[DUG] TListitems disappearing and reappearing causing access violations

David O'Brien Dave at iccs.co.nz
Thu Apr 11 16:30:34 NZST 2013 

What would happen to the program if the network drive "went away", even for a bit?

From: delphi-bounces at listserver.123.net.nz [mailto:delphi-bounces at listserver.123.net.nz] On Behalf Of Ross Levis
Sent: Thursday, 11 April 2013 3:55 p.m.
To: 'NZ Borland Developers Group - Delphi List'
Subject: Re: [DUG] TListitems disappearing and reappearing causing access violations

I believe they are networked files, but I don't see that being responsible for Windows deleting all the listitems.

From: delphi-bounces at listserver.123.net.nz<mailto:delphi-bounces at listserver.123.net.nz> [mailto:delphi-bounces at listserver.123.net.nz] On Behalf Of David O'Brien
Sent: Thursday, 11 April 2013 10:07 a.m.
To: NZ Borland Developers Group - Delphi List
Subject: Re: [DUG] TListitems disappearing and reappearing causing access violations

Are the tracks on a networked drive, or local? Sounds like a hardware (drive) problem to me.

From: delphi-bounces at listserver.123.net.nz<mailto:delphi-bounces at listserver.123.net.nz> [mailto:delphi-bounces at listserver.123.net.nz] On Behalf Of Ross Levis
Sent: Wednesday, 10 April 2013 11:24 p.m.
To: 'NZ Borland Developers Group - Delphi List'
Subject: Re: [DUG] TListitems disappearing and reappearing causing access violations

I've heard today that TeamViewer isn't in use when the problem occurs.  They often use it to resolve the issue after the problem has occurred.

They tell me they have seen it in mid-song where all the tracks disappear and the app then becomes unresponsive or crashes because of the app still accessing items that don't exist.

There are no deletion routines in the program which delete every item from the listview, so that's not what's going on.  Considering there are around 5000 users who have no problems at all, I don't think it can be a coding problem.  There has been the very rare issue like this reported over the years but this one user is getting it regularly, like every couple of days.

I thought I was on to something with remote control app but it appears not.

Another reported crash a couple of weeks ago was here...
i := Item.Index;
while (i < PlayView.Items.Count) and (PlayView.Items[i].Caption = EventTimeStr) do
...
The "while" line crashed with Access Violation in ComCtrls.TListItems.GetItem, and the screen shot again showed no items.  This should prove that it cannot be the software removing the items as otherwise Items.Count would be 0.

Someone else with dual monitors had a crash on the same line about 6 months ago and it has now been modified to validate items[i] is a real item before retrieving the caption.  This will stop the error but the software will not function correctly.

Cheers.

From: delphi-bounces at listserver.123.net.nz<mailto:delphi-bounces at listserver.123.net.nz> [mailto:delphi-bounces at listserver.123.net.nz] On Behalf Of Ross Levis
Sent: Wednesday, 10 April 2013 2:38 a.m.
To: 'NZ Borland Developers Group - Delphi List'
Subject: Re: [DUG] TListitems disappearing and reappearing causing access violations

I guess I am.  It makes the most sense.  I'll have to try a test with TeamViewer to duplicate it.

From: delphi-bounces at listserver.123.net.nz<mailto:delphi-bounces at listserver.123.net.nz> [mailto:delphi-bounces at listserver.123.net.nz] On Behalf Of Kyley Harris
Sent: Wednesday, 10 April 2013 12:48 a.m.
To: NZ Borland Developers Group - Delphi List
Subject: Re: [DUG] TListitems disappearing and reappearing causing access violations

I think the problem is that right now you are running on the Theory that they are being reconstructed as you state, and you have not created a test scenario to validate what you are saying.
it could just be a coding error, and the delete event will tell you when a list item is being destroyed. That list item will not be destroyed if the window handle is reconstructed. Not in the source code I can see.,

On Wed, Apr 10, 2013 at 12:41 AM, Ross Levis <ross at stationplaylist.com<mailto:ross at stationplaylist.com>> wrote:
Actually that doesn't help so much.  I really need to know when Windows is deleting the items.  My app also deletes items.  I guess I could remove the OnDeletion event property before coding a delete and reinstate if afterwards.  A little messy.

I really need to associate the individual deleted items with the identical new items that will be created just afterwards.  Ideally I would want my app to stop processing all timers and functions while the deletions and recreations are done.

Getting too complicated.

From: delphi-bounces at listserver.123.net.nz<mailto:delphi-bounces at listserver.123.net.nz> [mailto:delphi-bounces at listserver.123.net.nz<mailto:delphi-bounces at listserver.123.net.nz>] On Behalf Of Kyley Harris

Sent: Wednesday, 10 April 2013 12:06 a.m.
To: NZ Borland Developers Group - Delphi List
Subject: Re: [DUG] TListitems disappearing and reappearing causing access violations

the listview has an Ondeletion event. when that fires, you should check the index, and remove any listitem references from your tlist.

The issue seems to be that you are mixing view objects with data objects. the system might go more smoothly if you were containing your data objects disctinct from your view (tlistview) and synchronizing the view to the source.

