Thread Error: The Handle is Invalid (6) when trying to Free a suspended thread

Question

In a given example I am receiving an exception when calling AThread.Free.

program Project44;

{$APPTYPE CONSOLE}

uses
  SysUtils, Classes, Wind         


        

Answer 1

The situation is very complicated in your case.

First, you does not actually free a suspended thread; a thread is resumed in destructor:

  begin
    Terminate;
    if FCreateSuspended then
      Resume;
    WaitFor;
  end;

Since Terminate is called before Resume, the Execute method never runs, and thread terminates immediately after being resumed:

  try
    if not Thread.Terminated then
    try
      Thread.Execute;
    except
      Thread.FFatalException := AcquireExceptionObject;
    end;
  finally
    Result := Thread.FReturnValue;
    FreeThread := Thread.FFreeOnTerminate;
    Thread.DoTerminate;
    Thread.FFinished := True;
    SignalSyncEvent;
    if FreeThread then Thread.Free;

Now look at the last line - you call destructor (Thread.Free) from destructor itself! Fantastic bug!

---

To answer your questions:

1. You just can't use FreeOnTerminate:= True in your code;
2. You should ask Embarcadero why TThread is designed so; my guess - some code (DoTerminate method) should be executed in thread context while thread terminates.

---

You can send a feature request to QC: add FFreeOnTerminate:= False to TThread.Destroy implementation:

destructor TThread.Destroy;
begin
  FFreeOnTerminate:= False;
// everything else is the same
  ..
end;

That should prevent recursive desctructor call and make your code valid.