Review Request: Make Sonnet::Speller::setLanguage threadsafe, fixing segfaults in krunner

[Thomas Lübking]

> On Sept. 6, 2012, 12:29 p.m., David Faure wrote:
> > A number of comments on the implementation, but also a more general comment: does it even make sense to use a Speller in multiple threads, and to  change the language from one thread while another one is using the speller for spell-checking? It sounds like, even if a mutex/lock can prevent crashes, this is a weird thing to do anyway, since you have no idea at which point the spell-checking will switch to another language... could happen in the very middle of a sentence...
> > 
> > Maybe it would make more sense to "post" the language change operation to the spell-checking thread using the same mechanism as the one used to "post" spellchecking requests to it? (Disclaimer: I know nothing of the krunner architecture).
> 
> Simeon Bird wrote:
>     Thanks for the review. 
>     
>     I agree that this is a slightly weird thing to do, but, as you surmise, it was the only way I could think of to make the feature work properly. 
>     
>     As far as I understand it (from reading the documentation), the way krunner works is to call Plasma::AbstractRunner::match in a new thread for every runner every time input is entered. 
>     
>     So if I enter:
>     
>     "spell hello"
>     
>     I will be called with 's' 'sp' 'spe'...and so on until I get the whole match, without waiting for the previous threads to return. Thus match has to be thread-safe.
>     
>     The feature I'm trying to fix here is to be able to enter "spell french bonjour" and have it spell something. 
>     The runner is responsible for parsing the input and checking whether it should do anything else.
>     So we don't actually post spellchecking requests, we just post some input, and detect that we should spell-check, and change the language, by parsing strings. So I couldn't see a way to do this except to call Sonnet::Speller::setLanguage() in Plasma::AbstractRunner::match, 
>     which meant that for it to work I had to make match thread-safe. 
>     
>     Sorry. I'm open to suggestions if you have a better idea. 
>     
>     I've fixed your other comments in v2 of the diff, let me know if there is anything else.
> 
> Thomas Lübking wrote:
>     would it not make more sense to just kill the pending (and now worthless?) thread instead (no idea how expensive this spellchecking is to be threaded) to not waste cpu on a result that will be ignored anyway?
> 
> David Faure wrote:
>     Forcefully killing threads (QThread::terminate()) is a big no-no, it leaks memory, potentially worse: it can leave a mutex locked, so any further thread trying to lock it, will hang for ever.
>     
>     The only way to terminate a thread is to ask it nicely to terminate from within (i.e. to exit run()), which means it has to finish what it's currently doing, first, before it looks up for the next command or posted event which tells it to quit.

That is generally true and i admit to not know how the plasma spell checker is implemented (or even "where" ;-), but would expect some sort of ro non-joinable threading, operating on shared data (as the only purpose seems to prevent blocking the main event loop - not running two checks at the same  time and merge their results)  ... or does QThread alone bring structures to forbid such approach (The API doc does not mention inevitable leaks for QThread itself on ::terminate())?

(PS: I just start to wonder whether the speller threads are then tagged to catch 2,1 exits...)


- Thomas


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On Sept. 9, 2012, 10:06 p.m., Simeon Bird wrote:
> 
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> http://git.reviewboard.kde.org/r/106242/
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> 
> (Updated Sept. 9, 2012, 10:06 p.m.)
> 
> 
> Review request for kdelibs and Plasma.
> 
> 
> Description
> -------
> 
> Krunner's spellcheck plugin has been pretty broken since bd291d21f096a714a171e7af3a534ba345ca5659 (about two years ago) because it called Sonnet::Speller::setLanguage every time the spellchecker was invoked, which was (very much) not thread-safe.
> 
> This patch makes Sonnet::Speller::setLanguage threadsafe by protecting all access to the internal dict pointer using QReadWriteLock. 
> 
> A related review request is 106244, which adds more fixes to the spellcheck feature.
> 
> 
> This addresses bugs 264779 and 303831.
>     http://bugs.kde.org/show_bug.cgi?id=264779
>     http://bugs.kde.org/show_bug.cgi?id=303831
> 
> 
> Diffs
> -----
> 
>   kdecore/sonnet/speller.cpp b19e74d 
> 
> Diff: http://git.reviewboard.kde.org/r/106242/diff/
> 
> 
> Testing
> -------
> 
> Compiled, installed, used for a week or so, spellchecked a bunch of things.
> 
> 
> Thanks,
> 
> Simeon Bird
> 
>

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