blackbox: do not prevent 'chg init' from working Previously, blackbox always appends to blackbox.log and creates the directory for that file on demand. That could be an issue if: 1. chg starts from `$REPO` directory, so `ui._bbrepo` is set. 2. `rm -rf $REPO`. 3. `chg init $REPO`, blackbox writes something and `init` will fail because `$REPO` directory is non-empty. This patch fixes that by verifying whether vfs exists before re-using it. Differential Revision: https://phab.mercurial-scm.org/D768

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

from __future__ import absolute_import
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 extsetup(ui):
    signal.signal(signal.SIGQUIT, sigshow)
    try:
        signal.signal(signal.SIGINFO, sigshow)
    except AttributeError:
        pass