bug#21435: 25.0.50; file-notify has problems after renames

[Tassilo Horn <tsdh@gnu.org>]

Eli Zaretskii <eliz@gnu.org> writes:

>> I stepped through inotify_callback in inotify.c which can read 64
>> byte from inotifyfd where the size of one inotify event is 32 and
>> that produces 2 emacs events (although I cannot see which events are
>> created).  The Locals GUD frame always shows just nil for the `event'
>> local variable even after the assignments to it and I stepped into
>> the then-branch of an `if (!NILP(event.arg))' which makes it pretty
>> clear that it cannot be nil.  Why is that?
>
> Could be some gud bug.  Do you see the same when you invoke GDB from
> the shell?

Oh, my GDB foo on the command line ends with making backtraces. ;-)

But just typing in "info locals" in the *gud-bootstrap-emacs* buffer is
about the same, no?  And that shows

  event = {kind = FILE_NOTIFY_EVENT, part = scroll_bar_nowhere, code = 0, modifiers = 0, x = 0, y = 0, timestamp = 0, frame_or_window = 0, arg = 0}

whereas the relevant line in the locals GUD window is just

  struct input_event event        nil

So that looks like a bug.  I guess I should report this, or will you fix
this right now?

>> Is that expected that when edebugging event handlers one might miss
>> events?
>
> I always use 'message' when debugging event-driven code.  I just don't
> trust edebug enough in these cases.

Ok.

>> Anyway, eventually I found and fixed the culprit which simply was
>> that the watch descriptors of the pending and the current event were
>> compared with `eq' which is not valid because inotify descriptors are
>> conses.  I've changed the comparison to `equal' which fixes the
>> double-reporting issue.
>
> Thanks.  But meanwhile Michale explained that this behavior was on
> purpose, due to a recent change.  Do you see 2 events only when a file
> was moved to a different directory, or also when it is renamed in the
> same directory?

When renaming in the same directory.  I think Michaels case is where you
move a file from watched directory A to watched directory B, and then
you want the rename event reported to both handlers.  The case I fixed
is just that both source and target watch descriptor are actually the
same but not identical Lisp objects.

Bye,
Tassilo