Tachyon: Permission error on subprocesses are not propagated back to the user

Bug report

Bug description:

Running python -m profiling.sampling run --subprocesses <...> on some code that spawns subprocesses will ignore any PermissionError thrown in the child processes when doing a attach to the process, since stderr is forwarded to /dev/null, and the process exits with error code 0 (my mistake, it does exit with exit code 1, just the call is in the helper function). This causes confusion as the only user-visible effect is that the subprocesses sampling data files are not created.

This can happen when the user / python executable / kernel doesn't have the CAP_SYS_PTRACE capability, so the run subcommand works fine, but any attach will fail, which is what happens with the child processes.

CPython versions tested on:

CPython main branch

Operating systems tested on:

Linux

Linked PRs

• gh-155032

Start with Lib/profiling/sampling/_child_monitor.py at the subprocess attach path and Lib/profiling/sampling/main.py at the run command entry point. Reproduce the command on Linux without CAP_SYS_PTRACE, then verify that a child-process PermissionError is visible to the user rather than only appearing as missing sampling data. PR gh-155032 is already linked to this issue.