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Diffstat (limited to 'src/System.Private.CoreLib/shared/System/Collections/Concurrent/ConcurrentQueue.cs')
| -rw-r--r-- | src/System.Private.CoreLib/shared/System/Collections/Concurrent/ConcurrentQueue.cs | 834 |
1 files changed, 834 insertions, 0 deletions
diff --git a/src/System.Private.CoreLib/shared/System/Collections/Concurrent/ConcurrentQueue.cs b/src/System.Private.CoreLib/shared/System/Collections/Concurrent/ConcurrentQueue.cs new file mode 100644 index 000000000..63880b09f --- /dev/null +++ b/src/System.Private.CoreLib/shared/System/Collections/Concurrent/ConcurrentQueue.cs @@ -0,0 +1,834 @@ +// Licensed to the .NET Foundation under one or more agreements. +// The .NET Foundation licenses this file to you under the MIT license. +// See the LICENSE file in the project root for more information. + +using System.Collections.Generic; +using System.Diagnostics; +using System.Runtime.InteropServices; +using System.Threading; + +namespace System.Collections.Concurrent +{ + /// <summary> + /// Represents a thread-safe first-in, first-out collection of objects. + /// </summary> + /// <typeparam name="T">Specifies the type of elements in the queue.</typeparam> + /// <remarks> + /// All public and protected members of <see cref="ConcurrentQueue{T}"/> are thread-safe and may be used + /// concurrently from multiple threads. + /// </remarks> + [DebuggerDisplay("Count = {Count}")] + [DebuggerTypeProxy(typeof(IProducerConsumerCollectionDebugView<>))] + public class ConcurrentQueue<T> : IProducerConsumerCollection<T>, IReadOnlyCollection<T> + { + // This implementation provides an unbounded, multi-producer multi-consumer queue + // that supports the standard Enqueue/TryDequeue operations, as well as support for + // snapshot enumeration (GetEnumerator, ToArray, CopyTo), peeking, and Count/IsEmpty. + // It is composed of a linked list of bounded ring buffers, each of which has a head + // and a tail index, isolated from each other to minimize false sharing. As long as + // the number of elements in the queue remains less than the size of the current + // buffer (Segment), no additional allocations are required for enqueued items. When + // the number of items exceeds the size of the current segment, the current segment is + // "frozen" to prevent further enqueues, and a new segment is linked from it and set + // as the new tail segment for subsequent enqueues. As old segments are consumed by + // dequeues, the head reference is updated to point to the segment that dequeuers should + // try next. To support snapshot enumeration, segments also support the notion of + // preserving for observation, whereby they avoid overwriting state as part of dequeues. + // Any operation that requires a snapshot results in all current segments being + // both frozen for enqueues and preserved for observation: any new enqueues will go + // to new segments, and dequeuers will consume from the existing segments but without + // overwriting the existing data. + + /// <summary>Initial length of the segments used in the queue.</summary> + private const int InitialSegmentLength = 32; + /// <summary> + /// Maximum length of the segments used in the queue. This is a somewhat arbitrary limit: + /// larger means that as long as we don't exceed the size, we avoid allocating more segments, + /// but if we do exceed it, then the segment becomes garbage. + /// </summary> + private const int MaxSegmentLength = 1024 * 1024; + + /// <summary> + /// Lock used to protect cross-segment operations, including any updates to <see cref="_tail"/> or <see cref="_head"/> + /// and any operations that need to get a consistent view of them. + /// </summary> + private object _crossSegmentLock; + /// <summary>The current tail segment.</summary> + private volatile ConcurrentQueueSegment<T> _tail; + /// <summary>The current head segment.</summary> + private volatile ConcurrentQueueSegment<T> _head; + + /// <summary> + /// Initializes a new instance of the <see cref="ConcurrentQueue{T}"/> class. + /// </summary> + public ConcurrentQueue() + { + _crossSegmentLock = new object(); + _tail = _head = new ConcurrentQueueSegment<T>(InitialSegmentLength); + } + + /// <summary> + /// Initializes the contents of the queue from an existing collection. + /// </summary> + /// <param name="collection">A collection from which to copy elements.