Crash on application close - in Destroy of TCustomTimer or THintWindow
lazarus / 21-General

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rborsey · 2025-02-02 14:24

Hi all.
I've got a problem on closedown of my (rather large - about 90k lines
of code) form-based application. It has only appeared recently and
happens maybe once every 20 times I run the app so maybe some sort of
race condition during the shutdown. Anyway I catch the error and dump
the back trace and it appears to be when the application destroys all
its components (presumably during its own destruction) and it seems to
happen on trying to destroy either a CustomTimer (which I don't
directly use) or HintWindow (I do use hints).

I have written out all the application.components and immediately
after startup I get a list of all the forms created in my application.
If I run this dump very shortly after statup (with my first form
action) then a TCustomTimer has been added to the list (no idea what
has created that). If I pass over a control with a hint then the
THindWindow is created too.

I don't get an AV error report or invalid memory access - typically
the application hangs and then I either get a heaptrace (showing a
zillion blocks lost) or a windows style Searching for a Solution bow .
. .

Anybody have an idea what might be going on? Where to start looking?

Thanks
Richard

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Thaddy · 2025-02-02 14:34

Kill the timer(s) in form.onclosequery and only then give permission
to close the app.

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rborsey · 2025-02-02 23:17

Hi Thaddy. Yes I would do that (I free loads of stuff and close down
threads etc in CloseQuery and/or Clos. The problem os that it is not
my code which creates the CustomTimer - not directly at least. It
seems to appear spontaneously. I use timers from mmsystem (which I do
stop).

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rborsey · 2025-02-03 17:33

Aha. Solved

Thank-you Thaddy - your reference to onclosequery made me think. I had
been using closequery to decide if it was fine to close - saves done,
nothing critical underway, and using onclose to stop threads and free
resources. The documentation (what I found anyway) isn't super clear
on when to do this latter set of tasks an d so on reflectionI thought
maybe I was shutting things down too late and thus maybe creating some
race conditions between internal cleanup and the application itself
closing down. I moved all the thread termination and freeing actions
to onclosequery and Bingo - everything works fine. Job done . . .