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fix(vdisplay/gamescope): capture-loss rebuild probes must not steal the box session

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Observed live on a Deck host: the user switched Game Mode → Desktop, KDE came
up in 0.8 s — and the capture-loss rebuild's FIRST detection ran inside that
gap, still said Gaming, and its gamescope re-acquire restarted
gamescope-session.target. On SteamOS that steals the seat: the user was
yanked out of the KDE session they had just chosen, the two session managers
fought (job canceled / dependency failed), no gamescope node appeared within
30 s, and the stream died at the 40 s rebuild budget.

A rebuild probe acting on possibly-stale detection must never stop, relaunch,
or take over box sessions. New pf_vdisplay::rebuild_probe_scope (RAII,
counted): while held, the gamescope managed/takeover create paths only attach
to a live node and fail fast otherwise — no stop_autologin_sessions, no
session-plus relaunch, no gamescope-session.target restart. The capture-loss
loop holds it for the first 4 s after a loss (the detection-ambiguity
window); after that, destructive rebuilds are allowed again, so a genuine
switch INTO Game Mode still gets its headless takeover. Probe failures are
instant, so the loop now paces itself at ~2 Hz instead of spinning.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
This commit is contained in:
Enrico Bühler
2026-07-21 11:00:02 +02:00
parent 46d09ed973
commit 333c8979e9
3 changed files with 75 additions and 1 deletions
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crates/punktfunk-host/src/native/stream.rs
+14 -1
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@@ -1884,7 +1884,15 @@ pub(super) fn virtual_stream(ctx: SessionContext, prepared: Option<PreparedDispl
// connected, frozen on the last frame, and the stream resumes when the new output
// appears — no reconnect.
const REBUILD_BUDGET: std::time::Duration = std::time::Duration::from_secs(40);
let rebuild_deadline = std::time::Instant::now() + REBUILD_BUDGET;
// Attach-only holdoff: for the first seconds after a capture loss the session
// detection can be STALE (the new session isn't up yet), and a rebuild acting on
// a stale "Gaming" answer restarts gamescope-session.target — which on SteamOS
// steals the seat back from the session the user just switched to (observed
// live). While the holdoff lasts, builds run under a vdisplay rebuild-probe
// scope: attach to live outputs only, never stop/relaunch/take over sessions.
const PROBE_HOLDOFF: std::time::Duration = std::time::Duration::from_secs(4);
let loss_at = std::time::Instant::now();
let rebuild_deadline = loss_at + REBUILD_BUDGET;
let (new_cap, new_enc, new_frame, new_interval, new_node_id, new_display_gen) = loop {
// Follow the active session unless an explicit PUNKTFUNK_COMPOSITOR pin forbids
// retargeting (then we stick to the pinned backend and just rebuild it).
@@ -1918,6 +1926,8 @@ pub(super) fn virtual_stream(ctx: SessionContext, prepared: Option<PreparedDispl
}
}
}
let _probe = (loss_at.elapsed() < PROBE_HOLDOFF)
.then(crate::vdisplay::rebuild_probe_scope);
match build_pipeline_with_retry(
&mut vd,
cur_mode,
@@ -1948,6 +1958,9 @@ pub(super) fn virtual_stream(ctx: SessionContext, prepared: Option<PreparedDispl
}
tracing::warn!(error = %format!("{e2:#}"),
"capture lost — new session not up yet, retrying");
// Probe failures are instant (attach-only bail) — pace the loop so
// re-detection runs at ~2 Hz instead of spinning.
std::thread::sleep(std::time::Duration::from_millis(500));
}
}
};