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// 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;
using System.Diagnostics.CodeAnalysis;
using System.Threading;
using System.Threading.Tasks;
namespace Roslyn.Utilities
{
/// <summary>
/// A reference-counting wrapper which allows multiple uses of a single disposable object in code, which is
/// deterministically released (by calling <see cref="IDisposable.Dispose"/>) when the last reference is
/// disposed.
/// </summary>
/// <remarks>
/// <para>Each instance of <see cref="ReferenceCountedDisposable{T}"/> represents a counted reference (also
/// referred to as a <em>reference</em> in the following documentation) to a target object. Each of these
/// references has a lifetime, starting when it is constructed and continuing through its release. During
/// this time, the reference is considered <em>alive</em>. Each reference which is alive owns exactly one
/// reference to the target object, ensuring that it will not be disposed while still in use. A reference is
/// released through either of the following actions:</para>
///
/// <list type="bullet">
/// <item>The reference is explicitly released by a call to <see cref="Dispose"/>.</item>
/// <item>The reference is no longer in use by managed code and gets reclaimed by the garbage collector.</item>
/// </list>
///
/// <para>While each instance of <see cref="ReferenceCountedDisposable{T}"/> should be explicitly disposed when
/// the object is no longer needed by the code owning the reference, this implementation will not leak resources
/// in the event one or more callers fail to do so. When all references to an object are explicitly released
/// (i.e. by calling <see cref="Dispose"/>), the target object will itself be deterministically released by a
/// call to <see cref="IDisposable.Dispose"/> when the last reference to it is released. However, in the event
/// one or more references is not explicitly released, the underlying object will still become eligible for
/// non-deterministic release (i.e. finalization) as soon as each reference to it is released by one of the
/// two actions described previously.</para>
///
/// <para>When using <see cref="ReferenceCountedDisposable{T}"/>, certain steps must be taken to ensure the
/// target object is not disposed early.</para>
///
/// <list type="number">
/// <para>Use <see cref="ReferenceCountedDisposable{T}"/> consistently. In other words, do not mix code using
/// reference-counted wrappers with code that references to the target directly.</para>
/// <para>Only use the <see cref="ReferenceCountedDisposable{T}(T)"/> constructor one time per target object.
/// Additional references to the same target object must only be obtained by calling
/// <see cref="TryAddReference"/>.</para>
/// <para>Do not call <see cref="IDisposable.Dispose"/> on the target object directly. It will be called
/// automatically at the appropriate time, as described above.</para>
/// </list>
///
/// <para>All public methods on this type adhere to their pre- and post-conditions and will not invalidate state
/// even in concurrent execution.</para>
/// </remarks>
/// <typeparam name="T">The type of disposable object.</typeparam>
internal sealed class ReferenceCountedDisposable<T> : IReferenceCountedDisposable<T>, IDisposable
where T : class, IDisposable
{
/// <summary>
/// The target of this reference. This value is initialized to a non-<see langword="null"/> value in the
/// constructor, and set to <see langword="null"/> when the current reference is disposed.
/// </summary>
/// <remarks>
/// <para>This value is only cleared in order to support cases where one or more references is garbage
/// collected without having <see cref="Dispose"/> called.</para>
/// </remarks>
private T? _instance;
/// <summary>
/// The boxed reference count, which is shared by all references with the same <see cref="Target"/> object.
/// </summary>
/// <remarks>
/// <para>This field serves as the synchronization object for the current type, since it is shared among all
/// counted reference to the same target object. Accesses to <see cref="BoxedReferenceCount._referenceCount"/>
/// should only occur when this object is locked.</para>
///
/// <para>PERF DEV NOTE: A concurrent (but complex) implementation of this type with identical semantics is
/// available in source control history. The use of exclusive locks was not causing any measurable
/// performance overhead even on 28-thread machines at the time this was written.</para>
/// </remarks>
private readonly BoxedReferenceCount _boxedReferenceCount;
/// <summary>
/// Initializes a new reference counting wrapper around an <see cref="IDisposable"/> object.
