How can I make AllocateHwnd threadsafe?

This problem can be solved like so:

1. Obtain or implement a threadsafe version of AllocateHwnd and DeallocateHwnd.
2. Replace the VCL’s unsafe versions of these functions.

For item 1 I use Primož Gabrijelcic’s code, as described on his blog article on the subject. For item 2 I simply use the very well-known trick of patching the code at runtime and replacing the beginning of the unsafe routines with unconditional JMP instructions that redirect execution to the threadsafe functions.

Putting it all together results in the following unit.

(* Makes AllocateHwnd safe to call from threads. For example this makes TTimer
   safe to use from threads.  Include this unit as early as possible in your
   .dpr file.  It must come after any memory manager, but it must be included
   immediately after that before any included unit has an opportunity to call
   Classes.AllocateHwnd. *)
unit MakeAllocateHwndThreadsafe;

interface

implementation

{$IF CompilerVersion >= 23}{$DEFINE ScopedUnitNames}{$IFEND}
uses
  {$IFDEF ScopedUnitNames}System.SysUtils{$ELSE}SysUtils{$ENDIF},
  {$IFDEF ScopedUnitNames}System.Classes{$ELSE}Classes{$ENDIF},
  {$IFDEF ScopedUnitNames}Winapi.Windows{$ELSE}Windows{$ENDIF},
  {$IFDEF ScopedUnitNames}Winapi.Messages{$ELSE}Messages{$ENDIF};

const //DSiAllocateHwnd window extra data offsets
  GWL_METHODCODE = SizeOf(pointer) * 0;
  GWL_METHODDATA = SizeOf(pointer) * 1;

  //DSiAllocateHwnd hidden window (and window class) name
  CDSiHiddenWindowName="DSiUtilWindow";

var
  //DSiAllocateHwnd lock
  GDSiWndHandlerCritSect: TRTLCriticalSection;
  //Count of registered windows in this instance
  GDSiWndHandlerCount: integer;

//Class message dispatcher for the DSiUtilWindow class. Fetches instance's WndProc from
//the window extra data and calls it.
function DSiClassWndProc(Window: HWND; Message, WParam, LParam: longint): longint; stdcall;
var
  instanceWndProc: TMethod;
  msg            : TMessage;
begin
  {$IFDEF CPUX64}
  instanceWndProc.Code := pointer(GetWindowLongPtr(Window, GWL_METHODCODE));
  instanceWndProc.Data := pointer(GetWindowLongPtr(Window, GWL_METHODDATA));
  {$ELSE}
  instanceWndProc.Code := pointer(GetWindowLong(Window, GWL_METHODCODE));
  instanceWndProc.Data := pointer(GetWindowLong(Window, GWL_METHODDATA));
  {$ENDIF ~CPUX64}
  if Assigned(TWndMethod(instanceWndProc)) then
  begin
    msg.msg := Message;
    msg.wParam := WParam;
    msg.lParam := LParam;
    msg.Result := 0;
    TWndMethod(instanceWndProc)(msg);
    Result := msg.Result
  end
  else
    Result := DefWindowProc(Window, Message, WParam,LParam);
end; { DSiClassWndProc }

