fix: dynamically select timeout method to avoid PicklingError on macOS (#185)

[sbarbett]

This PR fixes a platform-specific issue on macOS where check_domains() fails with a PicklingError due to Python’s multiprocessing defaulting to the spawn start method. The decorated test_starttls function cannot be pickled, which causes the subprocess to fail.

See issue #185 for the problem report and stack trace.

Changes:

• Introduces a private helper function _get_timeout_method() that:

  • Uses signals on macOS (avoids multiprocessing entirely).
  • Uses multiprocessing on Windows (signals are not available).
  • Uses signals on Linux/Unix by default, unless multiple threads are detected (to avoid signal conflicts).

• Applies this selection dynamically to timeout_decorator.timeout(..., use_signals=...) in test_starttls.

Rationale:

Using signal-based timeouts on macOS avoids the need to pickle the decorated function, preventing PicklingError. This approach is cross-platform, does not change behavior for Linux, and gracefully handles multithreaded environments.

Testing:

• Verified on:

  • macOS Sonoma 14.x + Python 3.13.2 (Homebrew) — check_domains() now runs without raising PicklingError.
  • Linux (x86_64 + aarch64) — no regressions observed.

Related Issue

Closes #185.

[steinbrueckri]

@sbarbett THX for your fix! ❤️