Alban Hertroys wrote:
Notify is called before thread B (in this case) hits the condAllowed.wait() piece of code. So, it sits at that wait() for forever (because it doesn't get notified, because the notification already happened), waiting to be notified from the main thread, and the main thread is waiting on thread B (again, in this case) to call mainCond.notify(). This approach is a deadlock just wanting to happen (not waiting, because it already did happen). What is it exactly that you are trying to accomplish? I'm sure there is a better approach.
Hmm, I already learned something I didn't know by reading through my own version of the output.
I added an extra counter, printed before waiting in the for-loop in Main. My wrong assumption was that acquire() would block other threads from acquiring until release() was called. In that case the for-loop would wait 3 times (once for each thread), which is what I want. Unfortunately, in my output I see this:
T-A: acquire mainCond ... T-B: acquire mainCond ... T-B: released mainCond ... T-A: released mainCond
Which is exactly what I was trying to prevent...
But even then, as you pointed out, there is still the possibility that one of the threads sends a notify() while the main loop isn't yet waiting, no matter how short the timespan is that it's not waiting...
As for what I'm trying to do; I'm trying to merge three huge XML files into single seperate database records. Every database record contains related data from each of the XML files.
For practical purposes this is a two-stage process, where I first store an uncombined "record" from each XML file into the DB (marked as 'partial'), and then periodicaly merge the related records into one final record.
I could first store all data as 'partial' and then merge everything, but I consider it better to do this with relatively small batches at a time (queries are faster with smaller amounts of data, and the DB stays smaller too).
The reason I use threads for this is that (to my knowledge) it is not possible to pause an xml.parsers.xmlproc.xmlproc.Application object once it starts parsing XML, but I can pause a thread.
This is a timeline of what I'm trying to do:
Main start |combine XML |comb.
|next batch |next
Application A run>..............*| | >...........*| | etc.
Application B run>.........*| | >..............*|
Application C run>................*| >..........*|Legend:
> = thread is active
* = batch ready, wait()
| = timeline delimiter-- http://mail.python.org/mailman/listinfo/python-list
