Thread containing connection components to the database generates an exception when they are released from memory

Question

I needed to add some threads to carry out certain processes in the background.
This thread accesses the methods of a Webservice and also performs operations in the database.

I create all the objects I need to use in this thread within itself to avoid concurrent access with components outside it. Like IBDatabase and the IBTransaction (use IBX components).

So my thread’s kind of like this:

TThreadAtualizarDados = class(TThread)
private
  FIBDatabase: TIBDatabase;
  FIBTransaction: TIBTransaction;      
public
  constructor Create(AIBDatabase: TIBDatabase); reintroduce;
  destructor Destroy; override;
  procedure Execute; override;    
end;

Like I said, everything I need to use in the thread I instill in myself.

constructor TThreadAtualizarDados.Create(AIBDatabase: TIBDatabase);
begin
  inherited Create(false);
  FreeOnTerminate := true;

  // conexão com o banco de dados
  FIBDatabase := TIBDatabase.Create(nil);
  FIBDatabase.LoginPrompt := false;
  FIBDatabase.DatabaseName := AIBDatabase.DatabaseName;
  FIBDatabase.Params.Text := AIBDatabase.Params.Text;

  // controle transacional
  FIBTransaction := TIBTransaction.Create(nil);
  FIBTransaction.DefaultDatabase := FIBDatabase;

  FIBDatabase.DefaultTransaction := FIBTransaction;
end;

The processes I wish her to carry out are successfully carried out until her execution is complete.

destructor TThreadAtualizarDados.Destroy;
begin
  inherited;
  if FIBTransaction.Active then
    FIBTransaction.Commit;

  // Comentados porque está causando erro
  //
  // FIBDatabase.Free;  
  // FIBTransaction.Free;
end;

The lines that end the connection and the transaction are commented because if they are executed, a certain time later (short time) after the thread is finished (method output Destroy) an error is generated.

Error message:

Programa.exe faulted with message: 'application-defined Exception (code 0xc000041d) at 0x77071a91'.

What does not characterize a Access Violation.

What might be causing this error, and what to do to resolve it?

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Adding more information:

The thread is instilled this way:

procedure TFrmAtualizaDados.p_Iniciar_Atualizacao;
begin
  TThreadAtualizarDados.Create(Dm.IdbIntsys);
end;

That is, no external treatment is done in the thread. I’m not passing her reference to any variable so that I could be finishing it manually, or anything else. Since I have set her parameter to execute as soon as she is instituted inherited Create(false); and had her destroy herself automatically after finalizing her method Execute, with the FreeOnTerminate := true;, then I just instacing the parameters and let it work.

Even, I have even other classes in the thread that I instâncio, as TStringList and TList that do not give problems, only the instances of the components of the database generate problems if they are released from memory (.Free).

---

Another detail is that the error message is triggered only when I change the application to Delphi, in mode debug.

Of course the option "Stop on Delphi Exceptions", in "Debuger Options", is enabled.

Answer 1

As reported by Tiago Silva, if instantiated, IB**** objects within the scope of the Execute method do not occur. The logical explanation for this is that the components need to be created and destroyed within it Thread. As the method Create runs in Main Thread (Main Thread), and most likely threads are with FreeOnTerminate := True and thus the destructor is executed in the Thread created by the Tthread object, then the problem occurred.

Answer 2

Sometimes objects they inherit from TComponent has a strange behavior if destroyed within Threads.

To solve this type of problem, create a method to destroy the objects:

procedure TThreadAtualizaDados.DestruirObjetos;
begin
  FIBDatabase.Free;  
  FIBTransaction.Free;
end;

And in the Destroy of thread destroy them with Synchronize()

destructor TThreadAtualizarDados.Destroy;
begin
  inherited;
  if FIBTransaction.Active then
    FIBTransaction.Commit;

  Synchronize(DestruirObjetos);
end;