As documented in systemd.path(5):
When a service unit triggered by a path unit terminates (regardless
whether it exited successfully or failed), monitored paths are
checked immediately again, and the service accordingly restarted
instantly.
This commit implements this behaviour for PathExists=, PathExistsGlob=,
and DirectoryNotEmpty=. These predicates are essentially
"level-triggered": the service should be activated whenever the
predicate is true. PathChanged= and PathModified=, on the other hand,
are "edge-triggered": the service should only be activated when the
predicate *becomes* true.
The behaviour has been broken since at least as far back as commit
8fca6944c2 ("path: stop watching path specs once we triggered the target
unit"). This commit had systemd stop monitoring inotify whenever the
triggered unit was activated. Unfortunately this meant it never updated
the ->inotify_triggered flag, so it never rechecked the path specs when
the triggered unit deactivated.
With this commit, systemd rechecks all paths specs whenever the
triggered unit deactivates. If any PathExists=, PathExistsGlob= or
DirectoryNotEmpty= predicate passes, the triggered unit is reactivated.
If the target unit is activated by something outside of the path unit,
the path unit immediately transitions to a running state. This ensures
the path unit stops monitoring inotify in this situation.
With this change in place, commit d7cf8c24d4 ("core/path: fix spurious
triggering of PathExists= on restart/reload") is no longer necessary.
The path unit (and its triggered unit) is now always active whenever
the PathExists= predicate passes, so there is no spurious restart when
systemd is reloaded or restarted.
708961c701