</param> + private void InitializeFromCollection(IEnumerable<T> collection) + { + _crossSegmentLock = new object(); + + // Determine the initial segment size. We'll use the default, + // unless the collection is known to be larger than that, in which + // case we round its length up to a power of 2, as all segments must + // be a power of 2 in length. + int length = InitialSegmentLength; + var c = collection as ICollection<T>; + if (c != null) + { + int count = c.Count; + if (count > length) + { + length = Math.Min(ConcurrentQueueSegment<T>.RoundUpToPowerOf2(count), MaxSegmentLength); + } + } + + // Initialize the segment and add all of the data to it. + _tail = _head = new ConcurrentQueueSegment<T>(length); + foreach (T item in collection) + { + Enqueue(item); + } + } + + /// <summary> + /// Initializes a new instance of the <see cref="ConcurrentQueue{T}"/> class that contains elements copied + /// from the specified collection. + /// </summary> + /// <param name="collection"> + /// The collection whose elements are copied to the new <see cref="ConcurrentQueue{T}"/>. + /// </param> + /// <exception cref="System.ArgumentNullException">The <paramref name="collection"/> argument is null.</exception> + public ConcurrentQueue(IEnumerable<T> collection) + { + if (collection == null) + { + ThrowHelper.ThrowArgumentNullException(ExceptionArgument.collection); + } + + InitializeFromCollection(collection); + } + + /// <summary> + /// Copies the elements of the <see cref="ICollection"/> to an <see + /// cref="Array"/>, starting at a particular <see cref="Array"/> index. + /// </summary> + /// <param name="array"> + /// The one-dimensional <see cref="Array">Array</see> that is the destination of the + /// elements copied from the <see cref="ConcurrentQueue{T}"/>. <paramref name="array"/> must have + /// zero-based indexing. + /// </param> + /// <param name="index">The zero-based index in <paramref name="array"/> at which copying begins.</param> + /// <exception cref="ArgumentNullException"><paramref name="array"/> is a null reference (Nothing in + /// Visual Basic).</exception> + /// <exception cref="ArgumentOutOfRangeException"><paramref name="index"/> is less than + /// zero.</exception> + /// <exception cref="ArgumentException"> + /// <paramref name="array"/> is multidimensional. -or- + /// <paramref name="array"/> does not have zero-based indexing. -or- + /// <paramref name="index"/> is equal to or greater than the length of the <paramref name="array"/> + /// -or- The number of elements in the source <see cref="ICollection"/> is + /// greater than the available space from <paramref name="index"/> to the end of the destination + /// <paramref name="array"/>. -or- The type of the source <see + /// cref="ICollection"/> cannot be cast automatically to the type of the + /// destination <paramref name="array"/>. + /// </exception> + void ICollection.CopyTo(Array array, int index) + { + // Special-case when the Array is actually a T[], taking a faster path + T[] szArray = array as T[]; + if (szArray != null) + { + CopyTo(szArray, index); + return; + } + + // Validate arguments. + if (array == null) + { + ThrowHelper.ThrowArgumentNullException(ExceptionArgument.array); + } + + // Otherwise, fall back to the slower path that first copies the contents + // to an array, and then uses that array's non-generic CopyTo to do the copy. + ToArray().CopyTo(array, index); + } + + /// <summary> + /// Gets a value indicating whether access to the <see cref="ICollection"/> is + /// synchronized with the SyncRoot. + /// </summary> + /// <value>true if access to the <see cref="ICollection"/> is synchronized + /// with the SyncRoot; otherwise, false. For <see cref="ConcurrentQueue{T}"/>, this property always + /// returns false.</value> + bool ICollection.IsSynchronized => false; // always false, as true implies synchronization via SyncRoot + + /// <summary> + /// Gets an object that can be used to synchronize access to the <see + /// cref="ICollection"/>. This property is not supported. + /// </summary> + /// <exception cref="NotSupportedException">The SyncRoot property is not supported.</exception> + object ICollection.SyncRoot { get { ThrowHelper.ThrowNotSupportedException(ExceptionResource.ConcurrentCollection_SyncRoot_NotSupported); return default; } } + + /// <summary>Returns an enumerator that iterates through a collection.