线程可以安全地创建 FMX.Graphics.TBitmap.Canvas 吗？

Question

I'm maintaining a Delphi 10.2.3 Isapi application that uses FMX.Graphics.TBitmap. Multiple threads are creating their own private bitmap, draw on it, return the binary content to the webrequest handler, and free the bitmap. While debugging access violations are occuring in this stack trace:

:760c4742 KERNELBASE.RaiseException + 0x62
System.DynArraySetLength(nil,$407163,16,$F)
System.DynArraySetLength($6113648,$5DDE84,1,$6B6FE78)
System.Generics.Collections.TListHelper.InternalSetCapacity(8514146)
System.Generics.Collections.TListHelper.InternalGrow(???)
System.Generics.Collections.TListHelper.InternalGrowCheck(???)
System.Generics.Collections.TListHelper.InternalAddManaged((no value))
System.Messaging.TMessageManager.SubscribeToMessage(???,(FMX.Canvas.D2D.TCanvasD2D.ContextLostHandler,$6122F70))
FMX.Canvas.D2D.TCanvasD2D.CreateFromBitmap(???,SystemDefault)
FMX.Graphics.TBitmap.GetCanvas
Unit1.TWorker.Execute

I suspect that this code in the FMX framework code is not threadsafe:

// FMX.Canvas.D2D.pas:
constructor TCanvasD2D.CreateFromBitmap(const ABitmap: TBitmap; const AQuality: TCanvasQuality);
begin
  inherited;
  FLastBrushTransform := TMatrix.Identity;
  CreateResources;
  FContextLostId := TMessageManager.DefaultManager.SubscribeToMessage(TContextLostMessage, ContextLostHandler);
end;

It is calling the singleton TMessageManager.DefaultManager and adding a handler to it's internal dictionary, without any locking. That doesn't look very threadsafe. According to the documentation FMX bitmaps can be used in threads when using BeginScene and EndScene, which is fine. But actually creating/destroying a FMX canvas does not seem to be threadsafe because of the subscribe/unsubscribe to the singleton default MessageManager? Is this assumption correct?

The strange thing is that it might only raise access violations when any code has been paused and resumed somewhere in the debugger via a breakpoint. When the program is never paused by breakpoints it will run without problems.

Answer 1

I confirm you that code in FMX.Graphics.TBitmap.Canvas is not threadsafe. just look at it you will for exemple see that it's listen for message and messaging is not threadsafe

constructor TD2DBitmapHandle.Create(const AWidth, AHeight: Integer; const AAccess: TMapAccess);
begin
  inherited Create;
  FWidth := AWidth;
  FHeight := AHeight;
  FAccess := AAccess;
  FContextLostId := TMessageManager.DefaultManager.SubscribeToMessage(TContextLostMessage, ContextLostHandler);
end;

Answer 2

If you look the code of tcontext3d you will see that it use intensively global variable and thus can not be multi thread. Tcontext3d is use when you do for example bitmap.canvas.beginscene