Delphi Direct Oracle Access，DOA 4.0.7.1，TOracleEvent.Stop挂起，如何停止它？

Question

FOLLOW-UP (Apr 9, 2014): I cannot repeat this issue after replacing DOA 4.0.7.1 with DOA 4.1.1. My question is deprecated! However, I am still interested in receiving feedback from anyone that has implemented TOracleEvent in production software.

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Original Question:
How do I reliably implement the TOracleEvent for DBMS_ALERTs in Delphi 6 using DOA 4.0.7?

or

How do I work around the TOracleEvent.Stop method lockup without hacking the DOA source code?

If possible, I would prefer either a fix or a work around that does not need to modify the original AllRoundAutomations DOA source code.

I am working with legacy Delphi 6 and AllRoundAutomations Direct Oracle Access DOA 4.0.7. I have hit a show stopper where the DOA TOracleEvent instance cannot be stopped and freed, so the application's process cannot terminate itself.

My understanding from the DOA documentation is that I should call the myOracleEvent.Stop() to clean up the TOracleEvent instance. However, if I execute the Stop() method, my application will "Hang" at that method call. If I don't call Stop(), my application will close, but its Process will stay alive indefinitely until I kill it with taskmanager or other means.

There are 2 types of signals that the TOracleEvent can manage: dbms_alert and pipes. I am using the dbms_alert signal to enable multiple receivers.

A search on this issue finds that a few others have experienced this problem, in the AllRoundAutomations forum, but I have not found any Answers.

I do not think this is an Oracle server side issue because the Application's TOracleEvent process is causing the blockage. That is, the TOracleEvent instance is not "waiting" for anything, it is just hung. It is not released if I kill the "hanging" Oracle sessions server side. Of course, the issue could be that my program does not configure the TOracleEvent properties properly or clean it up properly for disposal.

The following is Delphi-6 code for a test case state machine that isolates the problem of the DOA.TOracleEvent.Stop() method freezing. If TOracleEvent.Stop() is fixed, then the state machine should display results similar to that shown after the code below. If not fixed, the Stop() method will freeze when called. I was not able to get any small test applications to work reliably without fixing (hacking) the DOA source code.

implementation
{$R *.dfm}

const
  ALERT_NAME__STACKOVERFLOW = 'STACKOVERFLOW';

/// This is the TOracleEvent.OnEvent handler that listens for DBMS_ALERT Signals.
procedure TForm1.OracleEvent1Event(Sender: TOracleEvent; const ObjectName: String; const Info: Variant);
var
  ii: integer;
begin
  Memo1.Lines.Add('*> Oracle Event Received!');
  Memo1.Lines.Add('   DBMS_ALERT.SIGNAL name = ' + ObjectName);
  if VarIsArray(Info) then begin
    for ii := 0 to VarArrayHighBound(Info, 1) do begin
      Memo1.Lines.Add('     Message= ' + Info[ii]);
    end;
  end;
  Memo1.Refresh();
end;

procedure TForm1.btnRunStateMachineClick(Sender: TObject);
begin
  // Button clicked to clear the display and start the state machine timer.
  AppState := 0;
  Memo1.Lines.Clear();
  Timer1.Enabled := true;
end;

procedure TForm1.Timer1Timer(Sender: TObject);
begin
  // Time Interval State Machine that steps through the test sequence. 

  // Stop the Timer for Failsafe
  Timer1.Enabled := false;

  if (AppState = 0) then begin
      try
        Memo1.Lines.Add('==========================');
        Memo1.Lines.Add('Connecting OracleSession2 to DB ...');
        // Check for, Close, and Destroy previous Sessions
        if (Assigned(OracleSession2) and OracleSession2.Connected) then begin
          // Close the Connection
          OracleSession2.LogOff();
          OracleSession2.Connected := false;
          OracleSession2.Free();
          OracleSession2 := nil;
        end;
        if (not Assigned(OracleSession2)) then begin
          OracleSession2 := TOracleSession.Create(self);
        end;
        // Configure Session #2 for general purpose routines
        OracleSession2.LogonDatabase := 'STACKOVERFLOW_DB';
        OracleSession2.LogonPassword := 'drowssap';
        OracleSession2.LogonUserName := 'answer';
        OracleSession2.Pooling := spInternal;
        OracleSession2.ThreadSafe := true;
        OracleSession2.Connected := true;
        Memo1.Lines[Memo1.Lines.Count - 1] := 'DB OracleSession2 Connected!';
        Inc(AppState);
      except
        on ex0: Exception do begin
          AppState := -1;
          Memo1.Lines.Add('!! ERROR !! ' + ex0.Message);
        end;
      end;
    end

  else if (AppState = 1) then begin
      try
        Memo1.Lines.Add('Connecting OracleSession1 to DB ...');
        if (Assigned(OracleSession1) and OracleSession1.Connected) then begin
          OracleSession1.LogOff();
          OracleSession1.Connected := false;
          OracleSession1.Free();
          OracleSession1 := nil;
        end;
        if (not Assigned(OracleSession1)) then begin
          OracleSession1 := TOracleSession.Create(self);
        end;

