fix: resolve pre-4.0 audit findings across streaming and cancellation paths (#362-#369)

Exception fidelity (#362): the Task returned by the IAsyncEnumerable
processors' ExecuteAsync is now built from a TaskCompletionSource, so
Task.Exception.InnerExceptions carries every failure (Task.WhenAll
fidelity) instead of discarding all but the first. Applied to the
worker pool, parallel, batch (whole failing batch preserved), and
one-at-a-time (multi-fault item task preserved) processors via a new
internal StreamingExecution helper.

Abandonment semantics (#363, #364): breaking out of a result stream
now cancels the processor's in-flight work, drains it within the 30s
disposal window, and observes failures so they neither mask the
propagating exception nor surface as UnobservedTaskException. The void
twin no longer masks a mid-enumeration source failure with an in-flight
task failure. The unbounded ProcessInParallel extension drains started
tasks on mid-enumeration failure instead of abandoning them
fire-and-forget.

Ordering contract (#365): documented per method - bounded streaming
and the awaitable extension yield source order, unbounded streaming
and GetResultsAsyncEnumerable yield completion order - in XML docs and
a new README section.

ConfigureAwait gaps (#366): added ConfigureAwait(false) to every
WithCancellation enumeration in the SelectMany/SelectManyAsync
iterator families.

Pre-cancelled token (#367): building a processor with an
already-cancelled token now yields cancelled per-item tasks (TPL
convention) instead of ArgumentException naming an internal
constructor parameter. The registration in ProcessorLifecycle.Start
fires synchronously on the fully built instance.

CancelAll/Dispose race (#368): CancelAllCore tolerates a concurrent
Dispose winning the race to the CancellationTokenSource, matching the
streaming processors' existing handling.

Single-use enforcement (#369): a second ExecuteAsync call throws
InvalidOperationException eagerly at the call site; a call after
disposal throws ObjectDisposedException naming the processor type.

Fixes #362, #363, #364, #365, #366, #367, #368, #369.