</summary> + /// <returns>An <see cref="IEnumerator"/> that can be used to iterate through the collection.</returns> + IEnumerator IEnumerable.GetEnumerator() => ((IEnumerable<T>)this).GetEnumerator(); + + /// <summary> + /// Attempts to add an object to the <see cref="Concurrent.IProducerConsumerCollection{T}"/>. + /// </summary> + /// <param name="item">The object to add to the <see + /// cref="Concurrent.IProducerConsumerCollection{T}"/>. The value can be a null + /// reference (Nothing in Visual Basic) for reference types. + /// </param> + /// <returns>true if the object was added successfully; otherwise, false.</returns> + /// <remarks>For <see cref="ConcurrentQueue{T}"/>, this operation will always add the object to the + /// end of the <see cref="ConcurrentQueue{T}"/> + /// and return true.</remarks> + bool IProducerConsumerCollection<T>.TryAdd(T item) + { + Enqueue(item); + return true; + } + + /// <summary> + /// Attempts to remove and return an object from the <see cref="Concurrent.IProducerConsumerCollection{T}"/>. + /// </summary> + /// <param name="item"> + /// When this method returns, if the operation was successful, <paramref name="item"/> contains the + /// object removed. If no object was available to be removed, the value is unspecified. + /// </param> + /// <returns>true if an element was removed and returned successfully; otherwise, false.</returns> + /// <remarks>For <see cref="ConcurrentQueue{T}"/>, this operation will attempt to remove the object + /// from the beginning of the <see cref="ConcurrentQueue{T}"/>. + /// </remarks> + bool IProducerConsumerCollection<T>.TryTake(out T item) => TryDequeue(out item); + + /// <summary> + /// Gets a value that indicates whether the <see cref="ConcurrentQueue{T}"/> is empty. + /// </summary> + /// <value>true if the <see cref="ConcurrentQueue{T}"/> is empty; otherwise, false.</value> + /// <remarks> + /// For determining whether the collection contains any items, use of this property is recommended + /// rather than retrieving the number of items from the <see cref="Count"/> property and comparing it + /// to 0. However, as this collection is intended to be accessed concurrently, it may be the case + /// that another thread will modify the collection after <see cref="IsEmpty"/> returns, thus invalidating + /// the result. + /// </remarks> + public bool IsEmpty + { + get + { + // IsEmpty == !TryPeek. We use a "resultUsed:false" peek in order to avoid marking + // segments as preserved for observation, making IsEmpty a cheaper way than either + // TryPeek(out T) or Count == 0 to check whether any elements are in the queue. + T ignoredResult; + return !TryPeek(out ignoredResult, resultUsed: false); + } + } + + /// <summary>Copies the elements stored in the <see cref="ConcurrentQueue{T}"/> to a new array.</summary> + /// <returns>A new array containing a snapshot of elements copied from the <see cref="ConcurrentQueue{T}"/>.</returns> + public T[] ToArray() + { + // Snap the current contents for enumeration. + ConcurrentQueueSegment<T> head, tail; + int headHead, tailTail; + SnapForObservation(out head, out headHead, out tail, out tailTail); + + // Count the number of items in that snapped set, and use it to allocate an + // array of the right size. + long count = GetCount(head, headHead, tail, tailTail); + T[] arr = new T[count]; + + // Now enumerate the contents, copying each element into the array. + using (IEnumerator<T> e = Enumerate(head, headHead, tail, tailTail)) + { + int i = 0; + while (e.MoveNext()) + { + arr[i++] = e.Current; + } + Debug.Assert(count == i); + } + + // And return it. + return arr; + } + + /// <summary> + /// Gets the number of elements contained in the <see cref="ConcurrentQueue{T}"/>. + /// </summary> + /// <value>The number of elements contained in the <see cref="ConcurrentQueue{T}"/>.