syslog-ng-mod-mongodb has deadlock problems

[Christian Franke]

Description: Ubuntu 12.04 LTS
Release: 12.04
Package: syslog-ng
Version: 3.3.4.dfsg-2ubuntu1

Having syslog-ng configured with mongodb as a destination, I happened to encounter deadlocks where syslog-ng would stop to process any messages. The impact is quite severe, as almost all applications which write to /dev/log will block indefinitely, making the system completely unusable.

Going through the fixes which had been commited since the version currently in Ubuntu 12.04, I found various fixes concerning mongodb.

There is:
c08618370fc018629d623148452403fb7d0e6f40 afmongodb: Fix a race condition in the queue & wakeup mechanism

And on the 3.4 branch there is also a fix concerning deadlocks:
d5f78c833a20d3eb5da7372dd70bc79f2f971814 afmongodb: Fix a queue notify<->wakeup deadlock

Having both of these patches applied, the issue did not reappear. As these patches only modify code of the mongodb destination module, the risk of introducing regression for users not deploying that modue is minimal, while for those that use syslog-ng-mod-mongodb, the patches help to avoid a very severe problem which, if it occurs, causes denial of service for most parts of the system. Therefore, I think that these patches should find their way into LTS.

Also, there are multiple other bugfixes that have been commited since 3.3.4. It might be a good idea to look at those as well.

[Gergely Nagy (algernon)]

In all honesty, I believe the best would be to push 3.3.6 (or better yet, 3.3.7 due on Oct 31) into the next LTS point release due to the large amount of fixes both small and large that went into that release. I'm not entirely sure whether that's possible, and if so, how, but with my upstream hat on, I strongly recommend 3.3.6+ for production use, instead of trying to port back patches on top of 3.3.4 (in the end, you'll end up with 3.3.6 anyway, save a few minor things).

Packages for 3.3.6 are already available - built for 12.04 as well as for other Ubuntu and Debian releases - from http://asylum.madhouse-project.org/projects/debian/

[Gergely Nagy (algernon)]

Trusty will have 3.5, which has these issues fixed.