Tag destructor throws an exception with an incorrect IP

[VitorKawao]

Hello,

I am using libplctag at version 1.6.0-alpha.2 and libplctag.NativeImport at version 2.0.0.0-alpha.5

I am trying to connect to Compact Logix PLC (Allen Bradley).
It works fine when the IP is correct, but when I use an invalid IP or the network is down, it generates an exception in the Tag destructor. (ErrNullPtr)

When I call tag.Initialize(); with an incorrect IP, it throws an exception that I can handle. However, after some time, the destructor is called asynchronously, and it throws the same exception again, causing my app to crash.

Is there a better way to check for a connection than using tag.Initialize()? Or is it possible to handle this exception?

I am attaching my debug logs

log.txt

Thank you,
Vítor Guedes

[kyle-github]

Is that throwing inside the destructor? Once the core library API call plc_tag_destroy() is called, the tag handle does not point to anything so trying to get the status on it may result in a NULL pointer warning from inside the library. It is not an actually NULL pointer reference, it is that the library noticed that it had a null pointer instead of a valid tag info object.

[timyhac]

Yep there are definitely some bugs here, thanks for raising it @VitorKawao.

There was some work done in #420 that introduced this issue and I'm surprised this hasn't surfaced before.

Below are some repros of the problem.

Test Harness

using libplctag;
using System;
using System.Diagnostics;

var tag = new Tag()
{
    Gateway = "10.10.10.10",
    Path = "1,0",
    PlcType = PlcType.ControlLogix,
    Protocol = Protocol.ab_eip,
    Name = "FAKE_DINT",
    Timeout = TimeSpan.FromSeconds(5)
};

var sw = new Stopwatch();
sw.Start();

Log("Start");

// ... Insert Test Case ...

Log("Finish");

void Log(string message) => Console.WriteLine($"[{sw.ElapsedMilliseconds:0000}ms] {message}");

Case 1: `InitializeAsync()` then `Dispose()`

Expectation is to see InitializeAsync() throw with Timeout after 5 seconds, and then the Dispose succesfully completes and does not perform any cleanup.

try
{
    Log("Init");
    await tag.InitializeAsync();
    Log("Init Success");
}
catch (Exception ex)
{
    Log($"Init Exception: {ex.GetType()} {ex.Message}");
}

try
{
    Log("Dispose");
    tag.Dispose();
    Log("Dispose Success");
}
catch (Exception ex)
{
    Log($"Dispose Exception: {ex.GetType()} {ex.Message}");
}

[0000ms] Start
[0015ms] Init
[5067ms] Init Exception: libplctag.LibPlcTagException ErrorTimeout
[5067ms] Dispose
[5069ms] Dispose Success
[5070ms] Finish

Case 2: `InitializeAsync(token)` then `Dispose()`, where token cancels after 1 second

Expectation is to see `InitializeAsync()` throw with `TaskCancelledException` after 1 second, and then the Dispose succesfully completes and does not perform any cleanup.

try
{
    Log("Init");
    var cts = new CancellationTokenSource();
    cts.CancelAfter(TimeSpan.FromSeconds(1));
    await tag.InitializeAsync(cts.Token);
    Log("Init Success");
}
catch (Exception ex)
{
    Log($"Init Exception: {ex.GetType()} {ex.Message}");
}

try
{
    Log("Dispose");
    tag.Dispose();
    Log("Dispose Success");
}
catch (Exception ex)
{
    Log($"Dispose Exception: {ex.GetType()} {ex.Message}");
}

[0000ms] Start
[0010ms] Init
[1067ms] Init Exception: System.Threading.Tasks.TaskCanceledException A task was canceled.
[1067ms] Dispose
[1099ms] Dispose Success
[1099ms] Finish

Case 3: `Initialize()` then `Dispose()`

Expectation is to see Initialize() throw with Timeout after 5 seconds, and then the Dispose succesfully completes and does not perform any cleanup.

try
{
    Log("Init");
    tag.Initialize();
    Log("Init Success");
}
catch (Exception ex)
{
    Log($"Init Exception: {ex.GetType()} {ex.Message}");
}

try
{
    Log("Dispose");
    tag.Dispose();
    Log("Dispose Success");
}
catch (Exception ex)
{
    Log($"Dispose Exception: {ex.GetType()} {ex.Message}");
}

[0000ms] Start
[0046ms] Init
[5146ms] Init Exception: libplctag.LibPlcTagException ErrorNotFound
[5146ms] Dispose
[5153ms] Dispose Exception: libplctag.LibPlcTagException ErrorNullPtr
[5153ms] Finish

Case 4: `Dispose()` without `Initialize()` or `InitializeAsync()`

Even though it would be unusual to dispose the tag without initializing, it is possible, and the Dispose should still complete succesfully without doing any cleanup.

try
{
    Log("Dispose");
    tag.Dispose();
    Log("Dispose Success");
}
catch (Exception ex)
{
    Log($"Dispose Exception: {ex.GetType()} {ex.Message}");
}

[0000ms] Start
[0011ms] Dispose
[0037ms] Dispose Exception: libplctag.LibPlcTagException ErrorNullPtr
[0038ms] Finish

Results

Case 1: PASS
Case 2: PASS
Case 3: FAIL
Case 4: FAIL

[timyhac]

PR #433 shuffles the logic around to resolve the above test cases.

[VitorKawao]

Thank you for the quick reply.
Would you like me to do some tests?

Do you intend to generate libplctag-v1.6.0-alpha.3 after this fix?

Thank you,
Vítor Guedes

[timyhac]

Do you intend to generate libplctag-v1.6.0-alpha.3 after this fix?

Yes

Would you like me to do some tests?

Yes I would appreciate if could test with the permutations of synchronous and asynchronous initialization, explicit and implicit disposal, and plcs/tags that exist and those that don't,

Explicit Disposal

var cts = new CancellationTokenSource();
cts.CancelAfter(60*1000); // 1 minute

while(!cts.IsCancellationRequested)
{
  var tag = new Tag()
  {
      ...
  };
  
  tag.Initialize(); // also test with `await tag.InitializeAsync()`
  tag.Dispose();
}

Implicit Disposal

while(!cts.IsCancellationRequested)
{
  var tag = new Tag()
  {
      ...
  };
  
  tag.Initialize();  // also test with `await tag.InitializeAsync()`
  // tag.Dispose();
}

[VitorKawao]

Hello,

I tested using the libplctag.dll generated by #433, and it seems to work fine for both my bug and the two tests mentioned above.
Even if the PLC/tag does not exist, it is not throwing an exception

Best regards,
Vítor Guedes

[timyhac]

Thanks! Released as v1.6.0-alpha.3

[kyle-github]

Can we close this?

[timyhac]

@kyle-github - I closed it, is there any work remaining?

[kyle-github]

Sorry, missed that it was already closed!