RE: ... and the pleasures of locking problems

Hi Philip,

What does this mean on a practical level? Ie.,
1) while getting headers for big folder A, I select folder B
2) while getting headers folder big folder A, I press 'Cancel'.

I noticed this is 'preparing for a Release Candidate' -
so no more semantic/architectural changes are expected before 

Best wishes,

> -----Original Message-----
> From: tinymail-devel-list-bounces gnome org 
> [mailto:tinymail-devel-list-bounces gnome org] On Behalf Of 
> ext Philip Van Hoof
> Sent: Wednesday, July 25, 2007 02:43
> To: tinymail-devel-list
> Subject: ... and the pleasures of locking problems
> Tinymail's _async functions where a design error right from the start.
> Therefore I drastically refactored them. I once also started 
> libtinymail-queues and the TnyQueue + libtinymail-asyncworker 
> work to emphasise that the _async functions are indeed a 
> problem. I figured that because a lot people are nonetheless 
> going to use them, the big f.
> design error in Tinymail needs a fix too.
> so ...
> I introduced an internal private type called TnyCamelQueue in 
> libtinymail-camel. Right now, in stead of simply launching 
> threads in the _async_default functions, I call 
> _tny_camel_queue_launch.
> The launch method of TnyCamelQueue is indeed the method to 
> add an item to the queue.
> I haven't defined any contract for this method as it's a 
> private function. The contract for it can be considered to be:
> "Will launch the function and pass data to it in future. Will 
> immediately return and wont block until the function is 
> handled. The function can happen in a thread therefore 
> enforcing you to use locking correctly in your 
> implementations. Items in the queue will happen serialised. 
> This means that no items in the queue will be processed in 
> parallel." ... or something like that.
> It does not specify when exactly in future, only that it will 
> happen in future. There's no priority setting, it's a simple FIFO.
> All the callbacks of the existing _async functions are 
> specified to be called in the mainloop. What makes this 
> indeed happen is (still) implemented by the execute_callback 
> function (which got recently refactored by Sergio, in case 
> you wonder about it) (differently put:
> nothing got changed).
> This might have broken some of the cancels and definitely has 
> broken support for parallel getting messages on IMAP. The 
> last one is easily solvable and fixable. The first one, in 
> case true, is less easy to fix.
> I noticed that the first one (broken cancels) doesn't always 
> occur. I noticed that the second one (broken parallel getting 
> of messages over
> IMAP) always does occur. I noticed that on POP parallel 
> getting of messages is still functional.
> Since now _async operations happen serialised, there's a more 
> clear and clean view over the locking order. The amount of 
> threads that can be active is also drastically lowered to ... 
> one for this queue.
> I know I'm going to be stormed by panicking project managers 
> and people with lots of questions. This is better, more 
> stable and worth temporarily breaking the two functions 
> (cancels and parallel getting messages).
> Although it wont fix all locking problems, it will or should 
> fix a lot of them or at least finding them and fixing it much 
> more easy.
> More work on also the account initialisation and connecting 
> will happen soon. This work attempts to get the alert_func, 
> get_pass_func and forget_pass_func to be ran in the GMainLoop 
> of your application. This could, at least in theory, also fix 
> a lot of ui hang difficulties.
> Last but not least: if people disagree that this is the right 
> way to fix the problems, they can ask me to create a branch 
> somewhere that doesn't redo the _async function's infrastructure.
> We can also agree to disagree by having multiple 
> implementations of TnyCamelQueue. The first-original 
> implementation simply calls the g_thread_create API exactly 
> like how the original code was, just wrapped by the 
> _tny_camel_queue_launch (makes absolutely no difference).
> Note, though, that this affects the behaviour of Tinymail 
> significantly and that people who disagree will have to 
> maintain the 'reverse of the refactor' themselves.
> --
> Philip Van Hoof, software developer
> home: me at pvanhoof dot be
> gnome: pvanhoof at gnome dot org
> _______________________________________________
> tinymail-devel-list mailing list
> tinymail-devel-list gnome org

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