</value> + /// <remarks> + /// For determining whether the collection contains any items, use of the <see cref="IsEmpty"/> + /// property is recommended rather than retrieving the number of items from the <see cref="Count"/> + /// property and comparing it to 0. + /// </remarks> + public int Count + { + get + { + var spinner = new SpinWait(); + while (true) + { + // Capture the head and tail, as well as the head's head and tail. + ConcurrentQueueSegment<T> head = _head; + ConcurrentQueueSegment<T> tail = _tail; + int headHead = Volatile.Read(ref head._headAndTail.Head); + int headTail = Volatile.Read(ref head._headAndTail.Tail); + + if (head == tail) + { + // There was a single segment in the queue. If the captured segments still + // match, then we can trust the values to compute the segment's count. (It's + // theoretically possible the values could have looped around and still exactly match, + // but that would required at least ~4 billion elements to have been enqueued and + // dequeued between the reads.) + if (head == _head && + tail == _tail && + headHead == Volatile.Read(ref head._headAndTail.Head) && + headTail == Volatile.Read(ref head._headAndTail.Tail)) + { + return GetCount(head, headHead, headTail); + } + } + else if (head._nextSegment == tail) + { + // There were two segments in the queue. Get the positions from the tail, and as above, + // if the captured values match the previous reads, return the sum of the counts from both segments. + int tailHead = Volatile.Read(ref tail._headAndTail.Head); + int tailTail = Volatile.Read(ref tail._headAndTail.Tail); + if (head == _head && + tail == _tail && + headHead == Volatile.Read(ref head._headAndTail.Head) && + headTail == Volatile.Read(ref head._headAndTail.Tail) && + tailHead == Volatile.Read(ref tail._headAndTail.Head) && + tailTail == Volatile.Read(ref tail._headAndTail.Tail)) + { + return GetCount(head, headHead, headTail) + GetCount(tail, tailHead, tailTail); + } + } + else + { + // There were more than two segments in the queue. Fall back to taking the cross-segment lock, + // which will ensure that the head and tail segments we read are stable (since the lock is needed to change them); + // for the two-segment case above, we can simply rely on subsequent comparisons, but for the two+ case, we need + // to be able to trust the internal segments between the head and tail. + lock (_crossSegmentLock) + { + // Now that we hold the lock, re-read the previously captured head and tail segments and head positions. + // If either has changed, start over. + if (head == _head && tail == _tail) + { + // Get the positions from the tail, and as above, if the captured values match the previous reads, + // we can use the values to compute the count of the head and tail segments. + int tailHead = Volatile.Read(ref tail._headAndTail.Head); + int tailTail = Volatile.Read(ref tail._headAndTail.Tail); + if (headHead == Volatile.Read(ref head._headAndTail.Head) && + headTail == Volatile.Read(ref head._headAndTail.Tail) && + tailHead == Volatile.Read(ref tail._headAndTail.Head) && + tailTail == Volatile.Read(ref tail._headAndTail.Tail)) + { + // We got stable values for the head and tail segments, so we can just compute the sizes + // based on those and add them. Note that this and the below additions to count may overflow: previous + // implementations allowed that, so we don't check, either, and it is theoretically possible for the + // queue to store more than int.MaxValue items. + int count = GetCount(head, headHead, headTail) + GetCount(tail, tailHead, tailTail); + + // Now add the counts for each internal segment. Since there were segments before these, + // for counting purposes we consider them to start at the 0th element, and since there is at + // least one segment after each, each was frozen, so we can count until each's frozen tail. + // With the cross-segment lock held, we're guaranteed that all of these internal segments are + // consistent, as the head and tail segment can't be changed while we're holding the lock, and + // dequeueing and enqueueing can only be done from the head and tail segments, which these aren't. + for (ConcurrentQueueSegment<T> s = head._nextSegment; s != tail; s = s._nextSegment) + { + Debug.Assert(s._frozenForEnqueues, "Internal segment must be frozen as there's a following segment."); + count += s._headAndTail.Tail - s.FreezeOffset; + } + + return count; + } + } + } + } + + // We raced with enqueues/dequeues and captured an inconsistent picture of the queue. + // Spin and try again. + spinner.SpinOnce(); + } + } + } + + /// <summary>Computes the number of items in a segment based on a fixed head and tail in that segment.</summary> + private static int GetCount(ConcurrentQueueSegment<T> s, int head, int tail) + { + if (head != tail && head != tail - s.FreezeOffset) + { + head &= s._slotsMask; + tail &= s._slotsMask; + return head < tail ? tail - head : s._slots.Length - head + tail; + } + return 0; + } + + /// <summary>Gets the number of items in snapped region.