GH-142591: Tachyon does not handle non-existent file/module

[savannahostrowski]

This validates script/module/PID exists before starting the profiler. If the target doesn't exist, we now fail fast with a clear error instead of launching an empty TUI.

• Issue: Tachyon does not handle non-existent file/module #142591

[savannahostrowski]

Hmm, there's a better fix here. Please hold :D

[pablogsal]

I am not sure this works on windows. On Windows, os.kill() only supports signals SIGTERM, CTRL_C_EVENT, and CTRL_BREAK_EVENT. Signal 0 (which is the Unix idiom for checking if a process exists without sending a signal) is not supported on Windows.

[pablogsal]

This also has a TOCTOU problem: the process may die after you check and before we attach

[pablogsal]

The fix probably needs to be done more on the line of https://github.com/python/cpython/pull/142655/changes (we should handle this there)

[pablogsal]

I've made some improvements to the implementation.

• Refactored the sync waiting logic by extracting _check_process_died() and _wait_for_ready_signal() helper functions for better readability. Replaced select.select with selectors.DefaultSelector which has a nicer interface.

• Changed stderr to always be captured so we can show clean error messages. Added try/except around _run_with_sync() calls to convert RuntimeError to clean sys.exit() messages so non-existent scripts/modules now show clean errors without tracebacks:

• Also fixed absolute path validation which was incorrectly prepending cwd to absolute paths.

[pablogsal]

LGTM

Thanks a lot for the PR @savannahostrowski ❤️