contrib/showstack.py

tests: fix a test hang on Windows when setting a debuglock

I have no idea why, but running the `hg -R auto-upgrade debuglock --set-lock`
command near the end of `test-upgrade-repo.t` hangs the test.  It does
background the process and `killdaemons.py` runs without error, but control
doesn't return to `run-tests.py` until the process is manually killed.  I did
notice that `$!` in MSYS is *not* the PID of the process that got backgrounded,
even when a simple `sleep 60 &` is run in MSYS without the *.t file.  When
`killdaemons.py` is run manually with the PID in ProcessExplorer, the
backgrounded process terminates immediately, and returns control to
`run-tests.py`.

This looks like it would be a race, but the test waits 10s for the lock file to
appear before attempting to kill the process, so there's time.  `hg serve` has a
`--pid-file` option to write the pid to the file, but this is only a debug
command, so I'm not bothering with cluttering the command line.

# showstack.py - extension to dump a Python stack trace on signal
#
# binds to both SIGQUIT (Ctrl-\) and SIGINFO (Ctrl-T on BSDs)
r"""dump stack trace when receiving SIGQUIT (Ctrl-\) or SIGINFO (Ctrl-T on BSDs)
"""

import signal
import sys
import traceback


def sigshow(*args):
    sys.stderr.write("\n")
    traceback.print_stack(args[1], limit=10, file=sys.stderr)
    sys.stderr.write("----\n")


def sigexit(*args):
    sigshow(*args)
    print('alarm!')
    sys.exit(1)


def extsetup(ui):
    signal.signal(signal.SIGQUIT, sigshow)
    signal.signal(signal.SIGALRM, sigexit)
    try:
        signal.signal(signal.SIGINFO, sigshow)
    except AttributeError:
        pass