</summary> + private static long GetCount(ConcurrentQueueSegment<T> head, int headHead, ConcurrentQueueSegment<T> tail, int tailTail) + { + // All of the segments should have been both frozen for enqueues and preserved for observation. + // Validate that here for head and tail; we'll validate it for intermediate segments later. + Debug.Assert(head._preservedForObservation); + Debug.Assert(head._frozenForEnqueues); + Debug.Assert(tail._preservedForObservation); + Debug.Assert(tail._frozenForEnqueues); + + long count = 0; + + // Head segment. We've already marked it as frozen for enqueues, so its tail position is fixed, + // and we've already marked it as preserved for observation (before we grabbed the head), so we + // can safely enumerate from its head to its tail and access its elements. + int headTail = (head == tail ? tailTail : Volatile.Read(ref head._headAndTail.Tail)) - head.FreezeOffset; + if (headHead < headTail) + { + // Mask the head and tail for the head segment + headHead &= head._slotsMask; + headTail &= head._slotsMask; + + // Increase the count by either the one or two regions, based on whether tail + // has wrapped to be less than head. + count += headHead < headTail ? + headTail - headHead : + head._slots.Length - headHead + headTail; + } + + // We've enumerated the head. If the tail is different from the head, we need to + // enumerate the remaining segments. + if (head != tail) + { + // Count the contents of each segment between head and tail, not including head and tail. + // Since there were segments before these, for our purposes we consider them to start at + // the 0th element, and since there is at least one segment after each, each was frozen + // by the time we snapped it, so we can iterate until each's frozen tail. + for (ConcurrentQueueSegment<T> s = head._nextSegment; s != tail; s = s._nextSegment) + { + Debug.Assert(s._preservedForObservation); + Debug.Assert(s._frozenForEnqueues); + count += s._headAndTail.Tail - s.FreezeOffset; + } + + // Finally, enumerate the tail. As with the intermediate segments, there were segments + // before this in the snapped region, so we can start counting from the beginning. Unlike + // the intermediate segments, we can't just go until the Tail, as that could still be changing; + // instead we need to go until the tail we snapped for observation. + count += tailTail - tail.FreezeOffset; + } + + // Return the computed count. + return count; + } + + /// <summary> + /// Copies the <see cref="ConcurrentQueue{T}"/> elements to an existing one-dimensional <see + /// cref="Array">Array</see>, starting at the specified array index. + /// </summary> + /// <param name="array">The one-dimensional <see cref="Array">Array</see> that is the + /// destination of the elements copied from the + /// <see cref="ConcurrentQueue{T}"/>. The <see cref="Array">Array</see> must have zero-based + /// indexing.</param> + /// <param name="index">The zero-based index in <paramref name="array"/> at which copying + /// begins.</param> + /// <exception cref="ArgumentNullException"><paramref name="array"/> is a null reference (Nothing in + /// Visual Basic).</exception> + /// <exception cref="ArgumentOutOfRangeException"><paramref name="index"/> is less than + /// zero.</exception> + /// <exception cref="ArgumentException"><paramref name="index"/> is equal to or greater than the + /// length of the <paramref name="array"/> + /// -or- The number of elements in the source <see cref="ConcurrentQueue{T}"/> is greater than the + /// available space from <paramref name="index"/> to the end of the destination <paramref + /// name="array"/>. + /// </exception> + public void CopyTo(T[] array, int index) + { + if (array == null) + { + ThrowHelper.ThrowArgumentNullException(ExceptionArgument.array); + } + if (index < 0) + { + ThrowHelper.ThrowArgumentOutOfRangeException(ExceptionArgument.index); + } + + // Snap for enumeration + ConcurrentQueueSegment<T> head, tail; + int headHead, tailTail; + SnapForObservation(out head, out headHead, out tail, out tailTail); + + // Get the number of items to be enumerated + long count = GetCount(head, headHead, tail, tailTail); + if (index > array.Length - count) + { + ThrowHelper.ThrowArgumentException(ExceptionResource.Arg_ArrayPlusOffTooSmall); + } + + // Copy the items to the target array + int i = index; + using (IEnumerator<T> e = Enumerate(head, headHead, tail, tailTail)) + { + while (e.MoveNext()) + { + array[i++] = e.Current; + } + } + Debug.Assert(count == i - index); + } + + /// <summary>Returns an enumerator that iterates through the <see cref="ConcurrentQueue{T}"/>.