//Thread-safe AllocateHwnd.
//  @author  gabr [based on http://fidoforum.ru/pages/new46s35o217746.ru.delphi and
//                 TIcsWndHandler.AllocateHWnd from ICS v6 (http://www.overbyte.be)]
//  @since   2007-05-30
function DSiAllocateHWnd(wndProcMethod: TWndMethod): HWND;
var
  alreadyRegistered: boolean;
  tempClass        : TWndClass;
  utilWindowClass  : TWndClass;
begin
  Result := 0;
  FillChar(utilWindowClass, SizeOf(utilWindowClass), 0);
  EnterCriticalSection(GDSiWndHandlerCritSect);
  try
    alreadyRegistered := GetClassInfo(HInstance, CDSiHiddenWindowName, tempClass);
    if (not alreadyRegistered) or (tempClass.lpfnWndProc <> @DSiClassWndProc) then begin
      if alreadyRegistered then
        {$IFDEF ScopedUnitNames}Winapi.{$ENDIF}Windows.UnregisterClass(CDSiHiddenWindowName, HInstance);
      utilWindowClass.lpszClassName := CDSiHiddenWindowName;
      utilWindowClass.hInstance := HInstance;
      utilWindowClass.lpfnWndProc := @DSiClassWndProc;
      utilWindowClass.cbWndExtra := SizeOf(TMethod);
      if {$IFDEF ScopedUnitNames}Winapi.{$ENDIF}Windows.RegisterClass(utilWindowClass) = 0 then
        raise Exception.CreateFmt('Unable to register DSiWin32 hidden window class. %s',
          [SysErrorMessage(GetLastError)]);
    end;
    Result := CreateWindowEx(WS_EX_TOOLWINDOW, CDSiHiddenWindowName, '', WS_POPUP,
      0, 0, 0, 0, 0, 0, HInstance, nil);
    if Result = 0 then
      raise Exception.CreateFmt('Unable to create DSiWin32 hidden window. %s',
              [SysErrorMessage(GetLastError)]);
    {$IFDEF CPUX64}
    SetWindowLongPtr(Result, GWL_METHODDATA, NativeInt(TMethod(wndProcMethod).Data));
    SetWindowLongPtr(Result, GWL_METHODCODE, NativeInt(TMethod(wndProcMethod).Code));
    {$ELSE}
    SetWindowLong(Result, GWL_METHODDATA, cardinal(TMethod(wndProcMethod).Data));
    SetWindowLong(Result, GWL_METHODCODE, cardinal(TMethod(wndProcMethod).Code));
    {$ENDIF ~CPUX64}
    Inc(GDSiWndHandlerCount);
  finally LeaveCriticalSection(GDSiWndHandlerCritSect); end;
end; { DSiAllocateHWnd }

//Thread-safe DeallocateHwnd.
//  @author  gabr [based on http://fidoforum.ru/pages/new46s35o217746.ru.delphi and
//                 TIcsWndHandler.AllocateHWnd from ICS v6 (http://www.overbyte.be)]
//  @since   2007-05-30
procedure DSiDeallocateHWnd(wnd: HWND);
begin
  if wnd = 0 then
    Exit;
  DestroyWindow(wnd);
  EnterCriticalSection(GDSiWndHandlerCritSect);
  try
    Dec(GDSiWndHandlerCount);
    if GDSiWndHandlerCount <= 0 then
      {$IFDEF ScopedUnitNames}Winapi.{$ENDIF}Windows.UnregisterClass(CDSiHiddenWindowName, HInstance);
  finally LeaveCriticalSection(GDSiWndHandlerCritSect); end;
end; { DSiDeallocateHWnd }

procedure PatchCode(Address: Pointer; const NewCode; Size: Integer);
var
  OldProtect: DWORD;
begin
  if VirtualProtect(Address, Size, PAGE_EXECUTE_READWRITE, OldProtect) then begin
    Move(NewCode, Address^, Size);
    FlushInstructionCache(GetCurrentProcess, Address, Size);
    VirtualProtect(Address, Size, OldProtect, @OldProtect);
  end;
end;

type
  PInstruction = ^TInstruction;
  TInstruction = packed record
    Opcode: Byte;
    Offset: Integer;
  end;

procedure RedirectProcedure(OldAddress, NewAddress: Pointer);
var
  NewCode: TInstruction;
begin
  NewCode.Opcode := $E9;//jump relative
  NewCode.Offset := NativeInt(NewAddress)-NativeInt(OldAddress)-SizeOf(NewCode);
  PatchCode(OldAddress, NewCode, SizeOf(NewCode));
end;

initialization
  InitializeCriticalSection(GDSiWndHandlerCritSect);
  RedirectProcedure(@AllocateHWnd, @DSiAllocateHWnd);
  RedirectProcedure(@DeallocateHWnd, @DSiDeallocateHWnd);

finalization
  DeleteCriticalSection(GDSiWndHandlerCritSect);

end.

This unit must be included very early in the .dpr file’s list of units. Clearly it cannot appear before any custom memory manager, but it should appear immediately after that. The reason being that the replacement routines must be installed before any calls to AllocateHwnd are made.

Update I have merged in the very latest version of Primož’s code which he kindly sent to me.