[Catalyst] Duplicate entries with C::P::Session::Store::DBIC and MySQL

[Tobias Kremer]

Hi list,

I've written about this at least once in the past and still haven't been able to
figure out a proper solution to the problem.

We're getting loads (up to 30-40 per day) of the following fatal error:

Duplicate entry 'flash:***' for key 1 [for Statement "INSERT INTO sessions2 (id)
VALUES (?)" with ParamValues: 0='flash:***'] at
/usr/local/lib/perl5/site_perl/5.8.8/Catalyst/Plugin/Session/Store/DBIC/Delegate.pm
line 52"

I suppose that most of you are never seeing this error because you're using

a) a sane database with proper transactions (although I'm
   unsure DBICs find_or_create makes use of them transparently). I'm
   (at least for the moment) stuck with MySQL and a MyISAM session table.
   Yeah, I know ...

b) a storage backend that doesn't bother if there are duplicate
   entries (FastMmap, Memcached, File, etc) and just overwrites.

The cause obviously seems to be concurrent requests by the same user - mostly
the impatient "F5-hitter"-type :)) So what happens is basically:

1) Request 1 comes in, selects for a flash-entry in the
   session table and doesn't find anything.

2) Request 2 comes in, does the same and comes back with empty hands, too.

3) Request 1 inserts its flash values into the table.

4) Request 2 tries to do the same, fails and the user gets a fatal error message
although nothing _that_ bad happened :(

Now, I know that the time window for this to happen is supposed to be quite
small but the amount of errors I'm getting is worrying. So, IMHO the error
should be handled by the application in a more gentle way (as opposed to
throwing a fatal error) because AFAICT nothing bad will happen if the session
data is just overwritten (which is what most other P::Session backends will do
anyway).

Is there anything speaking against wrapping line 52 of the DBIC backend class
with an eval to trap this duplicate error and ignore it silently (or just warn
about it) except the fact that it's trying to fix something that probably
should be the database's task?

Note that only the flash is causing this problem - I haven't had a single error
with just the session stuff.

--Tobias