`output_within` read stdout/stderr only after the child exited, and its doc justified that with
"these helpers emit at most a few hundred KiB, well under any real pipe pressure". A pipe holds
64 KiB. Anything past that blocks the helper in `write()`, so it never exits, the budget kills it,
and a successful query is reported to the caller as `TimedOut` with its answer discarded.
The busiest caller is the one that trips it: `pw-dump` on a populated PipeWire graph clears 64 KiB
routinely and is polled from the 45 s gamescope loops. Confirmed empirically — a child writing
1 MiB into an undrained pipe never exits.
Both pipes are now drained on their own threads, concurrently with the wait.
That makes the joins load-bearing, which exposed the second half: the Unix `tree::Guard` was an
empty stub whose doc claimed `Child::kill` "already ends the only process there is". It never did
for this crate's Linux helpers — `pkexec`, `systemd-run`, `systemctl --user` and the `sh -c`
wrappers all fork — and a surviving grandchild holds the pipes' write ends, so a reader would wait
for an EOF that never arrives. The child is now the leader of its own process group and the guard
`killpg`s it, which is the Unix shape of the Job object the Windows half already used.
Also gates `pf-frame`, `pf-gpu` and `pf-encode` to Windows: every use site of all three is
`cfg(windows)`, and between them they dragged FFmpeg, ash and openh264 into the Linux build for
nothing (sweep item 13.19).
enricobuehler
dd9bbaf1c5