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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.
// =+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+=+
//
//
//
// An abstraction for holding and aggregating exceptions.
//
// =-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
using System.Collections.Generic;
using System.Collections.ObjectModel;
using System.Diagnostics;
using System.Runtime.ExceptionServices;
namespace System.Threading.Tasks
{
/// <summary>
/// An exception holder manages a list of exceptions for one particular task.
/// It offers the ability to aggregate, but more importantly, also offers intrinsic
/// support for propagating unhandled exceptions that are never observed. It does
/// this by aggregating and calling UnobservedTaskException event event if the holder
/// is ever GC'd without the holder's contents ever having been requested
/// (e.g. by a Task.Wait, Task.get_Exception, etc).
/// </summary>
internal sealed class TaskExceptionHolder
{
/// <summary>The task with which this holder is associated.</summary>
private readonly Task m_task;
/// <summary>
/// The lazily-initialized list of faulting exceptions. Volatile
/// so that it may be read to determine whether any exceptions were stored.
/// </summary>
private volatile List<ExceptionDispatchInfo>? m_faultExceptions;
/// <summary>An exception that triggered the task to cancel.</summary>
private ExceptionDispatchInfo? m_cancellationException;
/// <summary>Whether the holder was "observed" and thus doesn't cause finalization behavior.</summary>
private volatile bool m_isHandled;
/// <summary>
/// Creates a new holder; it will be registered for finalization.
/// </summary>
/// <param name="task">The task this holder belongs to.</param>
internal TaskExceptionHolder(Task task)
{
Debug.Assert(task != null, "Expected a non-null task.");
m_task = task;
}
/// <summary>
/// A finalizer that repropagates unhandled exceptions.
/// </summary>
~TaskExceptionHolder()
{
if (m_faultExceptions != null && !m_isHandled)
{
// We will only propagate if this is truly unhandled. The reason this could
// ever occur is somewhat subtle: if a Task's exceptions are observed in some
// other finalizer, and the Task was finalized before the holder, the holder
// will have been marked as handled before even getting here.
// Publish the unobserved exception and allow users to observe it
AggregateException exceptionToThrow = new AggregateException(
SR.TaskExceptionHolder_UnhandledException,
m_faultExceptions);
UnobservedTaskExceptionEventArgs ueea = new UnobservedTaskExceptionEventArgs(exceptionToThrow);
TaskScheduler.PublishUnobservedTaskException(m_task, ueea);
}
}
/// <summary>Gets whether the exception holder is currently storing any exceptions for faults.</summary>
internal bool ContainsFaultList => m_faultExceptions != null;
/// <summary>
/// Add an exception to the holder. This will ensure the holder is
/// in the proper state (handled/unhandled) depending on the list's contents.
/// </summary>
/// <param name="representsCancellation">
/// Whether the exception represents a cancellation request (true) or a fault (false).
/// </param>
/// <param name="exceptionObject">
/// An exception object (either an Exception, an ExceptionDispatchInfo,
/// an IEnumerable{Exception}, or an IEnumerable{ExceptionDispatchInfo})
/// to add to the list.
/// </param>
/// <remarks>
/// Must be called under lock.
/// </remarks>
internal void Add(object exceptionObject, bool representsCancellation)
{
Debug.Assert(exceptionObject != null, "TaskExceptionHolder.Add(): Expected a non-null exceptionObject");
Debug.Assert(
exceptionObject is Exception || exceptionObject is IEnumerable<Exception> ||
exceptionObject is ExceptionDispatchInfo || exceptionObject is IEnumerable<ExceptionDispatchInfo>,
"TaskExceptionHolder.Add(): Expected Exception, IEnumerable<Exception>, ExceptionDispatchInfo, or IEnumerable<ExceptionDispatchInfo>");
if (representsCancellation) SetCancellationException(exceptionObject);
else AddFaultException(exceptionObject);
}
/// <summary>Sets the cancellation exception.</summary>
/// <param name="exceptionObject">The cancellation exception.</param>
/// <remarks>
/// Must be called under lock.
/// </remarks>
private void SetCancellationException(object exceptionObject)
{
Debug.Assert(exceptionObject != null, "Expected exceptionObject to be non-null.");
Debug.Assert(m_cancellationException == null,
"Expected SetCancellationException to be called only once.");
// Breaking this assumption will overwrite a previously OCE,
// and implies something may be wrong elsewhere, since there should only ever be one.
Debug.Assert(m_faultExceptions == null,
"Expected SetCancellationException to be called before any faults were added.");
// Breaking this assumption shouldn't hurt anything here, but it implies something may be wrong elsewhere.
// If this changes, make sure to only conditionally mark as handled below.
// Store the cancellation exception
if (exceptionObject is OperationCanceledException oce)
{
m_cancellationException = ExceptionDispatchInfo.Capture(oce);
}
else
{
var edi = exceptionObject as ExceptionDispatchInfo;
Debug.Assert(edi != null && edi.SourceException is OperationCanceledException,
"Expected an OCE or an EDI that contained an OCE");
m_cancellationException = edi;
}
// This is just cancellation, and there are no faults, so mark the holder as handled.
MarkAsHandled(false);
}
/// <summary>Adds the exception to the fault list.</summary>
/// <param name="exceptionObject">The exception to store.</param>
/// <remarks>
/// Must be called under lock.
