Two independent reasons a Nobara box could never stream from a gamescope session,
both found on glass (VM 123, Nobara 44, RTX 5070 Ti).
**1. The session ran a stock gamescope, so the host refused it.**
Nobara's `gamescope-session-plus` builds its command as
GAMESCOPECMD="/usr/bin/gamescope \
and reads `GAMESCOPE_BIN` NOWHERE. All three of our spawn levers miss at once: the env
var is ignored, and an absolute path cannot be redirected by a PATH shim. So the session
ran stock gamescope, the capability probe rejected it, and every session died with
"pipeline build failed (out of retries) … it ignored GAMESCOPE_BIN / the PATH shim".
`~/.gamescope-cmd.log` — which the script writes with the exact command it ran — settles
that in one line, and is the first thing to read on any such report.
Fixed by binding our wrapper over `/usr/bin/gamescope` inside the transient unit's mount
namespace (`BindReadOnlyPaths`). Deliberately a bind, not a replacement: punktfunk-gamescope
ships under its own name precisely so it sits BESIDE the distro package, and the bind is
scoped to the session — nothing outside it sees the redirect and nothing is written to
`/usr`. Skipped when the resolved binary already IS `/usr/bin/gamescope`.
**2. With the patched gamescope finally running, every Vulkan client died — black screen.**
The box's `VkLayer_FROG_gamescope_wsi` ships with the DISTRO's gamescope and speaks its
`gamescope_swapchain` protocol. Ours disagrees, so the compositor rejects the client's
`swapchain_feedback` ("message too short") and drops it. Steam never paints; there is no
other symptom, which is what makes it expensive to find.
Measured with `vkcube` under each build, layer on:
ours 3.16.25-17 ON -> 1 rejected client
ours 3.16.25-17 OFF -> 0
OLD pin 3.16.25-4 ON -> 1 rejected client
stock 3.16.23.2 ON -> 0
⭐ The upstream protocol XML is BYTE-IDENTICAL between the distro's commit (5cdb5b0) and
our pin — same interface version, same `uuuuuus` signature — so this is the distro patching
gamescope, not a version bump. Hence the gate is "do the upstream triples differ", not a
floor, and an unreadable version on either side leaves the layer alone rather than degrading
a box that works (Bazzite/SteamOS, where it has always been fine).
⚠⚠ The old pin fails identically, so REVERTING the pin bump fixes nothing here — this is
pre-existing, not a regression from 5fb8dce4.
Verified against the UNPATCHED distro script, reproducing exactly what this code emits:
the session's own log reports `punktfunk-gamescope version 3.16.25-17-ga87390d+pfhdr4`,
with 0 swapchain_feedback errors, 0 client-communication errors and 0 aborts.
Gate: `scripts/xcheck.sh linux clippy` clean (0 warning/error lines), `cargo fmt` clean.
Non-vacuity re-verified per the xcheck note — a planted type error in the new function
produced 3 errors, and removing it went back to Finished.
Still open, deliberately NOT addressed here: a 10-bit HDR stream aborts gamescope in
`destroy_buffer` (upstream `pipewire.cpp:88`), which is a separate defect.
enricobuehler
3500e95660