[xUnit 3] Fix parallel-class implicit-sender leak in Akka.TestKit.Xunit by Arkatufus · Pull Request #8174 · akkadotnet/akka.net

Arkatufus · 2026-04-22T20:23:53Z

Problem

Under xUnit v3 parallel-class scheduling, MaxConcurrencySyncContext dispatches test ctors and bodies onto a dedicated pool of reused threads. TestKitBase's constructor pins InternalCurrentActorCellKeeper.Current — a [ThreadStatic] — on its ctor thread. When a sibling test's body lands on that same reused thread, the pre-await synchronous prefix of the [Fact] reads the sibling's cell as its implicit sender, causing Tell() to use the wrong Sender. Replies then cross ActorSystem boundaries and land in the wrong TestActor's mailbox.

Originally reported in https://github.com/akkadotnet/Akka.Hosting by a user running tests with parallelizeTestCollections: true.

Fix

Add a [BeforeAfterTestAttribute] that runs synchronously on the test-body thread and:

Before: pins InternalCurrentActorCellKeeper.Current to the running test's TestActor cell (or null for INoImplicitSender), and installs ActorCellKeepingSynchronizationContext so await continuations re-pin the cell across worker-thread switches.
After: clears Current so the reused worker doesn't carry the cell into the next test.

Applied to Akka.TestKit.Xunit.TestKit with Inherited = true, so derived test classes (including Akka.Hosting.TestKit downstream) get parallel-safe behavior automatically via attribute inheritance.

Changes

Akka.TestKit/ActorCellKeepingSynchronizationContext.cs — promoted from internal to [InternalApi] public so the new attribute can install an equivalent wrapper without duplicating the save/pin/restore logic. All TBD XML docs replaced with real documentation.
Akka.TestKit.Xunit/Attributes/AkkaCleanAmbientContextAttribute.cs — new BeforeAfterTestAttribute. Uses TestContext.Current.TestClassInstance for direct instance access (xUnit v3 only).
Akka.TestKit.Xunit/TestKit.cs — decorated with [AkkaCleanAmbientContext].

Regression tests

ParallelAmbientContextSpec — 16 + 8 sibling test classes (distinct top-level classes so xUnit schedules them in separate collections). Verified: fails 15/16 without the attribute, passes all with it. Marked [LocalFact] because the shared src/xunit.runner.json disables parallelization — exercising the bug requires running with xUnit.ParallelizeTestCollections=true:
```
dotnet test --filter "FullyQualifiedName~ParallelAmbientContext" \
    -- xUnit.ParallelizeAssembly=true \
       xUnit.ParallelizeTestCollections=true
```
AkkaCleanAmbientContextAttributeSpec — reflection guards (always run) that the attribute is declared on TestKit, is inherited by derived classes, and has AttributeUsage.Inherited == true.

Scope

xUnit v3 only (Akka.TestKit.Xunit). Akka.TestKit.Xunit2 is untouched — the bug is specific to xUnit v3's MaxConcurrencySyncContext thread-reuse semantics.
No behavioral changes to Akka.TestKit.ActorCellKeepingSynchronizationContext — only visibility + docs.
No changes to Akka core (Akka.dll).

Validation

Akka.TestKit.Xunit.Tests: 31/31 pass (all three frameworks).
End-to-end via Akka.Hosting parallel repro (montrose issue): 15/15 pass 5× consecutively; fails without the attribute.

Under xUnit v3 parallel-class scheduling, MaxConcurrencySyncContext dispatches test ctors and bodies onto a dedicated pool of reused threads. TestKitBase's constructor pins InternalCurrentActorCellKeeper.Current (a ThreadStatic) on its ctor thread; when a sibling test's body lands on that same reused thread, the pre-await synchronous prefix of the [Fact] reads the sibling's cell as its implicit sender, causing Tell() to use the wrong Sender. Replies then cross ActorSystem boundaries. Fix: add a BeforeAfterTestAttribute that runs synchronously on the test- body thread and (a) pins Current to the running test's TestActor cell, (b) installs ActorCellKeepingSynchronizationContext so await continuations also re-pin the cell. After the test, Current is cleared so the reused worker doesn't carry the cell into the next test. Applied to Akka.TestKit.Xunit.TestKit so derived test classes (including Akka.Hosting.TestKit downstream) get parallel-safe behavior automatically via attribute inheritance. ActorCellKeepingSynchronizationContext promoted to [InternalApi] public so the attribute can install an equivalent wrapper without duplicating the save/pin/restore logic. TBD XML docs replaced with real documentation. Regression tests: - ParallelAmbientContextSpec: 16 + 8 sibling test classes asserting implicit-sender correctness and INoImplicitSender == null across awaits. Marked [LocalFact] — requires xUnit.ParallelizeTestCollections=true to exercise the bug (disabled in the shared src/xunit.runner.json). - AkkaCleanAmbientContextAttributeSpec: reflection guards that the attribute is declared on the base TestKit, is inherited by subclasses, and has Inherited=true in its AttributeUsage.

Arkatufus

self-review

Arkatufus · 2026-04-22T20:46:39Z

+/// mechanism and the ThreadStatic-vs-ExecutionContext rationale.
+/// </summary>
+[AttributeUsage(AttributeTargets.Assembly | AttributeTargets.Class | AttributeTargets.Method, AllowMultiple = false, Inherited = true)]
+public sealed class AkkaCleanAmbientContextAttribute : BeforeAfterTestAttribute


The fix. Since xUnit 3 can switch thread executor/context between the constructor, InitializeAsync() invocation, and the actual test method invocation, we pin the test actor into the ActorCellKeepingSynchronizationContext right before the test starts and clear it after when it is being teared down.