On Tue, Apr 9, 2013 at 11:58 PM, Ross Levis <ross at stationplaylist.com<mailto:ross at stationplaylist.com>> wrote:
It isn't a user initiated procedure.  It's a broadcast media player in automation mode and each track is a listitem.  Items can be dropped off the top and more added to the bottom as it plays automated.

There is a TTimer which accesses all the items in the list once a second to provide totals and such.  So the items are accessed very regularly.

Some of the list items are timed events which need to occur at a specific time.  These item pointers are stored in a TList so the software can easily access the next one without having to look for it in the list.

This user has had issues with the code that accesses this TList of TListitem pointers, so that sounds like the pointers to the listitems have been changed.

Ideally the listitems shouldn't be accessed directly and a separate data store should be used that is linked to each item via the Data property.  A bit too late for that now though.

So it sounds like I need to find the Windows message that is being sent to the app to recreate windows and do some housekeeping.

Any ideas what that might be?


From: delphi-bounces at listserver.123.net.nz<mailto:delphi-bounces at listserver.123.net.nz> [mailto:delphi-bounces at listserver.123.net.nz<mailto:delphi-bounces at listserver.123.net.nz>] On Behalf Of Kyley Harris
Sent: Tuesday, 9 April 2013 11:35 p.m.

To: NZ Borland Developers Group - Delphi List
Subject: Re: [DUG] TListitems disappearing and reappearing causing access violations

If that was the case.. then its unlikely to happen midstream during a very small short procedure call. Especially if that procedure call is a user initiated one.
if that is the case.. its extremely simple to test this with remote desktop and some logging of the construction/destruction of the list items.

On Tue, Apr 9, 2013 at 11:29 PM, Ross Levis <ross at stationplaylist.com<mailto:ross at stationplaylist.com>> wrote:
I've had someone suggest this elsewhere...

The user connects a monitor to a laptop, or connects remotely via
terminal services.
Basically, anything that changes the display resolution has that kind of
consequences.

I have seen a TeamViewer in one of the many screen shots.  So I guess Windows is recreating the windows and allocating new handles to the list items as I thought.  I'll check with them if the crashes coincide with use of TeamViewer.

Ross.

From: Ross Levis [mailto:ross at stationplaylist.com<mailto:ross at stationplaylist.com>]
Sent: Tuesday, 9 April 2013 11:17 p.m.
To: 'NZ Borland Developers Group - Delphi List'
Subject: RE: [DUG] TListitems disappearing and reappearing causing access violations

The list is populated by the software.  It's still in Delphi 7.

No threads are deleting any items.  Only the main thread adds and removes items.

There is no one procedure this has occurred in.  It can work for 2 days and suddenly this happens in a random procedure that accesses a list item.

In this particular case, it is in a procedure where one specific listitem is sent as a parameter.

Here are some of the relevant lines of code in the procedure being executed.

    if Item <> nil then
    begin
      if Item.Selected then UpdateButtons;
       ...
      Item.ImageIndex := 8;
    end;

There are no deletions in the procedure.

The Item.Selected test works fine but Item.ImageIndex := 8 crashes at TListItem.SetImage with an access violation.

The item existed and then no items exist based on the screen shot.

I've been having to put validations all over the place like this...
if ListView.Items.IndexOf(Item) >= 0 then Listitem.ImageIndex := 8.

This user has had a crash where it does this validation up the procedure somewhere which validated, but further down it will be accessing Item.Index and this crashes.

It's a nightmare.  I'm wondering if it could be corrupt RAM in the PC or something.

Cheers,
Ross.

From: delphi-bounces at listserver.123.net.nz<mailto:delphi-bounces at listserver.123.net.nz> [mailto:delphi-bounces at listserver.123.net.nz] On Behalf Of Kyley Harris
Sent: Tuesday, 9 April 2013 10:33 p.m.
To: NZ Borland Developers Group - Delphi List
Subject: Re: [DUG] TListitems disappearing and reappearing causing access violations

without seeing the 10lines above, and whats inbetween (if you were being literal) its very hard to say.
The only time I've ever had something dissapear from a list is when I deleted it though a callback, method, thread etc.. or freeing something.
what version of delphi is the code now? How does the list get populated or edited by the application..
I'd start by looking at how many locations of code insert or delete the list items, and verify none are being called while your procedure is executing its for-loop, (assuming the access violation is an index based one because of the list count change)


On Tue, Apr 9, 2013 at 10:21 PM, Ross Levis <ross at stationplaylist.com<mailto:ross at stationplaylist.com>> wrote:
Wondering if anyone has any ideas.

I've had this problem a long time from when I started writing an app about 7 or 8 years ago, but it happens so rarely it's not usually an issue. But one user in particularly is regularly sending me MadExcept reports which either have access violation errors or "the application appears to be frozen". In all cases it's during an access of a TListItem which was perfectly valid 10 lines of code up a procedure but suddenly doesn't exist.

The screenshot created at the same time shows the ListView has no visible items, but I know for a fact there are 30 or more items in the list. I believe sometimes the items visually disappear and reappear shortly afterwards, like a Desktop refresh.

I think I've heard something about Windows recreating the window handles or some such thing. I don't understand why it would do so. There are no changes to the listview that would require re-creating the window.

Any ideas?


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