ConcurrencyLimiter: a waiter cancelled during a release can leave a free permit that a queued request never gets

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Issue 类型
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csharp
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backend

调研方向

Start with ConcurrencyLimiter and its queue-processing and cancellation paths; the issue provides a self-contained reproduction using OldestFirst. Run the reproduction or add a focused regression test for the cancellation, release, and new-acquire interleaving. Done when the queued request receives the free permit and the regression test passes.

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描述

area-System.Threading untriaged
Description

With QueueProcessingOrder.OldestFirst, a ConcurrencyLimiter can end up with a free permit and a queued request that never gets it. No lease is held, so no Release ever comes to hand the permit out. The queued request waits until its own token is cancelled, and AttemptAcquire is refused while it waits.

It happens when three things coincide: a queued waiter's token is cancelled, the permit holder releases, and a new AcquireAsync arrives.

Reproduction Steps
using System.Diagnostics;
using System.Threading.RateLimiting;

// One permit, queue of 2, oldest first. Each round: A holds the permit and B waits with a token.
// Then, at the same moment, B's token is cancelled, A releases, and C calls AcquireAsync. However
// those three interleave, C should end up holding the permit, since nothing else does.
var budget = Stopwatch.StartNew();
for (var round = 1; budget.Elapsed < TimeSpan.FromMinutes(2); round++)
{
    var limiter = new ConcurrencyLimiter(new ConcurrencyLimiterOptions
    {
        PermitLimit = 1,
        QueueLimit = 2,
        QueueProcessingOrder = QueueProcessingOrder.OldestFirst,
    });

    var a = limiter.AttemptAcquire();
    using var cts = new CancellationTokenSource();
    var b = limiter.AcquireAsync(1, cts.Token).AsTask();

    using var start = new Barrier(3);
    Task<RateLimitLease>? c = null;
    await Task.WhenAll(
        Task.Run(() => { start.SignalAndWait(); cts.Cancel(); }),
        Task.Run(() => { start.SignalAndWait(); a.Dispose(); }),
        Task.Run(() => { start.SignalAndWait(); c = limiter.AcquireAsync(1).AsTask(); }));

    try { (await b).Dispose(); } catch (OperationCanceledException) { }

    if (await Task.WhenAny(c!, Task.Delay(TimeSpan.FromSeconds(1))) != c)
    {
        var stats = limiter.GetStatistics()!;
        Console.WriteLine($"Round {round}: C still waiting after 1 s, with nothing holding a permit.");
        Console.WriteLine($"  CurrentAvailablePermits = {stats.CurrentAvailablePermits}, CurrentQueuedCount = {stats.CurrentQueuedCount}");
        Console.WriteLine($"  A new AttemptAcquire is granted: {limiter.AttemptAcquire().IsAcquired}");
        return 1;
    }

    (await c!).Dispose();
}

Console.WriteLine("No stall within 2 minutes.");
return 0;

Output:

Round 17524: C still waiting after 1 s, with nothing holding a permit.
  CurrentAvailablePermits = 1, CurrentQueuedCount = 1
  A new AttemptAcquire is granted: False
Expected behavior

C is granted the permit, since nothing else holds it.

Actual behavior

The permit stays available and C stays queued until its own token is cancelled. With steady traffic, each new request queues behind the free permit and times out, so the limiter grants nothing until no request is queued.

Regression?

No response

Known Workarounds

No response

Configuration

Windows 11 x64
Repro'd on both .NET 11 and 10

Other information

No response

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