Dns.GetHostAddressesAsync gets stuck on Windows

[MartyIX]

Description

The following program is supposed to resolve multiple hostNameOrAddresses by Dns.GetHostAddressesAsync in parallel. However, the program gets stuck when run on Windows 10 (or 11) but it works as expected on the latest Ubuntu.

Reproduction Steps

App.csproj

<Project Sdk="Microsoft.NET.Sdk">
	<PropertyGroup>
		<OutputType>Exe</OutputType>
		<TargetFramework>net6.0</TargetFramework>
		<ImplicitUsings>enable</ImplicitUsings>
		<Nullable>enable</Nullable>
	</PropertyGroup>
</Project>

Program.cs

using System.Net;

namespace DnsCancelling;

public class Program
{
    public static async Task Main(string[] args)
    {
        string invalidAddress = "59FB::1005:CC57::";
        if (args.Length == 0)
        {
            bool result = await ResolveManyAsync(invalidAddress).ConfigureAwait(false);
            Console.WriteLine("Result 1 is {0}.", result);
            Console.WriteLine();
        }

        string[] addresses = new[]
        {
            invalidAddress,
            "localhost",
        };

        bool result2 = await ResolveManyAsync(addresses).ConfigureAwait(false);
        Console.WriteLine("Result 2 is {0}.", result2);
    }

    private static async Task<bool> ResolveManyAsync(params string[] addresses)
    {
        using CancellationTokenSource cts = new();

        Console.WriteLine("Going to resolve {0} addresses.", addresses.Length);
        List<Task<bool>> resolveTasks = new(capacity: addresses.Length);
        foreach (string address in addresses)
        {
            Task<bool> resolveTask = ResolveOneAsync(address, cts);
            resolveTasks.Add(resolveTask);
        }

        bool[] results = await Task.WhenAll(resolveTasks).ConfigureAwait(false);
        return results.All(r => r);
    }

    private static async Task<bool> ResolveOneAsync(string address, CancellationTokenSource cancellationTokenSource)
    {
        bool result = true;

        IPAddress[] ipAddresses;
        try
        {
            Console.WriteLine("Resolving address '{0}'.", address);
            ipAddresses = await Dns.GetHostAddressesAsync(address, cancellationTokenSource.Token).ConfigureAwait(false);
            if (ipAddresses.Length > 0) Console.WriteLine("'{0}' resolved to {1}.", address, ipAddresses[0]);
            else Console.WriteLine("'{0}' can not be resolved.", address);
        }
        catch (Exception e)
        {
            Console.WriteLine("Canceling all due to an exception when resolving '{0}': {1}", address, e.Message);
            cancellationTokenSource.Cancel();
            Console.WriteLine("Cancellation complete.");
            result = false;
        }

        return result;
    }
}

Run the sample using dotnet build && dotnet run using CLI.

Expected behavior

The program should terminate. I verified that the program terminates on the latest Ubuntu version.

Actual behavior

The program gets stuck and never finishes when it is run on Windows.

Regression?

No response

Known Workarounds

No response

Configuration

• .NET 6
• Windows 10, 11.

Other information

No response

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Issue Details

---

Description

The following program is supposed to resolve multiple hostNameOrAddresses by Dns.GetHostAddressesAsync in parallel. However, the program gets stuck when run on Windows 10 (or 11) but it works as expected on the latest Ubuntu.

Reproduction Steps

App.csproj

<Project Sdk="Microsoft.NET.Sdk">
	<PropertyGroup>
		<OutputType>Exe</OutputType>
		<TargetFramework>net6.0</TargetFramework>
		<ImplicitUsings>enable</ImplicitUsings>
		<Nullable>enable</Nullable>
	</PropertyGroup>
</Project>

Program.cs

using System.Net;

namespace DnsCancelling;

public class Program
{
    public static async Task Main(string[] args)
    {
        string invalidAddress = "59FB::1005:CC57::";
        if (args.Length == 0)
        {
            bool result = await ResolveManyAsync(invalidAddress).ConfigureAwait(false);
            Console.WriteLine("Result 1 is {0}.", result);
            Console.WriteLine();
        }

        string[] addresses = new[]
        {
            invalidAddress,
            "localhost",
        };

        bool result2 = await ResolveManyAsync(addresses).ConfigureAwait(false);
        Console.WriteLine("Result 2 is {0}.", result2);
    }

    private static async Task<bool> ResolveManyAsync(params string[] addresses)
    {
        using CancellationTokenSource cts = new();

