`CliRunner` terminates with `io.UnsupportedOperation` when using `faulthandler`

[ThomasLoke]

It looks like the output redirection for CliRunner doesn't play nicely with Python's faulthandler library, as it terminates prematurely with the following backtrace (slightly modified to hide local filesystem details):

Traceback (most recent call last):
  File "$CONDA_PREFIX/lib/python3.11/site-packages/click/testing.py", line 408, in invoke
    return_value = cli.main(args=args or (), prog_name=prog_name, **extra)
                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "$CONDA_PREFIX/lib/python3.11/site-packages/click/core.py", line 1078, in main
    rv = self.invoke(ctx)
         ^^^^^^^^^^^^^^^^
  File "$CONDA_PREFIX/lib/python3.11/site-packages/click/core.py", line 1434, in invoke
    return ctx.invoke(self.callback, **ctx.params)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "$CONDA_PREFIX/lib/python3.11/site-packages/click/core.py", line 783, in invoke
    return __callback(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "./test_click.py", line 12, in main
    faulthandler.enable()
io.UnsupportedOperation: fileno

Code for test_click.py that replicates the problem:

import click
from click.testing import CliRunner
import faulthandler
import traceback

@click.command()
@click.option('--flag', type=bool, default=True)
def main(flag):
    print("Executing main function...")
    if flag:
      print("Registering faulthandler")
      faulthandler.enable()
    print("Finished executing main function.")

if __name__ == '__main__':
    flag_value = True
    runner = CliRunner()
    result = runner.invoke(main, ["--flag", flag_value])
    print("Exit code = %d; contents of stdout:\n%s" % (result.exit_code, result.stdout))
    if result.exit_code != 0:
        traceback.print_exception(result.exc_info[1])

The code was then executed by running python test_click.py

I assume the problem is that the CliRunner redirected stderr doesn't implement some details that are required by the faulthandler signal handler registration. Ideally, it should just work; or at least, it should avoid crashing and just do nothing (and perhaps be documented somewhere).

Environment:

• Python version: 3.11.10
• Click version: 8.1.7

[kdeldycke]

@ThomasLoke I proposed a fix in #3244 , can you test it?

[ThomasLoke]

@kdeldycke Took it for a spin, and it works for me! And from a quick peek of the PR, the changes make sense to me, so all good from my side :)