bug#26126: 26.0.50; file-notify-rm-watch removes arbitrary watches

[Michael Albinus <michael.albinus@gmx.de>]

Andreas Politz <politza@hochschule-trier.de> writes:

Hi Andreas,

Thanks for the bug report.

> The descriptor returned by file-notify-add-watch is of the form (ID . DIR)
> (at least when using inotify) and these descriptors are stored in a
> hash-table using equal as comparator.

Yes.

> If multiple clients watch the same file and one of them removes its watch
> via file-notify-rm-watch, the result is unpredictable. I.e. the function
> removes some watch (I believe it is the last one added), but not necessarily
> the one associated with the client calling file-notify-rm-watch on the
> descriptor it got from calling file-notify-add-watch.

With the returned descriptors, it cannot be decided which watch has to
be removed, because both descriptors are equal. So we could decide
either to remove all watches for a given file if such a descriptor is
passed to `file-notify-rm-watch', or we must adapt
`file-notify-add-watch' to return different descriptors. I'm undecided
yet.

Note, that this problem seems to be specific to inotify. Other libraries
do not suffer from this problem, I believe.

> I've attached a test case for this.

Thanks. I've added this, modified, to `file-notify-test02-rm-watch' for
my internal tests. Will be pushed to master once I have a solution for
the problem.

> -ap

Best regards, Michael.