Tickets :: Comment on [#7712] Memcache Session handler, shouldn't unlock if no data in memcache

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Comment	> From looking through the current memcache session handler in CVS HEAD > (and the FW 3 Branch), it seems that there is a locking issue that > can occur on new sessions. > > > > In the code you > > 1) create a lock > > 2) try and read the data from memcache. (in case of a new session > this will be nothing) > > 3) if there is no data then you release the lock > > .... Later on page close > > 1) write the data (creating a new session) > > 2) release lock if set. > > > > Now, the issue is if two requests happen to occur around the same > time when there is no session data (ie.. expired) then there is a > high probability that they will overwrite each other. > > > > Example > > Req 1> New Request > > Req 1> Fetch Session (no data, expired, release lock) > > Req 2> New Request > > Req 2> Fetch Session (No data, expired, release lock) > > Req 2> Processing > > Req 2> Store Session > > Req 2> End Request > > Req 1> Store Session > > Req 1> End Request > > > > Because there is no lock, Req 1 overwrites Req 2s data (and Req 2 is > allowed to actually run even though it should have waited for Req 1). > > > > The worse case is this, (ie login scenario) > > R 1> New Request > > R 1> Fetch Session (no data, expired, release lock) > > R 1> Login attempt > > R 2> Nww Request > > R 2> Fetch Session (no data, expired, release lock) > > R 1> Process Login > > R 2> Do something else > > R 1> Store Session > > R 1> End Request > > R 2> Store Session (Overwriting Requests 1's login session) > > R 2> End Request > > > > This results in the users session being overwritten by another > request that user pushed through and thus, they are not logged in. > > > > The solution would be to NOT release the lock if there is no > session, because we ARE going to be writing at the end of the request.
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