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From: Michael Albinus <michael.albinus@gmx.de>
To: "Mattias Engdegård" <mattiase@acm.org>
Cc: 35418@debbugs.gnu.org
Subject: bug#35418: [PATCH] Don't poll auto-revert files that use notification
Date: Mon, 29 Apr 2019 09:19:58 +0200	[thread overview]
Message-ID: <87bm0pqnvl.fsf@gmx.de> (raw)
In-Reply-To: <DB035878-69EB-4006-A926-B703F0ABFCE0@acm.org> ("Mattias \=\?utf-8\?Q\?Engdeg\=C3\=A5rd\=22's\?\= message of "Sat, 27 Apr 2019 18:19:36 +0200")

Mattias Engdegård <mattiase@acm.org> writes:

> Actually, it is (arguably) a bug. With two buffers referring to
> distinct hard links for the same file, surely we want a change in that
> file to trigger notification for both! (It's quite an exotic case, not
> the least because Emacs normally recognises hard links as if they were
> the same file name.)

By design, in filenotify.el, we want see only events which are related
to the file *name*. If you want to be notified for both buffers, you
need to watch both file (names).

(Well, re-reading the docstring and the manual for `file-notify-add-watch',
this isn't said explicitly. Likely, we shall precise this.)

> However, with the kqueue back-end, file-notify watches do trigger for
> both, as expected.

Hmm, this is inconsistent. Worth a buig report?

> The reason is that file-notify does not call inotify-add-watch on
> individual files, as in your example above, but on their containing
> directory ("/tmp" in your example). When monitoring a directory with
> two hard links to the same file, and the file is changed, inotify
> (sensibly) only reports a change to one of the links (the one employed
> for the change). Thus, the logic is in the Linux kernel, not in
> filenotify.
>
> For kqueue it is different: here, changes to files are not reported
> when a watch is monitoring their directory, so filenotify.el sets
> kqueue watches on each file instead. The same could be done with
> inotify (and w32notify, if I read the code right), but watching
> directories has certain advantages.

It was a design decision, that filenotify.el implements directory
watching. Since kqueue does not support this, it must be emulated, somehow.

>> One alternative approach could be to analyze the file system device
>> number, as returned by `file-attributes'. By this, we could detect
>> mounted file systems.
>
> Sort of; the interpretation is tricky, and as Eli commented, quite
> platform-specific.

I'm also not in favor of this approach, I just wanted to mention it.

>> But I don't believe that this information is always trustworty, given it
>> isn't used anywhere. And at least for remote files it doesn't tell you
>> anything. Furthermore, mounted file systems are not the only reason that
>> file notification doesn't work, and we need to poll.
>
> What other reasons are you thinking about?

The reasons you have already quoted somewhere else: sometimes, file
notification is not applicable; there are not enough descriptors left; a
file might have been deleted; a file notification process has been
killed silently; you name it ...

Best regards, Michael.





  parent reply	other threads:[~2019-04-29  7:19 UTC|newest]