</summary> + /// <returns>An enumerator for the contents of the <see + /// cref="ConcurrentQueue{T}"/>.</returns> + /// <remarks> + /// The enumeration represents a moment-in-time snapshot of the contents + /// of the queue. It does not reflect any updates to the collection after + /// <see cref="GetEnumerator"/> was called. The enumerator is safe to use + /// concurrently with reads from and writes to the queue. + /// </remarks> + public IEnumerator<T> GetEnumerator() + { + ConcurrentQueueSegment<T> head, tail; + int headHead, tailTail; + SnapForObservation(out head, out headHead, out tail, out tailTail); + return Enumerate(head, headHead, tail, tailTail); + } + + /// <summary> + /// Gets the head and tail information of the current contents of the queue. + /// After this call returns, the specified region can be enumerated any number + /// of times and will not change. + /// </summary> + private void SnapForObservation(out ConcurrentQueueSegment<T> head, out int headHead, out ConcurrentQueueSegment<T> tail, out int tailTail) + { + lock (_crossSegmentLock) // _head and _tail may only change while the lock is held. + { + // Snap the head and tail + head = _head; + tail = _tail; + Debug.Assert(head != null); + Debug.Assert(tail != null); + Debug.Assert(tail._nextSegment == null); + + // Mark them and all segments in between as preserving, and ensure no additional items + // can be added to the tail. + for (ConcurrentQueueSegment<T> s = head; ; s = s._nextSegment) + { + s._preservedForObservation = true; + if (s == tail) break; + Debug.Assert(s._frozenForEnqueues); // any non-tail should already be marked + } + tail.EnsureFrozenForEnqueues(); // we want to prevent the tailTail from moving + + // At this point, any dequeues from any segment won't overwrite the value, and + // none of the existing segments can have new items enqueued. + + headHead = Volatile.Read(ref head._headAndTail.Head); + tailTail = Volatile.Read(ref tail._headAndTail.Tail); + } + } + + /// <summary>Gets the item stored in the <paramref name="i"/>th entry in <paramref name="segment"/>.</summary> + private T GetItemWhenAvailable(ConcurrentQueueSegment<T> segment, int i) + { + Debug.Assert(segment._preservedForObservation); + + // Get the expected value for the sequence number + int expectedSequenceNumberAndMask = (i + 1) & segment._slotsMask; + + // If the expected sequence number is not yet written, we're still waiting for + // an enqueuer to finish storing it. Spin until it's there. + if ((segment._slots[i].SequenceNumber & segment._slotsMask) != expectedSequenceNumberAndMask) + { + var spinner = new SpinWait(); + while ((Volatile.Read(ref segment._slots[i].SequenceNumber) & segment._slotsMask) != expectedSequenceNumberAndMask) + { + spinner.SpinOnce(); + } + } + + // Return the value from the slot. + return segment._slots[i].Item; + } + + private IEnumerator<T> Enumerate(ConcurrentQueueSegment<T> head, int headHead, ConcurrentQueueSegment<T> tail, int tailTail) + { + Debug.Assert(head._preservedForObservation); + Debug.Assert(head._frozenForEnqueues); + Debug.Assert(tail._preservedForObservation); + Debug.Assert(tail._frozenForEnqueues); + + // Head segment. We've already marked it as not accepting any more enqueues, + // so its tail position is fixed, and we've already marked it as preserved for + // enumeration (before we grabbed its head), so we can safely enumerate from + // its head to its tail. + int headTail = (head == tail ? tailTail : Volatile.Read(ref head._headAndTail.Tail)) - head.FreezeOffset; + if (headHead < headTail) + { + headHead &= head._slotsMask; + headTail &= head._slotsMask; + + if (headHead < headTail) + { + for (int i = headHead; i < headTail; i++) yield return GetItemWhenAvailable(head, i); + } + else + { + for (int i = headHead; i < head._slots.Length; i++) yield return GetItemWhenAvailable(head, i); + for (int i = 0; i < headTail; i++) yield return GetItemWhenAvailable(head, i); + } + } + + // We've enumerated the head. If the tail is the same, we're done. + if (head != tail) + { + // Each