgrp, ids)
AssertionError: 14270 not found in (14225, 4030)
[12/20/1] test_ioctl
test_ioctl skipped -- Neither the process group nor the session are attached to /dev/tty
[13/20/1] test_ioctl
[14/20/1] test_ioctl
[15/20/1] test_ioctl
[16/20/1] test_ioctl
[17/20/1] test_ioctl
test_ioctl skipped -- Neither the process group nor the session are attached to /dev/tty
[18/20/1] test_ioctl
[19/20/1] test_ioctl
[20/20/1] test_gdb
16 tests OK.
1 test failed:
 test_ioctl
3 tests skipped:
 test_ioctl test_ioctl test_ioctl

Nice catch, Serhiy. I wouldn't have suspected test_gdb.

Perhaps we need a possibility to mark a test that it can't run in parallel. 
I suspect some multiprocess tests fail for same reason.

> Perhaps we need a possibility to mark a test that it can't run in parallel. 
> I suspect some multiprocess tests fail for same reason.
Well, first I would like to know what the underlying problem is.
-j doesn't use multiprocessing, it spawns standalone processes using the subprocess module.

Ah, I see, that's because of the "Skip if another process is in foreground" thing?

Given what _check_ioctl_mutate_len() seems to be designed to check, perhaps we should relax:
 self.assertIn(rpgrp, ids)
into:
 self.assertNotEqual(rpgrp, fill)
or perhaps simply:
 self.assertGreater(rpgrp, 0)

Ah, and the test_ioctl() method needs to be fixed too.

@Antoine/Serhiy I believe that you'd want to follow up on this.

I found that if you redirect GDB’s stdin if fixes all the problems. I don’t fully understand what GDB is doing to change the terminal’s foreground process group.
In my patch I set stdin to a pipe. It is like using DEVNULL, but should be compatible with Python 2 as well.

New changeset 8b8ac7adbf49 by Martin Panter in branch '2.7':
Issue #14771: Redirect GDB's stdin to avoid messing the terminal settings
https://hg.python.org/cpython/rev/8b8ac7adbf49
New changeset c87cc05af8e7 by Martin Panter in branch '3.5':
Issue #14771: Redirect GDB's stdin to avoid messing the terminal settings
https://hg.python.org/cpython/rev/c87cc05af8e7