Delphi - random access violation in DLL Thread - multithreading

Question

I do something wrong, but what exactly?
I have simple DLL with thread. From App I put message in DLL queue, and DLL thread put message to callback.
When I use push in the application not in main thread everything fine.
Also everything is fine if I don't use DLL at all, and use thread with queue just in the application (or use DLL without TThread).
But when I push from main thread - I get random AV (in random time, in a random place)
uTestThread.pas
unit uTestThread;
interface
uses
Winapi.Windows, System.Classes, System.SyncObjs,
System.Generics.Collections;
type
TTestCallback = procedure(Data: Pointer);
TTestThread = class(TThread)
private
FLock: TRTLCriticalSection;
FQueue: TQueue<Pointer>;
FCallback: TTestCallback;
protected
procedure Execute; override;
public
constructor Create;
destructor Destroy; override;
procedure Push(Data: Pointer);
property Callback: TTestCallback read FCallback write FCallback;
end;
implementation
{ TTestThread }
constructor TTestThread.Create;
begin
InitializeCriticalSection(FLock);
FQueue:=TQueue<Pointer>.Create;
Inherited Create(True);
end;
destructor TTestThread.Destroy;
begin
FQueue.Free;
DeleteCriticalSection(FLock);
inherited;
end;
procedure TTestThread.Execute;
var
O: Pointer;
begin
while not Terminated do
begin
EnterCriticalSection(FLock);
try
if FQueue.Count > 0 then
begin
if Assigned(FCallback) then
begin
O:=FQueue.Dequeue;
FCallback(O);
end;
end;
except //on E: Exception do
//OutputDebugString(PChar('==='+E.Message+'==='));
end;
LeaveCriticalSection(FLock);
end;
end;
procedure TTestThread.Push(Data: Pointer);
begin
EnterCriticalSection(FLock);
try
FQueue.Enqueue(Data);
finally
LeaveCriticalSection(FLock);
end;
end;
end.
DLL
library lib;
uses
uTestThread in 'src\test\uTestThread.pas';
{$R *.res}
var
TT: TTestThread;
procedure Push(Data: TTestDataOut);
begin
TT.Push(Data);
end;
procedure TestThreadCreate(Callback: TTestCallback);
begin
TT:=TTestThread.Create;
TT.Callback:=Callback;
TT.Start;
end;
exports
TestThreadCreate,
Push;
begin
end.
App form
unit frmMain;
interface
uses
Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls, uTestThread;
type
TForm2 = class(TForm)
Button1: TButton;
Button2: TButton;
procedure Button1Click(Sender: TObject);
procedure Button2Click(Sender: TObject);
private
procedure AddX;
public
{ Public declarations }
end;
var
Form2: TForm2;
procedure TestThreadCreate(Data: TTestCallback); external 'lib.dll';
procedure Push(Data: Pointer); external 'lib.dll';
implementation
{$R *.dfm}
procedure TestCallback(Data: TTestDataOut);
begin
// OutputDebugString(PChar(TStringStream(Data).DataString));
TObject(Data).Free;
end;
{ TForm2 }
procedure TForm2.AddX;
var
I: Integer;
S: string;
O: Pointer;
begin
for I:=0 to 9999 do
begin
// S:='F'+IntToStr(I)+' zzzzzzzzzzzzzzzzzzzzzzzzzWWzzzzzzzzzzzsssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssssszzzx';
// O:=TStringStream.Create(S);
O:=TObject.Create;
Push(O);
end;
OutputDebugString(PChar('End'));
end;
procedure TForm2.Button1Click(Sender: TObject);
begin
TestThreadCreate(TestCallback);
end;
procedure TForm2.Button2Click(Sender: TObject);
begin
// Everything fine !!!
//
// TThread.CreateAnonymousThread(procedure
// begin
// AddX;
// end).Start;
// Get random AV !!!
AddX;
end;
end.
AV example
---------------------------
GExperts Debugger Exception Notification
---------------------------
Project lib.exe raised exception class EAccessViolation with message 'Access violation at address 004059B1. Read of address FFFFFFFC'.
---------------------------
ThreadId=2644
ProcessId=130
ThreadName=""
ExceptionMessage="Access violation at address 004059B1. Read of address FFFFFFFC"
ExceptionName="EAccessViolation"
ExceptionDisplayName="$C0000005"
ExceptionAddress=004059B1
FileName="GETMEM.INC"
LineNumber=1973
---------------------------
To demonstrate the problem, I use "while True do"
I know that FCallback should also be protected, but for demonstration I skip this
I dont create a thread in DllMain because:
https://learn.microsoft.com/en-us/windows/win32/dlls/dynamic-link-library-best-practices
As lock I tried to use TRTLCriticalSection / TCriticalSection / TMonitor / TMutex
As message I tried to use Pointer / WideString / PChar (and copy string)
As callback I tried to use Interface and Pointer to Procedure
As queue I tried TQueue<> / TList<> / TList / Array
I suspect something wrong with callback, because if I use queued item in DLL memory everything also fine