/// </summary>
/// <remarks>
/// <para>The reference count is initialized to 1.</para>
/// </remarks>
/// <param name="instance">The object owned by this wrapper.</param>
/// <exception cref="ArgumentNullException">
/// If <paramref name="instance"/> is <see langword="null"/>.
/// </exception>
public ReferenceCountedDisposable(T instance)
: this(instance, new BoxedReferenceCount(1))
{
}
private ReferenceCountedDisposable(T instance, BoxedReferenceCount referenceCount)
{
_instance = instance ?? throw new ArgumentNullException(nameof(instance));
// The reference count has already been incremented for this instance
_boxedReferenceCount = referenceCount;
}
/// <summary>
/// Gets the target object.
/// </summary>
/// <remarks>
/// <para>This call is not valid after <see cref="Dispose"/> is called. If this property or the target
/// object is used concurrently with a call to <see cref="Dispose"/>, it is possible for the code to be
/// using a disposed object. After the current instance is disposed, this property throws
/// <see cref="ObjectDisposedException"/>. However, the exact time when this property starts throwing after
/// <see cref="Dispose"/> is called is unspecified; code is expected to not use this property or the object
/// it returns after any code invokes <see cref="Dispose"/>.</para>
/// </remarks>
/// <value>The target object.</value>
public T Target => _instance ?? throw new ObjectDisposedException(nameof(ReferenceCountedDisposable<T>));
/// <summary>
/// Increments the reference count for the disposable object, and returns a new disposable reference to it.
/// </summary>
/// <remarks>
/// <para>The returned object is an independent reference to the same underlying object. Disposing of the
/// returned value multiple times will only cause the reference count to be decreased once.</para>
/// </remarks>
/// <returns>A new <see cref="ReferenceCountedDisposable{T}"/> pointing to the same underlying object, if it
/// has not yet been disposed; otherwise, <see langword="null"/> if this reference to the underlying object
/// has already been disposed.</returns>
public ReferenceCountedDisposable<T>? TryAddReference()
=> TryAddReferenceImpl(_instance, _boxedReferenceCount);
IReferenceCountedDisposable<T>? IReferenceCountedDisposable<T>.TryAddReference()
=> TryAddReference();
/// <summary>
/// Provides the implementation for <see cref="TryAddReference"/> and
/// <see cref="WeakReference.TryAddReference"/>.
/// </summary>
private static ReferenceCountedDisposable<T>? TryAddReferenceImpl(T? target, BoxedReferenceCount referenceCount)
{
lock (referenceCount)
{
if (referenceCount._referenceCount == 0)
{
// The target is already disposed, and cannot be reused
return null;
}
if (target == null)
{
// The current reference has been disposed, so even though it isn't disposed yet we don't have a
// reference to the target
return null;
}
checked
{
referenceCount._referenceCount++;
}
// Must return a new instance, in order for the Dispose operation on each individual instance to
// be idempotent.
return new ReferenceCountedDisposable<T>(target, referenceCount);
}
}
/// <summary>
/// Releases the current reference, causing the underlying object to be disposed if this was the last
/// reference.
/// </summary>
/// <remarks>
/// <para>After this instance is disposed, the <see cref="TryAddReference"/> method can no longer be used to
/// obtain a new reference to the target, even if other references to the target object are still in
/// use.</para>
/// </remarks>
public void Dispose()
{
var instanceToDispose = DisposeImpl();
instanceToDispose?.Dispose();
}
public ValueTask DisposeAsync()
{
var instanceToDispose = DisposeImpl();
if (instanceToDispose == null)
return ValueTaskFactory.CompletedTask;
#if !CODE_STYLE
if (instanceToDispose is IAsyncDisposable asyncDisposable)
return asyncDisposable.DisposeAsync();
#endif
instanceToDispose.Dispose();
return ValueTaskFactory.CompletedTask;
}
private T? DisposeImpl()
{
T? instanceToDispose = null;
lock (_boxedReferenceCount)
{
if (_instance == null)
{
// Already disposed; allow multiple without error.
return null;
}
_boxedReferenceCount._referenceCount--;
if (_boxedReferenceCount._referenceCount == 0)
{
instanceToDispose = _instance;
}
// Ensure multiple calls to Dispose for this instance are a NOP.
