tests/regress.py: prevent WiFiDriverDemo process leaks

[josephnef]

Why

Long-running Popens spawned by run_cell can outlive the script if:

• an outer timeout / SIGKILL hits Python before run_cell's finally block executes (orphan reparents to init, PPID=1)
• an unhandled exception aborts the run

Leaked WiFiDriverDemo processes keep the USB device claimed, which manifests in the kernel as:

usbfs: process N (WiFiDriverDemo) did not claim interface 0 before use

flooding dmesg, and prevents aircrack-ng/88XXau from binding in the VM on subsequent cells (can't set config #1, error -32).

A leaked process during the 2026-05-30 / -31 8821AU 5GHz UNII-2 investigation ran for 2:58 hours against the 8812AU and wedged the chip for the whole session — every "hardware-wedge" symptom was downstream of that.

What changes

Defense in depth (two independent layers):

1. atexit + SIGTERM/SIGINT/SIGHUP handler walks a module-level _ACTIVE_LOCAL_PROCS set and SIGKILLs every Popen that's still running. Re-raises the signal at default disposition so the exit code reflects how the script died.

2. preexec_fn for each spawned Popen calls prctl(PR_SET_PDEATHSIG, SIGKILL) + os.setsid(). The PDEATHSIG asks the kernel to kill the child the moment its parent dies — catches the pathological SIGKILL-from-outer-timeout case where Python has zero opportunity to run cleanup code. Replaces the previous start_new_session=True (which only handled the setsid half).

Wired into _spawn_devourer_rx, _spawn_devourer_tx, _spawn_sniffer (all local Popens). _spawn_kernel_rx / _spawn_kernel_tx remain untouched — those run remotely via ssh and die naturally when the ssh tunnel breaks.

_install_cleanup_handlers() is called once at the top of main().

Test plan

• PR_SET_PDEATHSIG works: kill -KILL \$(python_pid) mid-spawn — WiFiDriverDemo child dies within ms (verified by ps --ppid).
• Signal-handler path works: kill -TERM \$(python_pid) mid-spawn — handler runs _kill_all_local_procs, child gone before harness exit.
• End-to-end matrix: full 4-cell matrix at ch100 — zero leaks after the run (ps -ef | grep WiFiDriver | wc -l == 0).
• CI matrix

🤖 Generated with Claude Code