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

オープン
  • 何か問題がありますか？
#136,745 コメント 1 件 リアクション 0 件 担当者 0 名 GitHub で見る

メンテナーはふだん 1 日以内に返信

まだ誰も着手していません。

評価

難易度
4/5
見積もり時間
3〜5日
初心者へのやさしさ
55/100
issue の種類
バグ
明瞭さ
明確に書かれている
活発さ
活発
技術スタック
csharp
領域
backend

調査の方向性

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.

索引モデルが issue の本文から書いたものです。

説明

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

主要言語
C#
スター
18.4k
フォーク
5.8k
平均マージ
2日 13時間
マージ済み PR（30日）
588

環境構築

Codespaces で開く

このプロジェクトの開発コンテナを、あなたの GitHub アカウントでブラウザ上に起動します。

はじめの一歩

  1. issue を最後まで読み、次にプロジェクトのコントリビューションガイドを読みます。
  2. 着手することを issue にコメントします — 二人が同じ作業をするのを防げます。
  3. リポジトリをフォークし、ブランチを切って変更します。
  4. issue 番号を参照したプルリクエストを送ります。

dotnet/runtime のほかの issue

dotnet/runtime の issue をすべて見る

似ている issue

C# の issue をもっと見る

新しい issue をメールで受け取る

初心者向けの GitHub issue を短くまとめたダイジェスト。