The session probe decided "is a desktop live?" by reading /proc/<pid>/comm for
every process of our uid and exact-matching it against "kwin_wayland" /
"gamescope" / "gnome-shell" / "Hyprland". comm is the kernel's name for the
executed FILE, truncated to 15 bytes — not argv[0].
nixpkgs wraps essentially every graphical binary: wrapProgram moves the real
ELF aside to `.<name>-wrapped` and installs a wrapper under the original name,
which then `exec -a "$0"`s the hidden file. So on NixOS the kernel reports
`.kwin_wayland-w` (15 bytes of `.kwin_wayland-wrapped`) while ps/pgrep -a show
a perfectly ordinary `kwin_wayland`, because they read argv. Measured against a
live kernel: `.kwin_wayland-w`, `.kwin_wayland_w` (KWin's own
kwin_wayland_wrapper), `.gamescope-wrap`, all 15 bytes.
Nothing downstream could recover from that one string comparison:
- detect_active_session returned ActiveKind::None on a *running* KDE desktop;
- wayland_display is only resolved for a detected kind, so the connect log
reported wayland="-" even though WAYLAND_DISPLAY was correct;
- pick_compositor's Auto arm returns the DETECTED backend, so a live, fully
working KWin sitting in available() was never chosen — every connect died
"no usable compositor";
- and PUNKTFUNK_COMPOSITOR could not rescue it: pinned_at_a_dead_session
consults the same probe, turning the miss into a hard error instead.
No environment variable reached the comparison — the XDG_CURRENT_DESKTOP
fallback in detect() is only on the pinned path. Capture itself was never at
fault: a decoy process merely NAMED kwin_wayland satisfied the probe and the
stream came up against the real KWin.
Resolve the name through /proc/<pid>/exe (the full, untruncated file name) and
strip the nixpkgs decoration. Both the leading `.` and the trailing `-wrapped`
are required before anything is stripped, so KWin's own real
`kwin_wayland_wrapper` binary keeps its name rather than collapsing into
`kwin_wayland` and handing the probe the parent's PID. The comm fast path is
kept for every ordinary distro — one read, no readlink, and no name that
matched before can stop matching.
Also applied to foreign_gamescope_running, which had the same defect: nixpkgs
wraps gamescope too, so the attach-vs-spawn ladder saw no foreign session.
Tests are fixture-driven rather than spawn-driven on purpose: a stand-in has to
be a real ELF that tolerates being renamed, and /bin/sleep is not one — modern
coreutils is a multi-call binary that dispatches on the executable's own name,
so a copy called `.kwin_wayland-wrapped` exits instantly and /proc/<pid>/exe is
gone before it can be read. That failure looks exactly like this resolver being
broken; it cost one debugging round here and the same trap is already recorded
in punktfunk-host's /proc matcher.
enricobuehler
002702bcec