kill, a closed window, a reboot: no SessionEnd arrives and the state file
stays. Each case was tried rather than reasoned about, and one of the
three was broken.
A killed session was already handled -- the process is gone, so the file
and its lock are removed within the 20 s liveness tick. An interrupted
hook write left its temporary file behind forever; those are now swept
once they are five minutes old, which is late enough that a hook part-way
through writing one does not lose the update.
The reboot case was the broken one. State files outlive a reboot and pids
are handed out afresh, so "does /proc/<pid> exist" only answers "is some
process wearing that number". Verified by giving an unrelated live process
the pid of a dead session: the ghost sat in the panel as a session waiting
for input, and would have stayed there forever, asking for an answer
nobody could give. The pid is now pinned to the process start time from
/proc/<pid>/stat, recorded when the state is written and compared when it
is read.
Files written before that field existed compare only on existence, as
before, so a session open across the upgrade is not evicted.
An abandoned flock needed nothing: the kernel drops it when the holder
dies, so there is no deadlock to recover from.
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