Summary

qt-light egg: timers may put chicken into an inconsistent state

Metadata

Description

I have found that a Qt::Timer may be used to make callbacks into chicken faster than the callbacks themselves can finish. This puts chicken into an inconsistent state and causes a crash. Here is a test case - callback rate and count may need to be adjusted for faster machines.

 (import chicken scheme)
 
 (use qt-light)
 
 (qt:init)
 
 (let* ((thread-rate 0.025)
        (timer (qt:timer thread-rate)))
   (define (timer-callback)
     (let loop ((i 0))
       (if (< i 1e6)
           (loop (+ i 1))
           #t)))
   (qt:connect timer "timeout()" timer-callback)
   (qt:start timer)
   (qt:run))



 $ ./minimal.scm 
 
 Warning: excluded identifier doesn't exist in module foreign: foreign-declare
 
 Warning: excluded identifier doesn't exist in module foreign: foreign-declare
 csi: runtime.c:2797: C_save_and_reclaim: Assertion `av > C_temporary_stack_bottom || av < C_temporary_stack_limit' failed.
 Aborted

Since Qt must be used for the main event loop in a Qt program, making timers safe to use would be fairly important for the qt-light egg. Without knowing much about the internals, I think that maybe a semaphore could be used to block a callback if a previous callback has not finished. When this occurs, it would be nice if the api also exposed a way for a chicken program to know that a callback was dropped, so that it could take an appropriate action.

Changes and comments

[2017-08-25 14:22:59 UTC] sjamaan wrote:

It sounds like these callbacks are executed in a different thread. This is explicitly not supported. You could set up something in pure C which writes to a mailbox or some such (perhaps with the concurrent-native-callbacks egg)

So I don't think this is a bug. Can you maybe provide some more information about how this works in QT?