Delphi TThread.CurrentThread and EAccessViolation - Is This a Bug or My Incompetence..?

Question

In Delphi 2009 I'm finding that any time I use TThread.CurrentThread in an application, I'll get an error message like the following when the application closes:

Exception EAccessViolation in module ntdll.dll at 0003DBBA.
Access violation at address 7799DBBA in module 'ntdll.dll'.  Write of
address 00000014.

Unless it's just my machine, you can replicate this in a few seconds: create a new Delphi Forms Application, add a button to the form, and use something like the following for the button's event handler:

procedure TForm1.Button1Click(Sender: TObject);
begin
  TThread.CurrentThread;
end;

On both my Vista machine and my XP machine I'm finding that, if I don't click the button everything's fine, but if I do click the button I get the above error message when I close the application.

So... I'm wondering if this is a bug, but at the same time I think it's rather likely that I'm simply not understanding something very basic about how you're supposed to work with TThreads in Delphi. I am a bit of a Delphi newbie I'm afraid.

Is there something obviously wrong with using TThread.CurrentThread like that?

If not, and you have Delphi 2009, do you get the same problem if you implement my simple sample project?

---

Update: As François noted below, this actually is a bug in Delphi 2009 at the moment - you can vote for it here.

---

Update: This bug was fixed in Delphi 2010.

Answer 1

I think CurrentThread is added in 2009 (or 2007). I have 2006 at home. But are you sure it is a class property?

Answer 2

Unfortunately it seems like a bug linked to the call order of the finalization section in the Classes unit:

DoneThreadSynchronization clears the ThreadLock structure, then
FreeExternalThreads wants to destroy the Thread object you just created when calling CurrentThread, and
that requires the ThreadLock to be already initialized in the call to
EnterCriticalSection(ThreadLock) in TThread.RemoveQueuedEvents...

UPDATE:
There is now a workaround patch in the QC report.

Answer 3

Until CodeGear issues a fix, you can use the patch below. Save it into a standalone unit and use it anywhere in your program. I'll try to add it to the QC, too.

This version works with D2009 (original), update 1 and update 2.

{ Fix Delphi 2009's invalid finalization order in Classes.pas.
  Written by Primoz Gabrijelcic, http://gp.17slon.com.
  No rights reserved - released to public domain.
}
unit FixD2009Classes;

interface

implementation

uses
  Windows,
  SysUtils,
  Classes;

type
  TCode = array [0..109] of byte;

{$WARN SYMBOL_PLATFORM OFF}

procedure PatchClasses;
{$IFDEF ConditionalExpressions}
{$IF RTLVersion = 20}
var
  i         : integer;
  oldProtect: cardinal;
  pCode     : ^TCode;
  tmp       : DWORD;
const
  COffsets_Call: array [1..12] of integer = (0, 15, 24, 34, 49, 59, 69, 79, 89, 94, 99, 109);
  COffset_UnRegisterModuleClasses = 106;
  COffset_DoneThreadSynchronization = 94;
  COffset_FreeExternalThreads = 99;
  CCallDelta = COffset_FreeExternalThreads - COffset_DoneThreadSynchronization;
{$IFEND}
{$ENDIF}
begin
{$IFDEF ConditionalExpressions}
{$IF RTLVersion = 20}
  pCode := pointer(cardinal(@TStreamReader.ReadToEnd) + COffset_UnRegisterModuleClasses);
  Win32Check(VirtualProtect(pCode, COffsets_Call[High(COffsets_Call)], PAGE_READWRITE, oldProtect));
  try
    for i := Low(COffsets_Call) to High(COffsets_Call) do
      if pCode^[COffsets_Call[i]] <> $E8 then
        raise Exception.Create('Unexpected version of Classes - cannot patch');
    tmp := PDword(@pCode^[COffset_DoneThreadSynchronization+1])^;
    PDword(@pCode^[COffset_DoneThreadSynchronization+1])^ :=
      PDword(@pCode^[COffset_FreeExternalThreads+1])^ + CCallDelta;
    PDword(@pCode^[COffset_FreeExternalThreads+1])^ := tmp - CCallDelta;
  finally VirtualProtect(pCode, COffsets_Call[High(COffsets_Call)], oldProtect, oldProtect); end;
{$IFEND}
{$ENDIF}
end;

initialization
  PatchClasses;
end.

Answer 4

Patch unit for Delphi 2009 Update 3.

{ Fix Delphi 2009's invalid finalization order in Classes.pas.
  Written by Primoz Gabrijelcic, http://gp.17slon.com.
  No rights reserved - released to public domain.

  D2009 update 3 only.
}
unit FixD2009Classes;

interface

implementation

uses
  Windows,
  SysUtils,
  Classes;

type
  TCode = array [0..144] of byte;

{$WARN SYMBOL_PLATFORM OFF}

procedure PatchClasses;
{$IFDEF ConditionalExpressions}
{$IF RTLVersion = 20}
var
  i         : integer;
  oldProtect: cardinal;
  pCode     : ^TCode;
  tmp       : DWORD;
const
  COffsets_Call: array [1..12] of integer = (0, 15, 24, 42, 47, 58, 73, 91, 101, 111, 134, 139);
  COffset_UnRegisterModuleClasses = 107;
  COffset_DoneThreadSynchronization = 134;
  COffset_FreeExternalThreads = 139;
  CCallDelta = COffset_FreeExternalThreads - COffset_DoneThreadSynchronization;
{$IFEND}
{$ENDIF}
begin
{$IFDEF ConditionalExpressions}
{$IF RTLVersion = 20}
  pCode := pointer(cardinal(@TStreamReader.ReadToEnd) + COffset_UnRegisterModuleClasses);
  Win32Check(VirtualProtect(pCode, COffsets_Call[High(COffsets_Call)], PAGE_READWRITE, oldProtect));
  try
    for i := Low(COffsets_Call) to High(COffsets_Call) do
      if pCode^[COffsets_Call[i]] <> $E8 then
        raise Exception.Create('Unexpected version of Classes - cannot patch');
    tmp := PDword(@pCode^[COffset_DoneThreadSynchronization+1])^;
    PDword(@pCode^[COffset_DoneThreadSynchronization+1])^ :=
      PDword(@pCode^[COffset_FreeExternalThreads+1])^ + CCallDelta;
    PDword(@pCode^[COffset_FreeExternalThreads+1])^ := tmp - CCallDelta;
  finally VirtualProtect(pCode, COffsets_Call[High(COffsets_Call)], oldProtect, oldProtect); end;
{$IFEND}
{$ENDIF}
end;

initialization
  PatchClasses;
end.

Answer 5

The patch didn't work for me if Build with runtime packages is checked.