Rust live_marker_owner now REMOVES a stale marker (dead pid, past ceiling,
unparseable) on read — previously stale bytes were only ignored, so a crashed
updater whose Drop never ran refused every later acquire until the 20-minute
ceiling (#77259). pid_is_alive gains pid-0 and zombie handling (Linux /proc
state, macOS ps stat), closing the kill(pid,0) false-positive that kept a
dead-but-unreaped updater 'live'.
Python _pid_is_running gains the same zombie awareness via a new
_process_state probe (Linux /proc, macOS ps), which read_live_update inherits
through update_lock._pid_alive — so the CLI gate self-heals a zombie-owned
marker in seconds instead of 20 minutes (#120635, #125932).
Electron readLiveUpdateMarker consults a new posixProcessState probe
(injectable, fail-open to the signal-0 verdict) so the desktop boot gate no
longer parks on a zombie-owned marker.
Rust fix cherry-picked from #77885 (authorship kept); Python and Electron
are new parity work.
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