contrib/showstack.py

demandimport: don't delay threading import

A recent cpython change breaks demandimport by importing threading
locally in importlib.util.LazyLoader.exec_module; add it (plus warnings
and _weakrefset, which are imported by threading) to demandimport's
ignore list.

```
Traceback (most recent call last):
  File "/usr/bin/hg", line 57, in <module>
    from mercurial import dispatch
  File "<frozen importlib._bootstrap>", line 1360, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1331, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 935, in _load_unlocked
  File "/usr/lib/python3/dist-packages/hgdemandimport/demandimportpy3.py", line 52, in exec_module
    super().exec_module(module)
  File "<frozen importlib.util>", line 257, in exec_module
  File "<frozen importlib._bootstrap>", line 1360, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1331, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 935, in _load_unlocked
  File "/usr/lib/python3/dist-packages/hgdemandimport/demandimportpy3.py", line 52, in exec_module
    super().exec_module(module)
  File "<frozen importlib.util>", line 267, in exec_module
AttributeError: partially initialized module 'threading' has no attribute 'RLock' (most likely due to a circular import)
```

Ref: https://github.com/python/cpython/issues/117983
     https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1076449
     https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1076747

# 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