[Akonadi] [Bug 399167] Akonadi enters deadlock state which makes it completely unuseable

[David Faure]

https://bugs.kde.org/show_bug.cgi?id=399167

--- Comment #24 from David Faure <faure at kde.org> ---
Git commit 15c91a0ac93051465b37807efceb6e9fd36cb73b by David Faure.
Committed on 16/08/2019 at 09:27.
Pushed by dfaure into branch 'Applications/18.12'.

Akonadi: fix dangling transaction after itemsync failure

Summary:
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.

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

Tags: #kde_pim

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

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

M  +0    -2    src/core/jobs/transactionsequence.cpp

https://commits.kde.org/akonadi/15c91a0ac93051465b37807efceb6e9fd36cb73b

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