Akonadi: fix dangling transaction after itemsync failure

Authored by dfaure on Mar 3 2019, 10:10 AM.


Akonadi: fix dangling transaction after itemsync failure

TransactionSequence was emitting result() twice when rolling back.

  • How did this happen?

The TransactionRollbackJob is (automatically) added as a subjob of the
TransactionSequence, so when it finishes, slotResult is called (like for
all subjobs), as well as rollbackResult().
Since the latter emits result() already [mostly for symmetry with
commitResult()], we don't need to do that in slotResult (which doesn't do
it for the case of committing, either).

  • Why is it a problem to emit result() twice?

Well, first, it's against the law in KJob world. In practice,
ItemSyncPrivate::slotTransactionResult was called twice (for the same
TransactionSequence job) which made it decrement mTransactionJobs one
time too many.
As a result, checkDone() finished too early and didn't go into the
"commit transaction" branch for other transactions.
Leaving a transaction "open" is a good recipe for database deadlocks further
down the line.

  • Why did the TransactionSequence roll back in the first place?

In my case because of the infamous and not-yet fixed "Multiple merge
candidates" problem, but it seems that it can also happen when having
items without a part, according to Volker's investigations.
All of these issues still need to be fixed, but at least akonadi seems
to be still usable after they happen.

CCBUG: 399167

Test Plan: Ctrl+L in kmail, with a folder having multiple items for the same RID

Reviewers: dvratil, vkrause

Reviewed By: dvratil

Subscribers: kfunk, kde-pim


Differential Revision: https://phabricator.kde.org/D19487

(cherry picked from commit f1281cf18f40fd69acd61c31b48f5ce43e138eea)
(cherry picked from commit 8ff596c4fe15199b66262c624d8b7c8d8ec7368f)


dfaureAug 16 2019, 9:27 AM
Differential Revision
D19487: Akonadi: fix dangling transaction after itemsync failure
R165:53ad3b6d73d9: Fix collection detaching at the wrong time in attribute()