[Nepomuk] Review Request 106748: FileWatch: Use KAuth to automatically raise the inotify watch limit if we run out of watches.

[Simeon Bird]

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http://git.reviewboard.kde.org/r/106748/
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(Updated May 4, 2013, 3:35 p.m.)


Review request for Nepomuk, Sebastian Trueg and Vishesh Handa.


Changes
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Fix post-review induced craziness


Description (updated)
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FileWatch: Use KAuth to automatically raise the inotify watch limit if we run out of watches.

When we run out of watches, use a KAuth action to double the inotify
watch limit (by writing to /proc/sys/fs/inotify/max_user_watches).
At the same time, make the new setting persist across reboots by writing
/etc/sysctl.d/97-kde-nepomuk-filewatch-inotify.conf.

If for some reason raising the limit does not work, print a message to
syslog.

While the limit is being raised, no new watches will be added, only
queued.

==Potential issues==

1. At the moment there is no way to turn this off, except by not using
nepomuk or denying the user the requisite kauth permissions. This is the
sort of thing that people complain about, but I can't really see any
reason to want to do that - you'd be running nepomuk in a "known broken"
configuration, which makes no sense.

2. the action description string is "To avoid missing file changes,
raise the folder watch limit", which could probably be improved.

3. The method of making the change persist across reboots is to write a
file to /etc/sysctl.d, which is a bit anti-social. (note that if /etc is
not writable, it simply does nothing). /etc/sysctl.d should work on all
systemd distros, debian (including derivatives such as ubuntu) and
gentoo.

Part of me wants to make this a separate action, but as I understand it
this would require a second prompt and a second authorization, which
would be a bit annoying. Also, the user's file system isn't going away -
if they wanted a larger limit once, they almost certainly want it again,
so there are limited reasons for not wanting it permanent. But finer
grained permissions are a Good Thing, so I'm not so sure about this.

4. If the user has manually set the watch limit to a too-low number in
sysctl.conf, it could potentially over-ride the file in /etc/sysctl.d,
leading to the prompt appearing on every boot.

Also, I'd just like to mention that I was quite impressed at how easy to
KAuth was to work with.


Diffs (updated)
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  services/filewatch/CMakeLists.txt 338fe8c2b008b1c898d71934e4de3028c0078fca 
  services/filewatch/kinotify.h e795371d922d483bce29e9eea03c1eeb97738355 
  services/filewatch/kinotify.cpp 94babfe437ddfa8c9318b8b29dd8c8a03a4e71b1 
  services/filewatch/nepomukfilewatch.h 66e0112d909a2abefed48d0959323e7f32a5ff9b 
  services/filewatch/nepomukfilewatch.cpp fbbf3db619516e296bc1e4aa07f53808fbe4a4c0 
  services/filewatch/org.kde.nepomuk.filewatch.actions PRE-CREATION 
  services/filewatch/raiselimit.h PRE-CREATION 
  services/filewatch/raiselimit.cpp PRE-CREATION 

Diff: http://git.reviewboard.kde.org/r/106748/diff/


Testing
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Compiled, ran, raised and lowered the limit a few times.


Thanks,

Simeon Bird

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