Author: thomhurst

EnumerableAsyncProcessor.UnitTests/AsyncEnumerableExceptionFidelityTests.cs

@@ -0,0 +1,240 @@
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
+using EnumerableAsyncProcessor.Extensions;
+using TUnit.Assertions;
+using TUnit.Core;
+
+namespace EnumerableAsyncProcessor.UnitTests;
+
+/// <summary>
+/// Guards exception fidelity and abandonment semantics on the IAsyncEnumerable paths:
+/// - the Task from void ExecuteAsync must carry every failure via Task.Exception.InnerExceptions,
+///   matching the IEnumerable processors (issue #362),
+/// - a mid-enumeration source failure must stay the primary exception instead of being masked
+///   by an in-flight task failure (issue #363),
+/// - breaking out of an unbounded result stream must cancel in-flight work instead of silently
+///   blocking until it finishes naturally (issue #363),
+/// - the unbounded ProcessInParallel extension must drain started tasks on mid-enumeration
+///   failure instead of abandoning them fire-and-forget (issue #364).
+/// </summary>
+public class AsyncEnumerableExceptionFidelityTests
+{
+    private static async IAsyncEnumerable<int> Source(int count)
+    {
+        for (var i = 0; i < count; i++)
+        {
+            await Task.Yield();
+            yield return i;
+        }
+    }
+
+    private static async IAsyncEnumerable<int> ThrowingSource(int yieldBeforeThrow)
+    {
+        for (var i = 0; i < yieldBeforeThrow; i++)
+        {
+            await Task.Yield();
+            yield return i;
+        }
+
+        throw new InvalidOperationException("source failed");
+    }
+
+    [Test]
+    public async Task Bounded_Void_ExecuteAsync_Task_Carries_Every_Failure()
+    {
+        var executeTask = Source(10)
+            .ForEachAsync(i => (i is 2 or 5 or 8)
+                ? Task.FromException(new InvalidOperationException($"item {i} failed"))
+                : Task.CompletedTask)
+            .ProcessInParallel(maxConcurrency: 2)
+            .ExecuteAsync();
+
+        Exception? caught = null;
+        try
+        {
+            await executeTask;
+        }
+        catch (Exception exception)
+        {
+            caught = exception;
+        }
+
+        await Assert.That(caught is InvalidOperationException).IsTrue();
+
+        var messages = executeTask.Exception!.InnerExceptions.Select(e => e.Message).OrderBy(m => m).ToList();
+        await Assert.That(messages.Count).IsEqualTo(3);
+        await Assert.That(messages).IsEquivalentTo(new[] { "item 2 failed", "item 5 failed", "item 8 failed" });
+    }
+
+    [Test]
+    public async Task Batch_Void_ExecuteAsync_Task_Carries_Every_Failure_In_The_Batch()
+    {
+        var executeTask = Source(2)
+            .ForEachAsync(i => Task.FromException(new InvalidOperationException($"item {i} failed")))
+            .ProcessInBatches(2)
+            .ExecuteAsync();
+
+        try
+        {
+            await executeTask;
+        }
+        catch (Exception)
+        {
+            // Expected
+        }
+
+        var messages = executeTask.Exception!.InnerExceptions.Select(e => e.Message).ToList();
+        await Assert.That(messages.Count).IsEqualTo(2);
+        await Assert.That(messages).IsEquivalentTo(new[] { "item 0 failed", "item 1 failed" });
+    }
+
+    [Test]
+    public async Task Unbounded_Void_Source_Failure_Stays_Primary_When_InFlight_Tasks_Also_Fail()
+    {
+        var taskStarted = new TaskCompletionSource(TaskCreationOptions.RunContinuationsAsynchronously);
+
+        var executeTask = ThrowingSource(1)
+            .ForEachAsync(async _ =>
+            {
+                taskStarted.TrySetResult();
+                await Task.Yield();
+                throw new ApplicationException("in-flight task failed");
+            })
+            .ProcessInParallel()
+            .ExecuteAsync();
+
+        await taskStarted.Task;
+
+        Exception? caught = null;
+        try
+        {
+            await executeTask;
+        }
+        catch (Exception exception)
+        {
+            caught = exception;
+        }
+
+        // The source failure is primary; the in-flight failure is preserved alongside it
+        await Assert.That(caught is InvalidOperationException).IsTrue();
+        await Assert.That(caught!.Message).IsEqualTo("source failed");
+
+        var messages = executeTask.Exception!.InnerExceptions.Select(e => e.Message).ToList();
+        await Assert.That(messages.Count).IsEqualTo(2);
+        await Assert.That(messages.Contains("in-flight task failed")).IsTrue();
+    }
+
+    [Test]
+    public async Task Unbounded_Result_Stream_Early_Break_Cancels_InFlight_Work()
+    {
+        var cancelledCount = 0;
+
+        var processor = Source(10)
+            .SelectAsync(async (i, cancellationToken) =>
+            {
+                if (i == 0)
+                {
+                    return i;
+                }
+
+                try
+                {
+                    await Task.Delay(Timeout.Infinite, cancellationToken);
+                }
+                catch (OperationCanceledException)
+                {
+                    Interlocked.Increment(ref cancelledCount);
+                    throw;
+                }
+
+                return i;
+            })
+            .ProcessInParallel();
+
+        var stopwatch = Stopwatch.StartNew();
+
+        await foreach (var _ in processor.ExecuteAsync())
+        {
+            break;
+        }
+
+        stopwatch.Stop();
+
+        // Before the fix this blocked forever (the drain waited for every in-flight task and
+        // nothing cancelled them). Well under the 30s disposal window proves cancellation ran.
+        await Assert.That(stopwatch.Elapsed < TimeSpan.FromSeconds(15)).IsTrue();
+        await Assert.That(cancelledCount).IsEqualTo(9);
+    }
+
+    [Test]
+    public async Task Bounded_Result_Stream_Early_Break_Cancels_InFlight_Work()
+    {
+        var cancelledCount = 0;
+
+        var processor = Source(10)
+            .SelectAsync(async (i, cancellationToken) =>
+            {
+                if (i == 0)
+                {
+                    return i;
+                }
+
+                try
+                {
+                    await Task.Delay(Timeout.Infinite, cancellationToken);
+                }
+                catch (OperationCanceledException)
+                {
+                    Interlocked.Increment(ref cancelledCount);
+                    throw;
+                }
+
+                return i;
+            })
+            .ProcessInParallel(maxConcurrency: 3);
+
+        var stopwatch = Stopwatch.StartNew();
+
+        await foreach (var _ in processor.ExecuteAsync())
+        {
+            break;
+        }
+
+        stopwatch.Stop();
+
+        await Assert.That(stopwatch.Elapsed < TimeSpan.FromSeconds(15)).IsTrue();
+        await Assert.That(cancelledCount > 0).IsTrue();
+    }
+
+    [Test]
+    public async Task Unbounded_Extension_Drains_Started_Tasks_On_MidEnumeration_Failure()
+    {
+        var completedCount = 0;
+
+        Exception? caught = null;
+        try
+        {
+            _ = await ThrowingSource(3).ProcessInParallel(async i =>
+            {
+                await Task.Delay(100);
+                Interlocked.Increment(ref completedCount);
+                return i;
+            });
+        }
+        catch (Exception exception)
+        {
+            caught = exception;
+        }
+
+        await Assert.That(caught is InvalidOperationException).IsTrue();
+        await Assert.That(caught!.Message).IsEqualTo("source failed");
+
+        // Before the fix the extension rethrew immediately and abandoned the started tasks
+        // fire-and-forget; now they are drained before the exception leaves the method.
+        await Assert.That(completedCount).IsEqualTo(3);
+    }
+}

