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| 1 | +// Copyright (c) Microsoft Corporation. All rights reserved. |
| 2 | +// Licensed under the MIT license. See LICENSE file in the project root for full license information. |
| 3 | + |
| 4 | +using System; |
| 5 | +using System.Collections.Concurrent; |
| 6 | +using System.Threading; |
| 7 | +using System.Threading.Tasks; |
| 8 | + |
| 9 | +namespace Microsoft.VisualStudio.Threading; |
| 10 | + |
| 11 | +/// <summary> |
| 12 | +/// A mutex that can be entered asynchronously. |
| 13 | +/// </summary> |
| 14 | +/// <remarks> |
| 15 | +/// <para> |
| 16 | +/// This class utilizes the OS mutex synchronization primitive, which is fundamentally thread-affinitized and requires synchronously blocking the thread that will own the mutex. |
| 17 | +/// This makes a native mutex unsuitable for use in async methods, where the thread that enters the mutex may not be the same thread that exits it. |
| 18 | +/// This class solves that problem by using a private dedicated thread to enter and release the mutex, but otherwise allows its owner to execute async code, switch threads, etc. |
| 19 | +/// </para> |
| 20 | +/// </remarks> |
| 21 | +/// <example><![CDATA[ |
| 22 | +/// using AsyncCrossProcessMutex mutex = new("Some-Unique Name"); |
| 23 | +/// using (await mutex.EnterAsync()) |
| 24 | +/// { |
| 25 | +/// // Code that must not execute in parallel with any other thread or process protected by the same named mutex. |
| 26 | +/// } |
| 27 | +/// ]]></example> |
| 28 | +public class AsyncCrossProcessMutex |
| 29 | + : IDisposable |
| 30 | +{ |
| 31 | +#pragma warning disable SA1310 // Field names should not contain underscore |
| 32 | + private const int STATE_READY = 0; |
| 33 | + private const int STATE_HELD_OR_WAITING = 1; |
| 34 | + private const int STATE_DISPOSED = 2; |
| 35 | +#pragma warning restore SA1310 // Field names should not contain underscore |
| 36 | + |
| 37 | + private static readonly Action ExitSentinel = new Action(() => { }); |
| 38 | + private readonly Thread namedMutexOwner; |
| 39 | + private readonly BlockingCollection<Action> mutexWorkQueue = new(); |
| 40 | + private readonly Mutex mutex; |
| 41 | + private int state; |
| 42 | + |
| 43 | + /// <summary> |
| 44 | + /// Initializes a new instance of the <see cref="AsyncCrossProcessMutex"/> class. |
| 45 | + /// </summary> |
| 46 | + /// <param name="name"> |
| 47 | + /// A non-empty name for the mutex, which follows standard mutex naming rules. |
| 48 | + /// This name will share a namespace with other processes in the system and collisions will result in the processes sharing a single mutex across processes. |
| 49 | + /// </param> |
| 50 | + /// <remarks> |
| 51 | + /// See the help docs on the underlying <see cref="Mutex"/> class for more information on the <paramref name="name"/> parameter. |
| 52 | + /// Consider when reading that the <c>initiallyOwned</c> parameter for that constructor is always <see langword="false"/> for this class. |
| 53 | + /// </remarks> |
| 54 | + public AsyncCrossProcessMutex(string name) |
| 55 | + { |
| 56 | + Requires.NotNullOrEmpty(name); |
| 57 | + this.namedMutexOwner = new Thread(this.MutexOwnerThread, 100 * 1024) |
| 58 | + { |
| 59 | + Name = $"{nameof(AsyncCrossProcessMutex)}-{name}", |
| 60 | + }; |
| 61 | + this.mutex = new Mutex(false, name); |
| 62 | + this.namedMutexOwner.Start(); |
| 63 | + this.Name = name; |
| 64 | + } |
| 65 | + |
| 66 | + /// <summary> |
| 67 | + /// Gets the name of the mutex. |
| 68 | + /// </summary> |
| 69 | + public string Name { get; } |
| 70 | + |
| 71 | + /// <summary> |
| 72 | + /// Disposes of the underlying native objects. |
| 73 | + /// </summary> |
| 74 | + public void Dispose() |
| 75 | + { |
| 76 | + int priorState = Interlocked.Exchange(ref this.state, STATE_DISPOSED); |
| 77 | + if (priorState != STATE_DISPOSED) |
| 78 | + { |
| 79 | + this.mutexWorkQueue.Add(ExitSentinel); |
| 80 | + this.mutexWorkQueue.CompleteAdding(); |
| 81 | + } |
| 82 | + } |
| 83 | + |
| 84 | + /// <inheritdoc cref="EnterAsync(TimeSpan)"/> |
| 85 | + public Task<LockReleaser> EnterAsync() => this.EnterAsync(Timeout.InfiniteTimeSpan); |
| 86 | + |
| 87 | + /// <summary> |
| 88 | + /// Acquires the mutex asynchronously. |
| 89 | + /// </summary> |
| 90 | + /// <param name="timeout">The maximum time to wait before timing out. Use <see cref="Timeout.InfiniteTimeSpan"/> for no timeout, or <see cref="TimeSpan.Zero"/> to acquire the mutex only if it is immediately available.</param> |
| 91 | + /// <returns>A value whose disposal will release the mutex.</returns> |
| 92 | + /// <exception cref="TimeoutException">Thrown from the awaited result if the mutex could not be acquired within the specified timeout.</exception> |
| 93 | + /// <exception cref="ArgumentOutOfRangeException">Thrown from the awaited result if the <paramref name="timeout"/> is a negative number other than -1 milliseconds, which represents an infinite timeout.</exception> |