segment between head and tail, not including head and tail. Since there were + // segments before these, for our purposes we consider it to start at the 0th element. + for (ConcurrentQueueSegment<T> s = head._nextSegment; s != tail; s = s._nextSegment) + { + Debug.Assert(s._preservedForObservation, "Would have had to been preserved as a segment part of enumeration"); + Debug.Assert(s._frozenForEnqueues, "Would have had to be frozen for enqueues as it's intermediate"); + + int sTail = s._headAndTail.Tail - s.FreezeOffset; + for (int i = 0; i < sTail; i++) + { + yield return GetItemWhenAvailable(s, i); + } + } + + // Enumerate the tail. Since there were segments before this, we can just start at + // its beginning, and iterate until the tail we already grabbed. + tailTail -= tail.FreezeOffset; + for (int i = 0; i < tailTail; i++) + { + yield return GetItemWhenAvailable(tail, i); + } + } + } + + /// <summary>Adds an object to the end of the <see cref="ConcurrentQueue{T}"/>.</summary> + /// <param name="item"> + /// The object to add to the end of the <see cref="ConcurrentQueue{T}"/>. + /// The value can be a null reference (Nothing in Visual Basic) for reference types. + /// </param> + public void Enqueue(T item) + { + // Try to enqueue to the current tail. + if (!_tail.TryEnqueue(item)) + { + // If we're unable to, we need to take a slow path that will + // try to add a new tail segment. + EnqueueSlow(item); + } + } + + /// <summary>Adds to the end of the queue, adding a new segment if necessary.</summary> + private void EnqueueSlow(T item) + { + while (true) + { + ConcurrentQueueSegment<T> tail = _tail; + + // Try to append to the existing tail. + if (tail.TryEnqueue(item)) + { + return; + } + + // If we were unsuccessful, take the lock so that we can compare and manipulate + // the tail. Assuming another enqueuer hasn't already added a new segment, + // do so, then loop around to try enqueueing again. + lock (_crossSegmentLock) + { + if (tail == _tail) + { + // Make sure no one else can enqueue to this segment. + tail.EnsureFrozenForEnqueues(); + + // We determine the new segment's length based on the old length. + // In general, we double the size of the segment, to make it less likely + // that we'll need to grow again. However, if the tail segment is marked + // as preserved for observation, something caused us to avoid reusing this + // segment, and if that happens a lot and we grow, we'll end up allocating + // lots of wasted space. As such, in such situations we reset back to the + // initial segment length; if these observations are happening frequently, + // this will help to avoid wasted memory, and if they're not, we'll + // relatively quickly grow again to a larger size. + int nextSize = tail._preservedForObservation ? InitialSegmentLength : Math.Min(tail.Capacity * 2, MaxSegmentLength); + var newTail = new ConcurrentQueueSegment<T>(nextSize); + + // Hook up the new tail. + tail._nextSegment = newTail; + _tail = newTail; + } + } + } + } + + /// <summary> + /// Attempts to remove and return the object at the beginning of the <see + /// cref="ConcurrentQueue{T}"/>. + /// </summary> + /// <param name="result"> + /// When this method returns, if the operation was successful, <paramref name="result"/> contains the + /// object removed. If no object was available to be removed, the value is unspecified. + /// </param> + /// <returns> + /// true if an element was removed and returned from the beginning of the + /// <see cref="ConcurrentQueue{T}"/> successfully; otherwise, false. + /// </returns> + public bool TryDequeue(out T result) => + _head.TryDequeue(out result) || // fast-path that operates just on the head segment + TryDequeueSlow(out result); // slow path that needs to fix up segments + + /// <summary>Tries to dequeue an item, removing empty segments as needed.