[Kde-pim] Review Request: Fix deadlock situation when open mail (problem in nepomuksearch.cpp)

[Andreas Cord-Landwehr]

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http://git.reviewboard.kde.org/r/105599/
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Review request for Akonadi.


Description
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(I posted my problems already at the kdepim list: http://lists.kde.org/?l=kde-pim&m=134236644810213&w=2, the following is copy&paste from that mail, concluding some infos about the patch)

Introduction:
In KMail I have a couple of mails that when I click on them (i.e. to display) I get the "Retrieving Folder Contents" bluescreen. With GDB I followed the problem into the Akonadi sources: When trying to display the mail at some point an Akonadi Connection is created as well as AkonadiConnection::run() is exucuted, which yields (after some more calls) to search.cpp:55 where a 
NepomukSearch object is created and (one line later) a search is executed. The following I give in detail to explain how there a threading issues emerges from the calls (especially here in case of a specific mail I try to display):

Step 1. Object creation of "NepomukSearch* service" in search.cpp:55
This causes the NepomukSearch ctor to create:
  mSearchService = new Nepomuk::Query::QueryServiceClient( this );
  mIdSearchService = new Nepomuk::Query::QueryServiceClient( this );
Futher in the ctor, for both there newEntries-signals are connected to methods 
of NepomukSearch, ie.
  connect( mSearchService, SIGNAL(newEntries(QList<Nepomuk::Query::Result>)),
           this, SLOT(hitsAdded(QList<Nepomuk::Query::Result>)) );
  mIdSearchService = new Nepomuk::Query::QueryServiceClient( this );
  connect( mIdSearchService, 
SIGNAL(newEntries(QList<Nepomuk::Query::Result>)),
           this, SLOT(idHitsAdded(QList<Nepomuk::Query::Result>)) );

Step 2. search:cpp:56 service->search(...) is called. There we get:
* mSearchService->blockingQuery(...) issues a query
* this query finds 6 results (in my example)
* by former connection of mSearchService, for each result 
NepomukSearch::hitsAdded is invoked
* FOR-EVERY-OF-THESE-RESULTS it happens in NepomukSearch::hitsAdded(...)
  * the result has akonadi ID => mIdSearchService->blockingQuery(...) is 
invoked
* THE-LAST-INVOKED-QUERY finishs and invokes by the, in the ctor created, 
connection NepomukSearch::idHitsAdded(...)
  * uses addHit(...) to add result to result-list
  * then idHitsAdded(...) returns
  * the blockingQuery THE-LAST-INVOKED-QUERY returns from the call from 
QueryServiceClient::blockingQuery
<<< AKONADI STALLS >>>

Conclusion:
I verfied that virtuoso finished all queries as well as that none of the other blockingQuery calls returned. (Akonadi is Git master, KDEPIM/KDEPIMlibs is KDE 4.9 RC2.)
I suspect that by return from the blockingQuery the still blocking thread is invoked. Hence, the next blockingQuery issued by mSearchService can never return.

Solution:
In the attached patch I move the corresponding code that was formerly used by mIdSearchService to request missing IDs in parallel to the exuecuted mSearchService blocking query is moved to a point where the initial blocking query is finished an the described deadlock cannot happen.


Diffs
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  server/src/nepomuksearch.h c502bd6 
  server/src/nepomuksearch.cpp da7b19a 

Diff: http://git.reviewboard.kde.org/r/105599/diff/


Testing
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I applied the patch to master and tested for several hours. Without the patch I got deadlockes deterministly when trying to open specific mails. With the patch I did not get any new deadlock.


Thanks,

Andreas Cord-Landwehr

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