Crash - "Start may not be called on a task that has completed" - at "OnProcessMessageReceived"

[eadan-y]

Hey
We see a crash from our end-users.
Wasn't able to reproduce this locally yet so no information other than the stack trace
CefSharp version - 90.6.50

System.InvalidOperationException: Start may not be called on a task that has completed.\r\n at System.Threading.Tasks.Task.Start(TaskScheduler scheduler)\r\n at CefSharp.Internals.ConcurrentMethodRunnerQueue.Enqueue(MethodInvocation methodInvocation)\r\n at CefSharp.Internals.ClientAdapter.OnProcessMessageReceived(ClientAdapter* , scoped_refptr* browser, scoped_refptr* frame, cef_process_id_t source_process, scoped_refptr* message)

[amaitland]

Nothing immediately stands out as to the cause. Do you have any clues as to what was happening when the exception occurred? Was Dispose being called? Has the exception occurred more than once?

My initial thoughts are we should switch to using Task.Run as we are simply queueing on the ThreadPool, no reason to use Task.Start (or at least I don't remember there being a reason).

I'll see if Google yields any possible causes based on the error message.

[amaitland]

Removing the cancellationTokenSource.IsCancellationRequested check from ConcurrentMethodRunnerQueue.Enqueue and running the below xUnit test I get an InvalidOperationException.

[Fact]
public void DisposeConcurrentMethodRunnerQueueThenQueueTaskDemo()
{
	var methodInvocation = new MethodInvocation(1, 1, 1, "Testing", 1);
	methodInvocation.Parameters.Add("Echo Me!");

	var objectRepository = new JavascriptObjectRepository
	{
		NameConverter = null
	};

	var methodRunnerQueue = new ConcurrentMethodRunnerQueue(objectRepository);

	methodRunnerQueue.Dispose();

	Assert.Throws<InvalidOperationException>(() => methodRunnerQueue.Enqueue(methodInvocation));
}

I created a couple of synthetic tests as well to compare the behaviour of using Task.Start vs Task.Run. Will switch to Task.Run as the result is a Task with TaskStatus.Canceled, no exception is thrown.

[Fact]
public void SimulateStartOnTaskAlreadyCompleted()
{
	var cts = new CancellationTokenSource();

	//Create a new Task
	var task = new Task(async () =>
	{
		await Task.Delay(100);

	}, cts.Token);

	//Cancel before started
	cts.Cancel();

	Assert.Throws<InvalidOperationException>(() => task.Start(TaskScheduler.Default));
}

[Fact]
public void SimulateTaskRunStartOnTaskAlreadyCompleted()
{
	var cts = new CancellationTokenSource();

	//Cancel before started
	cts.Cancel();

	var task = Task.Run(async () =>
	{
		await Task.Delay(100);
	}, cts.Token);

	Assert.NotNull(task);
	Assert.Equal(TaskStatus.Canceled, task.Status);
}

[amaitland]

I've created #3639 (so the issue can be assigned to a milestone) and committed my proposed fix in 4ab6d2a

[amaitland]

@eadan-y Let me know if you are happy and I can release a new version. Can confirm if you are using version 90 or 91?

[eadan-y]

@amaitland
We currently use 90 in production and 91 in the next version. Fixing it for 91 should be enough. Thanks for the quick response!

[amaitland]

Version 91.1.211 is now on Nuget.org