D18888: Akonadi: fix racy code in (MimeType|Resource)::retrieveByNameOrCreate

[David Faure]

dfaure created this revision.
dfaure added a reviewer: dvratil.
dfaure requested review of this revision.

REVISION SUMMARY
  If two threads (TA and TB) would call retrieveByNameOrCreate() with a
  *different* argument (say A and B), TA might acquire the lock to create A,
  and meanwhile TB will fail the tryLock() and never create B.
  
  Also the final lock/unlock (with nothing in between) was useless.
  
  Solution: lock around lookup+creation, don't use tryLock (which is
  almost always wrong IMHO...), simplify code by using QMutexLocker.
  
  I found this by reading the code, but indirectly this seems to also
  fix akonadiserver locking up in mysql queries during akonaditest
  startup (sometimes), probably because it reduces concurrent querying a bit...

TEST PLAN
  ctest, the "Resource synchronization timed out for akonadi_knut_resource_0"
  errors (that happened approx 30% of the time) are gone.

BRANCH
  Applications/18.12

REVISION DETAIL
  https://phabricator.kde.org/D18888

AFFECTED FILES
  src/server/storage/entities-source.xsl

To: dfaure, dvratil
Cc: kde-pim
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.kde.org/pipermail/kde-pim/attachments/20190209/ac94cfb6/attachment.html>