        // Configure Session #1 for the OracleEvent Handler
        OracleSession1.LogonDatabase := 'STACKOVERFLOW_DB';
        OracleSession1.LogonPassword := 'drowssap';
        OracleSession1.LogonUserName := 'answer';
        OracleSession1.Pooling := spInternal;
        OracleSession1.ThreadSafe := true;
        OracleSession1.Connected := true;
        Memo1.Lines[Memo1.Lines.Count - 1] := 'DB OracleSession1 Connected!';
        Inc(AppState);
      except
        on ex1: Exception do begin
          AppState := -1;
          Memo1.Lines.Add('!! ERROR !! ' + ex1.Message);
        end;
      end;
    end

  else if (AppState = 2) then begin
      try
        Memo1.Lines.Add('Configuring the Oracle Event Handler ...');
        if (not OracleEvent1.Started) then begin
          if (OracleEvent1.Session = nil) then begin
            OracleEvent1.Session := OracleSession1;
          end;
          OracleEvent1.KeepConnection := false;
          OracleEvent1.Synchronized := true;
          OracleEvent1.TimeOut := 1;
          OracleEvent1.ObjectNames := ALERT_NAME__STACKOVERFLOW;
          OracleEvent1.Start();
          Memo1.Lines[Memo1.Lines.Count -1] := 'Oracle Event Handler Started!';
        end;
        Inc(AppState);
      except
        on ex2: Exception do begin
          AppState := -1;
          Memo1.Lines.Add('!! ERROR !! ' + ex2.Message);
        end;
      end;
    end

  else if ((AppState >= 3) and (AppState <= 7)) then begin
      try
        Memo1.Lines.Add('Sending a DBMS_ALERT Signal to StackOverflow ...');
        try
          OracleSession1.DBMS_Alert.Signal(ALERT_NAME__STACKOVERFLOW, 'Hello StackOverflow! ' + FormatDateTime('HH:nn:ss', now));
          OracleSession1.Commit();
        except
          on ex: Exception do begin
            Memo1.Lines.Add('*> Oracle Event Signal ERRORT!');
            Memo1.Lines.Add(ex.Message);
          end;
        end;
        Inc(AppState);
      except
        on ex3: Exception do begin
          AppState := -1;
          Memo1.Lines.Add('!! ERROR !! ' + ex3.Message);
        end;
      end;
    end

  else if (AppState = 8) then begin
      try
        Memo1.Lines.Add('Disconnecting OracleSession2 from DB ...');
        if (Assigned(OracleSession2) and OracleSession2.Connected) then begin
          OracleSession2.LogOff();
          OracleSession2.Connected := false;
          OracleSession2.Free();
          OracleSession2 := nil;
        end;
        Memo1.Lines[Memo1.Lines.Count -1] := 'DB OracleSession2 Disconnected!';
        Inc(AppState);
      except
        on ex4: Exception do begin
          AppState := -1;
          Memo1.Lines.Add('!! ERROR !! ' + ex4.Message);
        end;
      end;
    end

  else if (AppState = 9) then begin
      try
        Memo1.Lines.Add('Stopping OracleEvent handler.');
        Memo1.Lines.Add(' * THIS IS WHERE THE HANGUP PROBLEM IS CALLED!');
        OracleEvent1.Stop();  // <<<< Freezes here if TOracleEvent.Stop() is not fixed!
        ////
        Memo1.Lines.Add('OracleEvent Handler Stopped OK!');
        Memo1.Lines.Add(' * If we got here, either there were no events...');
        Memo1.Lines.Add('   or the Bug is Fixed B-)');
        Inc(AppState);
      except
        on ex5: Exception do begin
          AppState := -1;
          Memo1.Lines.Add('!! ERROR !! ' + ex5.Message);
        end;
      end;
    end

  else if (AppState = 10) then begin
      try
        Memo1.Lines.Add('Disconnecting OracleSession1 from DB ...');
        if (Assigned(OracleSession1) and OracleSession1.Connected) then begin
          OracleSession1.LogOff();
          OracleSession1.Connected := false;
          OracleSession1.Free();
          OracleSession1 := nil;
        end;
        Memo1.Lines[Memo1.Lines.Count -1] := 'DB OracleSession1 Disconnected!';
        Inc(AppState);
      except
        on ex6: Exception do begin
          AppState := -1;
          Memo1.Lines.Add('!! ERROR !! ' + ex6.Message);
        end;
      end;
    end

  else begin
    AppState := 0;
  end;