Thread overview: 101+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-04-24 18:14 bug#35418: [PATCH] Don't poll auto-revert files that use notification Mattias Engdegård
2019-04-24 18:58 ` Eli Zaretskii
2019-04-24 19:36   ` Michael Albinus
2019-04-26 20:46     ` Mattias Engdegård
2019-04-27  9:40       ` Michael Albinus
2019-04-27 16:28         ` Mattias Engdegård
2019-04-25  9:56   ` Mattias Engdegård
2019-04-25 10:04     ` Eli Zaretskii
2019-04-25 18:07       ` Mattias Engdegård
2019-04-27  9:27       ` Michael Albinus
2019-04-27  9:54         ` Eli Zaretskii
2019-04-27 10:23           ` Michael Albinus
2019-04-27 16:19         ` Mattias Engdegård
2019-04-27 16:52           ` Eli Zaretskii
2019-04-28 10:21             ` Mattias Engdegård
2019-04-29  7:53               ` Michael Albinus
2019-04-29 11:06                 ` Mattias Engdegård
2019-04-29 12:18                   ` Michael Albinus
2019-04-29 16:24                     ` Eli Zaretskii
2019-04-29 18:29                     ` Mattias Engdegård
2019-04-29 20:17                       ` Michael Albinus
2019-04-30  3:57                         ` Eli Zaretskii
2019-04-30 11:41                           ` Mattias Engdegård
2019-04-30 12:59                             ` Michael Albinus
2019-04-30 13:56                               ` Mattias Engdegård
2019-04-30 14:19                                 ` Michael Albinus
2019-04-29 16:23                   ` Eli Zaretskii
2019-04-29 19:21                     ` Mattias Engdegård
2019-04-29 19:56                       ` Michael Albinus
2019-04-30 21:09                     ` Mattias Engdegård
2019-05-01 17:45                       ` Eli Zaretskii
2019-05-01 19:41                         ` Mattias Engdegård
2019-05-02 12:18                           ` Michael Albinus
2019-05-02 12:53                             ` Mattias Engdegård
2019-05-02 13:02                               ` Michael Albinus
2019-05-03 12:00                                 ` Mattias Engdegård
2019-05-03 13:44                               ` Eli Zaretskii
2019-05-03 14:47                                 ` Mattias Engdegård
2019-05-04  9:04                                   ` Eli Zaretskii
2019-05-04 11:21                                     ` Mattias Engdegård
2019-05-04 13:41                                       ` Eli Zaretskii
2019-05-04 16:53                                       ` Michael Albinus
2019-05-04 17:08                                         ` Eli Zaretskii
2019-05-04 18:50                                         ` Mattias Engdegård
2019-05-04 19:43                                           ` Michael Albinus
2019-05-04 20:31                                             ` Michael Albinus
2019-05-04 20:46                                               ` Mattias Engdegård
2019-05-05  8:22                                                 ` Michael Albinus
2019-05-05  9:58                                                   ` Mattias Engdegård
2019-05-08  8:34                                                     ` Mattias Engdegård
2019-05-08  8:47                                                       ` Eli Zaretskii
2019-05-08 10:18                                                         ` Mattias Engdegård
2019-05-08 10:58                                                           ` Eli Zaretskii
2019-05-08 11:48                                                             ` Mattias Engdegård
2019-05-08 12:35                                                               ` Eli Zaretskii
2019-05-08 12:58                                                                 ` Mattias Engdegård
2019-05-08 13:09                                                                   ` Michael Albinus
2019-05-08 13:28                                                                   ` Eli Zaretskii
2019-05-08 14:13                                                                     ` Mattias Engdegård
2019-05-08 17:24                                                                       ` Eli Zaretskii
2019-05-08 18:17                                                                         ` Michael Albinus
2019-05-09 11:50                                                               ` Michael Albinus
2019-05-10 15:22                                                                 ` Mattias Engdegård
2019-05-12  8:48                                                                   ` Michael Albinus
2019-05-12 19:49                                                                     ` Mattias Engdegård
2019-05-13 13:35                                                                       ` Michael Albinus
2019-05-14 12:41                                                                         ` Mattias Engdegård
2019-05-14 14:52                                                                           ` Michael Albinus
2019-05-08 10:23                                                       ` Mattias Engdegård
2019-05-09 10:00                                                     ` Mattias Engdegård
2019-05-09 10:48                                                       ` Eli Zaretskii
2019-05-09 11:15                                                         ` Mattias Engdegård
2019-05-10  9:49                                                       ` Michael Albinus
2019-05-10 12:27                                                         ` Mattias Engdegård
2019-05-10 12:43                                                           ` Michael Albinus
2019-05-13 11:34                                                             ` Mattias Engdegård
2019-05-13 15:08                                                               ` Michael Albinus
2019-05-18 17:39                                                                 ` Mattias Engdegård
2019-05-19  9:12                                                                   ` Michael Albinus
2019-05-19 20:25                                                                     ` Mattias Engdegård
2019-05-20  7:30                                                                       ` Michael Albinus
2019-05-20 19:19                                                                         ` Mattias Engdegård
2019-04-29  7:19           ` Michael Albinus [this message]
2019-04-29 11:54             ` Mattias Engdegård
2019-04-29 12:26               ` Michael Albinus
2019-04-29 18:58                 ` Mattias Engdegård
2019-04-29 20:04                   ` Michael Albinus
2019-04-30 15:14                   ` Eli Zaretskii
2019-04-24 19:59 ` Michael Albinus
2019-04-25  9:58   ` Mattias Engdegård
2019-04-25 11:04     ` Michael Albinus
2019-04-25 15:22       ` Mattias Engdegård
2019-04-30  1:03 ` Zhang Haijun
2019-04-30  7:06   ` Michael Albinus
2019-05-01  2:17     ` Zhang Haijun
2019-05-01  2:59       ` Zhang Haijun
2019-05-01  3:10         ` Zhang Haijun
2019-05-02 12:30           ` Michael Albinus
2019-05-02 13:24             ` Zhang Haijun
2019-05-02 12:28         ` Michael Albinus
2019-05-02 12:24       ` Michael Albinus

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