_instance = null;
}
return instanceToDispose;
}
/// <summary>
/// Represents a weak reference to a <see cref="ReferenceCountedDisposable{T}"/> which is capable of
/// obtaining a new counted reference up until the point when the object is no longer accessible.
/// </summary>
/// <remarks>
/// This value type holds a single field, which is not subject to torn reads/writes.
/// </remarks>
public readonly struct WeakReference
{
private readonly BoxedReferenceCount? _boxedReferenceCount;
public WeakReference(ReferenceCountedDisposable<T> reference)
: this()
{
if (reference == null)
{
throw new ArgumentNullException(nameof(reference));
}
var instance = reference._instance;
var referenceCount = reference._boxedReferenceCount;
if (instance == null)
{
// The specified reference is already not valid. This case is supported by WeakReference (not
// unlike `new System.WeakReference(null)`), but we return early to avoid an unnecessary
// allocation in this case.
//
// ⚠ Note that in cases where referenceCount._weakInstance is already non-null, it would be
// possible to reconstruct a strong reference even though the current reference instance was
// disposed. This case is intentionally not supported (by checking 'instance' before checking
// 'referenceCount._weakInstance'), since it would be confusing semantics if this constructor
// sometimes worked from a disposed reference and sometimes did not.
return;
}
// We only need to allocate a new WeakReference<T> for this reference if one has not already been
// created for it.
LazyInitialization.EnsureInitialized(ref referenceCount._weakInstance, static instance => new WeakReference<T>(instance), instance);
_boxedReferenceCount = referenceCount;
}
/// <summary>
/// Increments the reference count for the disposable object, and returns a new disposable reference to
/// it.
/// </summary>
/// <remarks>
/// <para>Unlike <see cref="ReferenceCountedDisposable{T}.TryAddReference"/>, this method is capable of
/// adding a reference to the underlying instance all the way up to the point where it is finally
/// disposed.</para>
///
/// <para>The returned object is an independent reference to the same underlying object. Disposing of
/// the returned value multiple times will only cause the reference count to be decreased once.</para>
/// </remarks>
/// <returns>A new <see cref="ReferenceCountedDisposable{T}"/> pointing to the same underlying object,
/// if it has not yet been disposed; otherwise, <see langword="null"/> if the underlying object has
/// already been disposed.</returns>
public ReferenceCountedDisposable<T>? TryAddReference()
{
// Ensure 'this' is only read once by this method.
var referenceCount = _boxedReferenceCount;
if (referenceCount == null)
{
// This is either a default(WeakReference), or the current instance was constructed from a reference
// that was already disposed. No target is available.
return null;
}
var weakInstance = referenceCount._weakInstance;
// _weakInstance is initialized by the constructor before assigning a value to _boxedReferenceCount.
// Since it latches in a non-null state, it cannot be null at this point.
Contract.ThrowIfNull(weakInstance);
if (!weakInstance.TryGetTarget(out var target))
{
// The weak reference has already been collected, so the target is no longer available.
return null;
}
return TryAddReferenceImpl(target, referenceCount);
}
}
/// <summary>
/// Holds the reference count associated with a disposable object.
/// </summary>
private sealed class BoxedReferenceCount
{
/// <summary>
/// Holds the weak reference used by instances of <see cref="WeakReference"/> to obtain a reference-counted
/// reference to the original object. This field is initialized the first time a weak reference is obtained
/// for the instance, and latches in a non-null state once initialized.
/// </summary>
/// <remarks>
/// DO NOT DISPOSE OF THE TARGET.
/// </remarks>
[DisallowNull]
public WeakReference<T>? _weakInstance;
public int _referenceCount;
public BoxedReferenceCount(int referenceCount)
{
_referenceCount = referenceCount;
}
}
}
}
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