</summary> + private bool TryDequeueSlow(out T item) + { + while (true) + { + // Get the current head + ConcurrentQueueSegment<T> head = _head; + + // Try to take. If we're successful, we're done. + if (head.TryDequeue(out item)) + { + return true; + } + + // Check to see whether this segment is the last. If it is, we can consider + // this to be a moment-in-time empty condition (even though between the TryDequeue + // check and this check, another item could have arrived). + if (head._nextSegment == null) + { + item = default; + return false; + } + + // At this point we know that head.Next != null, which means + // this segment has been frozen for additional enqueues. But between + // the time that we ran TryDequeue and checked for a next segment, + // another item could have been added. Try to dequeue one more time + // to confirm that the segment is indeed empty. + Debug.Assert(head._frozenForEnqueues); + if (head.TryDequeue(out item)) + { + return true; + } + + // This segment is frozen (nothing more can be added) and empty (nothing is in it). + // Update head to point to the next segment in the list, assuming no one's beat us to it. + lock (_crossSegmentLock) + { + if (head == _head) + { + _head = head._nextSegment; + } + } + } + } + + /// <summary> + /// Attempts to return an object from the beginning of the <see cref="ConcurrentQueue{T}"/> + /// without removing it. + /// </summary> + /// <param name="result"> + /// When this method returns, <paramref name="result"/> contains an object from + /// the beginning of the <see cref="Concurrent.ConcurrentQueue{T}"/> or default(T) + /// if the operation failed. + /// </param> + /// <returns>true if and object was returned successfully; otherwise, false.</returns> + /// <remarks> + /// For determining whether the collection contains any items, use of the <see cref="IsEmpty"/> + /// property is recommended rather than peeking. + /// </remarks> + public bool TryPeek(out T result) => TryPeek(out result, resultUsed: true); + + /// <summary>Attempts to retrieve the value for the first element in the queue.</summary> + /// <param name="result">The value of the first element, if found.</param> + /// <param name="resultUsed">true if the result is needed; otherwise false if only the true/false outcome is needed.</param> + /// <returns>true if an element was found; otherwise, false.</returns> + private bool TryPeek(out T result, bool resultUsed) + { + // Starting with the head segment, look through all of the segments + // for the first one we can find that's not empty. + ConcurrentQueueSegment<T> s = _head; + while (true) + { + // Grab the next segment from this one, before we peek. + // This is to be able to see whether the value has changed + // during the peek operation. + ConcurrentQueueSegment<T> next = Volatile.Read(ref s._nextSegment); + + // Peek at the segment. If we find an element, we're done. + if (s.TryPeek(out result, resultUsed)) + { + return true; + } + + // The current segment was empty at the moment we checked. + + if (next != null) + { + // If prior to the peek there was already a next segment, then + // during the peek no additional items could have been enqueued + // to it and we can just move on to check the next segment. + Debug.Assert(next == s._nextSegment); + s = next; + } + else if (Volatile.Read(ref s._nextSegment) == null) + { + // The next segment is null. Nothing more to peek at. + break; + } + + // The next segment was null before we peeked but non-null after. + // That means either when we peeked the first segment had + // already been frozen but the new segment not yet added, + // or that the first segment was empty and between the time + // that we peeked and then checked _nextSegment, so many items + // were enqueued that we filled the first segment and went + // into the next. Since we need to peek in order, we simply + // loop around again to peek on the same segment. The next + // time around on this segment we'll then either successfully + // peek or we'll find that next was non-null before peeking, + // and we'll traverse to that segment. + } + + result = default; + return false; + } + + /// <summary> + /// Removes all objects from the <see cref="ConcurrentQueue{T}"/>. + /// </summary> + public void Clear() + { + lock (_crossSegmentLock) + { + // Simply substitute a new segment for the existing head/tail, + // as is done in the constructor. Operations currently in flight + // may still read from or write to an existing segment that's + // getting dropped, meaning that in flight operations may not be + // linear with regards to this clear operation. To help mitigate + // in-flight operations enqueuing onto the tail that's about to + // be dropped, we first freeze it; that'll force enqueuers to take + // this lock to synchronize and see the new tail. + _tail.EnsureFrozenForEnqueues(); + _tail = _head = new ConcurrentQueueSegment<T>(InitialSegmentLength); + } + } + } +} |