  Memo1.Refresh();
  Timer1.Enabled := (AppState >= 0);
end;

procedure TForm1.btnStopClick(Sender: TObject);
begin
  // Stop Button Clicked to halt the State Machine
  Timer1.Enabled := false;
  Memo1.Lines.Add('State Machine Stopped.');
  Memo1.Refresh();
end;

/// Other handlers on the TOracleEvent instance
procedure TForm1.OracleEvent1Start(Sender: TOracleEvent);
begin
  Memo1.Lines.Add('*> Oracle Event START!');
end;

procedure TForm1.OracleEvent1Stop(Sender: TOracleEvent);
begin
  Memo1.Lines.Add('*> Oracle Event STOP!');
end;

procedure TForm1.OracleEvent1Error(Sender: TOracleEvent; const Error: Exception);
begin
  Memo1.Lines.Add('*> Oracle Event ERROR!');
  Memo1.Lines.Add('=> ' + Error.Message);
end;

procedure TForm1.OracleEvent1TimeOut(Sender: TOracleEvent; var Continue: Boolean);
begin
  Memo1.Lines.Add('*> Oracle Event TIMEOUT!');
end;

The following is the state machine's output:

==========================
The following are the results for successfully running 
the state machine with a DOA.TOracleEvent.Stop() "fixed".
Without fixing DOA.TOracleEvent.Stop(), the state machine will "freeze"
==========================

DB OracleSession2 Connected!
DB OracleSession1 Connected!
Configuring the Oracle Event Handler ...
Oracle Event Handler Started!
Sending a DBMS_ALERT Signal to StackOverflow ...
*> Oracle Event Received!
   DBMS_ALERT.SIGNAL name = STACKOVERFLOW
     Message= Hello StackOverflow! 21:55:41
*> Oracle Event TIMEOUT!
Sending a DBMS_ALERT Signal to StackOverflow ...
*> Oracle Event Received!
   DBMS_ALERT.SIGNAL name = STACKOVERFLOW
     Message= Hello StackOverflow! 21:55:42
*> Oracle Event TIMEOUT!
Sending a DBMS_ALERT Signal to StackOverflow ...
*> Oracle Event Received!
   DBMS_ALERT.SIGNAL name = STACKOVERFLOW
     Message= Hello StackOverflow! 21:55:43
*> Oracle Event TIMEOUT!
Sending a DBMS_ALERT Signal to StackOverflow ...
*> Oracle Event Received!
   DBMS_ALERT.SIGNAL name = STACKOVERFLOW
     Message= Hello StackOverflow! 21:55:44
*> Oracle Event TIMEOUT!
Sending a DBMS_ALERT Signal to StackOverflow ...
*> Oracle Event Received!
   DBMS_ALERT.SIGNAL name = STACKOVERFLOW
     Message= Hello StackOverflow! 21:55:45
DB OracleSession2 Disconnected!
*> Oracle Event TIMEOUT!
Stopping OracleEvent handler.
 * This is where the Hangup problem gets called!
*> Oracle Event STOP!
OracleEvent Handler Stopped OK!
 * If we got here, either there were no events...
   or the Bug is Fixed B-)
DB OracleSession1 Disconnected!
==========================

Answer 1

The following is a revision of the TOracleEvent.Stop routine from the DOA 4.1.1 source code. The unchanged bulk DOA code in this routine is replaced by '. . .'

This revised procedure is a functional work around (so far) for the TOracleEvent.Stop() call that intermittently causes my applications to hang.

procedure TOracleEvent.Stop;
var
  bLoggedOff: boolean;
begin
  . . .
  . . .
  // Skip the CriticalSection Enter/Leave toggle.
  // This is causing my application to get stuck here!
  //CriticalSection.Enter;
  //CriticalSection.Leave;
  . . .
  . . .
  // LogOff the duplicate sessions
  if (not KeepConnection) then begin
    bLoggedOff := false;
    // Keep trying the logoff until we actually get Logged Off or disconnected.
    while (not bLoggedOff) do begin
      try
        if ((InternalSession <> nil) and (InternalSession.Connected)) then begin
          InternalSession.LogOff;
        end;
        if ((StopSession <> nil) and (StopSession.Connected)) then begin
          StopSession.LogOff;
        end;
        bLoggedOff := true;
      except
        on exLogoff: Exception do begin
          // This error is typically ORA-24909: Call In Progress, Current Operation Cancelled.
          // Sink this hangover error by sleeping it off
          Sleep(1000);
        end;
      end;
    end;
  end;
end;