        Console.WriteLine("Going to resolve {0} addresses.", addresses.Length);
        List<Task<bool>> resolveTasks = new(capacity: addresses.Length);
        foreach (string address in addresses)
        {
            Task<bool> resolveTask = ResolveOneAsync(address, cts);
            resolveTasks.Add(resolveTask);
        }

        bool[] results = await Task.WhenAll(resolveTasks).ConfigureAwait(false);
        return results.All(r => r);
    }

    private static async Task<bool> ResolveOneAsync(string address, CancellationTokenSource cancellationTokenSource)
    {
        bool result = true;

        IPAddress[] ipAddresses;
        try
        {
            Console.WriteLine("Resolving address '{0}'.", address);
            ipAddresses = await Dns.GetHostAddressesAsync(address, cancellationTokenSource.Token).ConfigureAwait(false);
            if (ipAddresses.Length > 0) Console.WriteLine("'{0}' resolved to {1}.", address, ipAddresses[0]);
            else Console.WriteLine("'{0}' can not be resolved.", address);
        }
        catch (Exception e)
        {
            Console.WriteLine("Canceling all due to an exception when resolving '{0}': {1}", address, e.Message);
            cancellationTokenSource.Cancel();
            Console.WriteLine("Cancellation complete.");
            result = false;
        }

        return result;
    }
}

Expected behavior

The program gets stuck and never finishes when it is run on Windows.

Actual behavior

The program should terminate. I verified that the program terminates on the latest Ubuntu version.

Regression?

No response

Known Workarounds

No response

Configuration

Tested on:

• .NET 6
• Windows 10, 11.

Other information

No response

Author:	MartyIX
Assignees:	-
Labels:	area-System.Net, untriaged
Milestone:	-

[tomrus88]

The program gets stuck and never finishes when it is run on Windows.

I can't repro, program finishes nearly instantly for me on Win11.

[MartyIX]

@tomrus88 Ah, I forgot to add that I run the program using dotnet build && dotnet run in cmd / Terminal (maybe ideally using Release configuration). In VS2022, I cannot reproduce it either.

[MartyIX]

Also it's good idea to run the sample 10 times in a row.

[stephentoub]

@dotnet/ncl, took a quick look, and it seems likely this is a deadlock caused by lock inversion between a lock we take in managed and a critical section used in the Windows implementation:

[geoffkizer]

It certainly does look like that. That said, I'm surprised that the GetAddrInfoExCancel call is blocking waiting on the callback to complete.

Regardless, it seems like we need to avoid taking our managed lock during the callback.

[antonfirsov]

Would it remove the need for the lock, if we used _cancellationRegistration.Dispose() instead of _cancellationRegistration.Unregister()? Dispose should wait for outstanding cancellation callbacks.

runtime/src/libraries/System.Net.NameResolution/src/System/Net/NameResolutionPal.Windows.cs

Lines 407 to 429 in 9529c35

	_cancellationRegistration = cancellationToken.UnsafeRegister(o =>
	{
	var @this = (GetAddrInfoExState)o!;
	int cancelResult = 0;
	lock (@this)
	{
	GetAddrInfoExContext* context = @this._cancellationContext;
	if (context != null)
	{
	// An outstanding operation will be completed with WSA_E_CANCELLED, and GetAddrInfoExCancel will return NO_ERROR.
	// If this thread has lost the race between cancellation and completion, this will be a NOP
	// with GetAddrInfoExCancel returning WSA_INVALID_HANDLE.
	cancelResult = Interop.Winsock.GetAddrInfoExCancel(&context->CancelHandle);
	}
	}
	if (cancelResult != 0 && cancelResult != Interop.Winsock.WSA_INVALID_HANDLE && NetEventSource.Log.IsEnabled())
	{
	NetEventSource.Info(@this, $"GetAddrInfoExCancel returned error {cancelResult}");
	}
	}, this);

runtime/src/libraries/System.Net.NameResolution/src/System/Net/NameResolutionPal.Windows.cs

Lines 433 to 442 in 9529c35

	public CancellationToken UnregisterAndGetCancellationToken()
	{
	lock (this)
	{
	_cancellationContext = null;
	_cancellationRegistration.Unregister();
	}
	return _cancellationRegistration.Token;
	}

/cc @scalablecory as author of #33420.

[karelz]

Triage: Deadlock in new API in .NET 6.0. We should fix it and backport it to .NET 6.0 servicing if we have customers hitting it.
@MartyIX is it affecting your production in any way?

[MartyIX]

@karelz Our project is not yet released but we would love to use the code snippet in the original post once we are production ready.

[antonfirsov]

Reopening to track 6.0.* backport. We need a validation with the original console app.

[karelz]

Fixed in 7.0 (main) in PR #63904 and in 6.0.5 in PR #67291.