EnumerableAsyncProcessor.UnitTests/PreCancelledTokenTests.cs

@@ -0,0 +1,91 @@
+using System;
+using System.Linq;
+using System.Threading;
+using System.Threading.Tasks;
+using EnumerableAsyncProcessor.Extensions;
+using TUnit.Assertions;
+using TUnit.Core;
+
+namespace EnumerableAsyncProcessor.UnitTests;
+
+/// <summary>
+/// Guards the pre-cancelled token contract (issue #367):
+/// building a processor with an already-cancelled token must produce a processor whose
+/// per-item tasks are cancelled - matching TPL convention - instead of throwing
+/// ArgumentException from an internal constructor parameter the caller never passed.
+/// </summary>
+public class PreCancelledTokenTests
+{
+    [Test]
+    public async Task Void_Processor_Built_With_PreCancelled_Token_Yields_Cancelled_Tasks()
+    {
+        using var cancellationTokenSource = new CancellationTokenSource();
+        cancellationTokenSource.Cancel();
+
+        var invoked = 0;
+
+        await using var processor = Enumerable.Range(0, 5).ToList()
+            .ForEachAsync(_ =>
+            {
+                Interlocked.Increment(ref invoked);
+                return Task.CompletedTask;
+            }, cancellationTokenSource.Token)
+            .ProcessInParallel(maxConcurrency: 2);
+
+        Exception? caught = null;
+        try
+        {
+            await processor.WaitAsync();
+        }
+        catch (Exception exception)
+        {
+            caught = exception;
+        }
+
+        await Assert.That(caught is OperationCanceledException).IsTrue();
+        await Assert.That(processor.GetEnumerableTasks().All(t => t.IsCanceled)).IsTrue();
+        await Assert.That(invoked).IsEqualTo(0);
+    }
+
+    [Test]
+    public async Task Result_Processor_Built_With_PreCancelled_Token_Yields_Cancelled_Tasks()
+    {
+        using var cancellationTokenSource = new CancellationTokenSource();
+        cancellationTokenSource.Cancel();
+
+        await using var processor = Enumerable.Range(0, 5).ToList()
+            .SelectAsync(i => Task.FromResult(i), cancellationTokenSource.Token)
+            .ProcessInParallel(maxConcurrency: 2);
+
+        Exception? caught = null;
+        try
+        {
+            _ = await processor.GetResultsAsync();
+        }
+        catch (Exception exception)
+        {
+            caught = exception;
+        }
+
+        await Assert.That(caught is OperationCanceledException).IsTrue();
+        await Assert.That(processor.GetEnumerableTasks().All(t => t.IsCanceled)).IsTrue();
+    }
+
+    [Test]
+    public async Task OneAtATime_And_Batch_Built_With_PreCancelled_Token_Yield_Cancelled_Tasks()
+    {
+        using var cancellationTokenSource = new CancellationTokenSource();
+        cancellationTokenSource.Cancel();
+
+        await using var oneAtATime = new[] { 1, 2, 3 }
+            .ForEachAsync(_ => Task.CompletedTask, cancellationTokenSource.Token)
+            .ProcessOneAtATime();
+
+        await using var batched = new[] { 1, 2, 3 }
+            .ForEachAsync(_ => Task.CompletedTask, cancellationTokenSource.Token)
+            .ProcessInBatches(2);
+
+        await Assert.That(oneAtATime.GetEnumerableTasks().All(t => t.IsCanceled)).IsTrue();
+        await Assert.That(batched.GetEnumerableTasks().All(t => t.IsCanceled)).IsTrue();
+    }
+}