enricobuehler
fdef4c90ce
A managed gamescope session on Nobara 44 (VM 123) died on essentially every
client connect. The visible symptom was a black screen; underneath,
`punktfunk-gamescope` was SIGABRT crash-looping — 11 coredumps in three minutes
— until `gamescope-session-plus` ran out of retries and came up on the *stock*
`/usr/bin/gamescope` at its default 1920x1080, which looks like a working game
mode and carries none of our capture patches.
punktfunk-gamescope: ../src/pipewire.cpp:88: void destroy_buffer(
pipewire_buffer*): Assertion `false' failed.
#4 __assert_fail
#5 destroy_buffer(pipewire_buffer*).cold
The abort is a use-after-free wearing an `assert(false); // unreachable`.
`pw_buffer->user_data` is associated with its `pipewire_buffer` in exactly one
place, at the bottom of `stream_handle_add_buffer` — after all four `goto error`
paths, whose label is a bare `delete buffer`. And `stream_handle_remove_buffer`
clears `buffer->buffer`, the only route back to the `pw_buffer`, while a still-
`copying` buffer is deleted later on the steamcompmgr thread with no way to
reach the slot. PipeWire recycles `pw_buffer` slots across renegotiations, so
the next remove reads `buffer->type` out of freed memory, falls off the end of
the switch and aborts.
The host sets the session to the client's mode on connect, and that mode change
is what renegotiates the stream — which is why "every connect" was the trigger.
Patch 0007 fixes the association rather than the symptom: set `user_data` at
allocation so it is valid on every path out of `add_buffer` and clear it on the
error path; clear it in `remove_buffer`, the last point both halves are known;
null-check the two consumers. The `default:` arm then logs instead of aborting.
Offered upstream — nothing about it is punktfunk-specific.
Two traps this cost time on, both now written down in the README:
* It is NOT HDR-specific. The abort was first seen right after a 10-bit
stream negotiated, so `PUNKTFUNK_GAMESCOPE_HDR=0` looked like a workaround.
The failing argv carries no `--hdr-enabled` at all.
* `gamescope-session-plus` hides it by falling back to stock gamescope, so a
session existing proves nothing — read the banner.
`.pfhdrN` moves to 5 even though no capability moved: every deployed pfhdr4
binary crash-loops, so an operator needs to be able to tell them apart. All
`>=` thresholds in the host's probe are unaffected.
Also documents `libstdc++-static` as a build dependency — it is punktfunk's
requirement (the script links the C++ runtime statically on purpose), so no
`dnf builddep` will ever pull it, and without it meson fails with a message
naming neither the flag nor the package.
Verified on VM 123 with the patched binary installed: 5 rapid connect/
disconnect cycles plus 3 further sessions, zero new gamescope coredumps (43
before, 43 after), Steam game mode streaming real content at 5120x1440, and
`/tmp/chimeraos-short-session-tracker` never created — the short-session latch
that used to strand the box in plasma was downstream of this crash.