| 94 | + /// <exception cref="InvalidOperationException">Thrown if called before a prior call to this method has completed, with its releaser disposed if the mutex was entered.</exception> |
| 95 | + public async Task<LockReleaser> EnterAsync(TimeSpan timeout) => await this.TryEnterAsync(timeout) ?? throw new TimeoutException(); |
| 96 | + |
| 97 | + /// <summary> |
| 98 | + /// Acquires the mutex asynchronously, allowing for timeouts without throwing exceptions. |
| 99 | + /// </summary> |
| 100 | + /// <param name="timeout">The maximum time to wait before timing out. Use <see cref="Timeout.InfiniteTimeSpan"/> for no timeout, or <see cref="TimeSpan.Zero"/> to acquire the mutex only if it is immediately available.</param> |
| 101 | + /// <returns> |
| 102 | + /// If the mutex was acquired, the result is a value whose disposal will release the mutex. |
| 103 | + /// In the event of a timeout, the result in a <see langword="null" /> value. |
| 104 | + /// </returns> |
| 105 | + /// <exception cref="ArgumentOutOfRangeException">Thrown from the awaited result if the <paramref name="timeout"/> is a negative number other than -1 milliseconds, which represents an infinite timeout.</exception> |
| 106 | + /// <exception cref="InvalidOperationException">Thrown if called before a prior call to this method has completed, with its releaser disposed if the mutex was entered.</exception> |
| 107 | + public Task<LockReleaser?> TryEnterAsync(TimeSpan timeout) |
| 108 | + { |
| 109 | + int priorState = Interlocked.CompareExchange(ref this.state, STATE_HELD_OR_WAITING, STATE_READY); |
| 110 | + switch (priorState) |
| 111 | + { |
| 112 | + case STATE_HELD_OR_WAITING: |
| 113 | + throw new InvalidOperationException(); |
| 114 | + case STATE_DISPOSED: |
| 115 | + throw new ObjectDisposedException(this.GetType().FullName); |
| 116 | + } |
| 117 | + |
| 118 | + // Pass `this` as the state simply to assist in debugging dumps. |
| 119 | + TaskCompletionSource<LockReleaser?> tcs = new(this, TaskCreationOptions.RunContinuationsAsynchronously); |
| 120 | + this.mutexWorkQueue.Add(delegate |
| 121 | + { |
| 122 | + try |
| 123 | + { |
| 124 | + if (this.mutex.WaitOne(timeout)) |
| 125 | + { |
| 126 | + tcs.SetResult(new LockReleaser(this)); |
| 127 | + } |
| 128 | + else |
| 129 | + { |
| 130 | + Assumes.True(Interlocked.CompareExchange(ref this.state, STATE_READY, STATE_HELD_OR_WAITING) == STATE_HELD_OR_WAITING); |
| 131 | + tcs.SetResult(null); |
| 132 | + } |
| 133 | + } |
| 134 | + catch (AbandonedMutexException) |
| 135 | + { |
| 136 | + tcs.SetResult(new LockReleaser(this, abandoned: true)); |
| 137 | + } |
| 138 | + catch (Exception ex) |
| 139 | + { |
| 140 | + Assumes.True(Interlocked.CompareExchange(ref this.state, STATE_READY, STATE_HELD_OR_WAITING) == STATE_HELD_OR_WAITING); |
| 141 | + tcs.SetException(ex); |
| 142 | + } |
| 143 | + }); |
| 144 | + |
| 145 | + return tcs.Task; |
| 146 | + } |
| 147 | + |
| 148 | + private void Release() |
| 149 | + { |
| 150 | + Assumes.True(Interlocked.CompareExchange(ref this.state, STATE_READY, STATE_HELD_OR_WAITING) == STATE_HELD_OR_WAITING); |
| 151 | + this.mutexWorkQueue.Add(this.mutex.ReleaseMutex); |
| 152 | + } |
| 153 | + |
| 154 | + private void MutexOwnerThread() |
| 155 | + { |
| 156 | + try |
| 157 | + { |
| 158 | + while (!this.mutexWorkQueue.IsCompleted) |
| 159 | + { |
| 160 | + Action work = this.mutexWorkQueue.Take(); |
| 161 | + if (work == ExitSentinel) |
| 162 | + { |
| 163 | + // We use an exit sentinel to avoid an exception having to be thrown and caught on disposal when we call Take() and CompleteAdding() is called. |
| 164 | + break; |
| 165 | + } |
| 166 | + |
| 167 | + work(); |
| 168 | + } |
| 169 | + } |
| 170 | + finally |
| 171 | + { |
| 172 | + this.mutex.Dispose(); |
| 173 | + } |
| 174 | + } |
| 175 | + |
| 176 | + /// <summary> |
| 177 | + /// The value returned from <see cref="EnterAsync(TimeSpan)"/> that must be disposed to release the mutex. |
| 178 | + /// </summary> |
| 179 | + public struct LockReleaser : IDisposable |
| 180 | + { |
| 181 | + private AsyncCrossProcessMutex? owner; |
| 182 | + |
| 183 | + internal LockReleaser(AsyncCrossProcessMutex mutex, bool abandoned = false) |
| 184 | + { |
| 185 | + this.owner = mutex; |
| 186 | + this.IsAbandoned = abandoned; |
| 187 | + } |
| 188 | + |
| 189 | + /// <summary> |
| 190 | + /// Gets a value indicating whether the mutex was abandoned by its previous owner. |
| 191 | + /// </summary> |
| 192 | + public bool IsAbandoned { get; } |
| 193 | + |
| 194 | + /// <summary> |
| 195 | + /// Releases the named mutex. |
| 196 | + /// </summary> |
| 197 | + public void Dispose() |
| 198 | + { |
| 199 | + Interlocked.Exchange(ref this.owner, null)?.Release(); |
| 200 | + } |
| 201 | + } |
| 202 | +} |
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