[systemd-devel] Using *.path units without races?

[Michael Chapman]

On Fri, 20 Mar 2020, Uwe Geuder wrote:
[...]
> The manual page is not very specific about how that is supposed to work
> IMHO, but I could imagine the following distinction:
> 
> PathExists=, PathExistsGlob=, and DirectoryNotEmpty= are absolute
> predicates. When setting the path unit to waiting one can just check
> whether they are true or not. (After arming inotify of course.) With
> your patch my limited testing was successful.
> 
> However, PathChanged= and PathModified= are relative predicates. You
> cannot just check whether they are true or not. Wouldn't the correct
> implementation
> 
> 1. need to store the applicable timestamp of the path involved when the
>    path unit is set to started and
> 
> 2. when the path unit is set to waiting again it would need to compare
>    the stored timestamp with the current timestamp (again after arming
>    inotify) to catch modifications that happened while the unit was
>    running/inotify not armed
> 
> I don't think I see the timestamps stored at all. So how was this
> supposed to work? Was the intended semantics different?

PathChanged= and PathModified= each map down to a set of inotify 
events. It's the kernel's inotify system that determines whether the 
file changed or modified, not systemd.

For example, consider the IN_MODIFY event. This is the only event that 
distinguishes PathModified= from PathChanged=. inotify generates this 
event on any kind of "data write" operation to the file.

See the inotify(7) manpage for details.