/// </remarks>
private void AddFaultException(object exceptionObject)
{
Debug.Assert(exceptionObject != null, "AddFaultException(): Expected a non-null exceptionObject");
// Initialize the exceptions list if necessary. The list should be non-null iff it contains exceptions.
List<ExceptionDispatchInfo>? exceptions = m_faultExceptions ??= new List<ExceptionDispatchInfo>(1);
// Handle Exception by capturing it into an ExceptionDispatchInfo and storing that
if (exceptionObject is Exception exception)
{
exceptions.Add(ExceptionDispatchInfo.Capture(exception));
}
else
{
// Handle ExceptionDispatchInfo by storing it into the list
if (exceptionObject is ExceptionDispatchInfo edi)
{
exceptions.Add(edi);
}
else
{
// Handle enumerables of exceptions by capturing each of the contained exceptions into an EDI and storing it
if (exceptionObject is IEnumerable<Exception> exColl)
{
#if DEBUG
int numExceptions = 0;
#endif
foreach (Exception exc in exColl)
{
#if DEBUG
Debug.Assert(exc != null, "No exceptions should be null");
numExceptions++;
#endif
exceptions.Add(ExceptionDispatchInfo.Capture(exc));
}
#if DEBUG
Debug.Assert(numExceptions > 0, "Collection should contain at least one exception.");
#endif
}
else
{
// Handle enumerables of EDIs by storing them directly
if (exceptionObject is IEnumerable<ExceptionDispatchInfo> ediColl)
{
exceptions.AddRange(ediColl);
#if DEBUG
Debug.Assert(exceptions.Count > 0, "There should be at least one dispatch info.");
foreach (ExceptionDispatchInfo tmp in exceptions)
{
Debug.Assert(tmp != null, "No dispatch infos should be null");
}
#endif
}
// Anything else is a programming error
else
{
throw new ArgumentException(SR.TaskExceptionHolder_UnknownExceptionType, nameof(exceptionObject));
}
}
}
}
if (exceptions.Count > 0)
MarkAsUnhandled();
}
/// <summary>
/// A private helper method that ensures the holder is considered
/// unhandled, i.e. it is registered for finalization.
/// </summary>
private void MarkAsUnhandled()
{
// If a thread partially observed this thread's exceptions, we
// should revert back to "not handled" so that subsequent exceptions
// must also be seen. Otherwise, some could go missing. We also need
// to reregister for finalization.
if (m_isHandled)
{
GC.ReRegisterForFinalize(this);
m_isHandled = false;
}
}
/// <summary>
/// A private helper method that ensures the holder is considered
/// handled, i.e. it is not registered for finalization.
/// </summary>
/// <param name="calledFromFinalizer">Whether this is called from the finalizer thread.</param>
internal void MarkAsHandled(bool calledFromFinalizer)
{
if (!m_isHandled)
{
if (!calledFromFinalizer)
{
GC.SuppressFinalize(this);
}
m_isHandled = true;
}
}
/// <summary>
/// Allocates a new aggregate exception and adds the contents of the list to
/// it. By calling this method, the holder assumes exceptions to have been
/// "observed", such that the finalization check will be subsequently skipped.
/// </summary>
/// <param name="calledFromFinalizer">Whether this is being called from a finalizer.</param>
/// <param name="includeThisException">An extra exception to be included (optionally).</param>
/// <returns>The aggregate exception to throw.</returns>
internal AggregateException CreateExceptionObject(bool calledFromFinalizer, Exception? includeThisException)
{
List<ExceptionDispatchInfo>? exceptions = m_faultExceptions;
Debug.Assert(exceptions != null, "Expected an initialized list.");
Debug.Assert(exceptions.Count > 0, "Expected at least one exception.");
// Mark as handled and aggregate the exceptions.
MarkAsHandled(calledFromFinalizer);
// If we're only including the previously captured exceptions,
// return them immediately in an aggregate.
if (includeThisException == null)
return new AggregateException(exceptions);
// Otherwise, the caller wants a specific exception to be included,
// so return an aggregate containing that exception and the rest.
Exception[] combinedExceptions = new Exception[exceptions.Count + 1];
for (int i = 0; i < combinedExceptions.Length - 1; i++)
{
combinedExceptions[i] = exceptions[i].SourceException;
}
combinedExceptions[^1] = includeThisException;
return new AggregateException(combinedExceptions);
}
/// <summary>
/// The holder assumes exceptions to have been "observed", such that the finalization
/// check will be subsequently skipped.
/// </summary>
internal List<ExceptionDispatchInfo> GetExceptionDispatchInfos()
{
List<ExceptionDispatchInfo>? exceptions = m_faultExceptions;
Debug.Assert(exceptions != null, "Expected an initialized list.");
Debug.Assert(exceptions.Count > 0, "Expected at least one exception.");
MarkAsHandled(false);
return exceptions;
}
/// <summary>
/// Gets the ExceptionDispatchInfo representing the singular exception
/// that was the cause of the task's cancellation.
/// </summary>
/// <returns>
/// The ExceptionDispatchInfo for the cancellation exception. May be null.
/// </returns>
internal ExceptionDispatchInfo? GetCancellationExceptionDispatchInfo()
{
ExceptionDispatchInfo? edi = m_cancellationException;
Debug.Assert(edi == null || edi.SourceException is OperationCanceledException,
"Expected the EDI to be for an OperationCanceledException");
return edi;
}
}
}
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