BeforeAfterTestAttribute is a new attribute in xUnit 3 for test setup and tear down.

Arkatufus · 2026-04-22T20:48:35Z

 /// This class represents an Akka.NET TestKit that uses <a href="https://xunit.github.io/">xUnit</a>
 /// as its testing framework.
 /// </summary>
+[AkkaCleanAmbientContext]


Mark TestKit with the new AkkaCleanAmbientContext. This attribute is marked as Inherited so all classes that inherits TestKit will automatically have this attribute.

…Arkatufus/akka.net into xunit-3-implicit-sender-leak-D2

Keep ActorCellKeepingSynchronizationContext internal via friend assembly access and run Akka.TestKit.Xunit.Tests with parallel collections so CI exercises the implicit-sender leak regression by default.

Aaronontheweb

Generally looks good but have some nitpicks

Aaronontheweb · 2026-04-23T15:33:11Z

@@ -1,6 +1,4 @@
 <Project Sdk="Microsoft.NET.Sdk">
-  <Import Project="../../../xunitSettings.props" />


Going to use a unique, project-specific xunit settings file for this so we can enable test parallelism and see about reproducing issues like akkadotnet/Akka.Hosting#733 in CI/CD.

Aaronontheweb · 2026-04-23T15:48:20Z

+    // Forces the post-ctor continuation onto a different SC worker — the
+    // thread pollution only manifests when the body thread differs from the
+    // ctor thread.
+    public async ValueTask InitializeAsync() => await Task.Yield();


Aaronontheweb · 2026-04-23T15:48:52Z

+    public ValueTask DisposeAsync() => default;
+
+    [Fact]
+    public async Task Implicit_sender_should_resolve_to_own_TestActor()


Aaronontheweb · 2026-04-23T16:06:42Z


 // The following GUID is for the ID of the typelib if this project is exposed to COM
 [assembly: Guid("ae01b790-1478-4917-9299-b4855ba997cb")]
+[assembly: InternalsVisibleTo("Akka.TestKit.Xunit")]


Make Akka.TestKit.Xunit a friend assembly - that's preferable to exposing more things on the public API.

Aaronontheweb · 2026-04-23T16:07:06Z

These are just TBD cleanup, which is appreciated.

Aaronontheweb · 2026-04-23T16:07:36Z

+{
+  "$schema": "https://xunit.github.io/schema/current/xunit.runner.schema.json",
+  "longRunningTestSeconds": 60,
+  "parallelizeAssembly": true,


Ensures we run the Akka.TestKit.Xunit.Tests assembly in parallel to try to get some CI/CD coverage over the test isolation guarantees.

Stop swallowing NullReferenceException when reading TestActor and drop the reflection-only attribute spec in favor of the parallel behavior tests that now run in CI.

Aaronontheweb

LGTM - addressed my requested changes from the previous review.

Aaronontheweb

Looks like this approach may not work at all for the Hosting.TestKit, so I'm investigating that separately.

Aaronontheweb · 2026-04-23T22:21:40Z

Related: Wrapping SynchronizationContext for Hosting Compatibility

PR #8182 builds on this PR's ActorCellKeepingSynchronizationContext + AkkaCleanAmbientContextAttribute to support wrapping the outer SynchronizationContext rather than replacing it. This is needed for consumers like Akka.Hosting.TestKit whose IHost async startup depends on xUnit v3's MaxConcurrencySyncContext scheduling — raw ThreadPool.QueueUserWorkItem dispatch causes hangs.

The wrapping approach:

Captures SynchronizationContext.Current in Before() and passes it as an _inner SC to ActorCellKeepingSynchronizationContext
Post()/Send() delegate scheduling to the inner SC (preserving xUnit's worker-thread management) while wrapping callbacks with the cell-pinning save/restore window
Falls back to ThreadPool dispatch when no outer SC exists (identical to the current behavior)

Proven in Akka.Hosting PR akkadotnet/Akka.Hosting#735 — 303/303 tests pass under real xUnit v3 parallel class execution.

See issue akkadotnet/Akka.Hosting#733 for the original bug report.

Aaronontheweb · 2026-05-07T23:29:13Z

superseded via #8182

Arkatufus added 2 commits April 23, 2026 03:23

Merge branch 'dev' into xunit-3-implicit-sender-leak-D2

3404dee

Arkatufus commented Apr 22, 2026

View reviewed changes

Arkatufus and others added 6 commits April 23, 2026 04:12

Merge branch 'dev' into xunit-3-implicit-sender-leak-D2

2e9b786

Update API approval list

80d4dc1

Merge branch 'xunit-3-implicit-sender-leak-D2' of https://github.com/…

0c7bb19

…Arkatufus/akka.net into xunit-3-implicit-sender-leak-D2

Merge branch 'dev' into xunit-3-implicit-sender-leak-D2

449b125

Merge branch 'dev' into xunit-3-implicit-sender-leak-D2

b52beba

Keep xUnit ambient context plumbing internal

1fb7272

Keep ActorCellKeepingSynchronizationContext internal via friend assembly access and run Akka.TestKit.Xunit.Tests with parallel collections so CI exercises the implicit-sender leak regression by default.

Aaronontheweb added akka-testkit Akka.NET Testkit issues confirmed bug labels Apr 23, 2026

Aaronontheweb requested changes Apr 23, 2026

View reviewed changes

Remove exception-driven ambient context checks

29356e1

Stop swallowing NullReferenceException when reading TestActor and drop the reflection-only attribute spec in favor of the parallel behavior tests that now run in CI.

Aaronontheweb approved these changes Apr 23, 2026

View reviewed changes

Merge branch 'dev' into xunit-3-implicit-sender-leak-D2

a5b3ad1

Aaronontheweb requested changes Apr 23, 2026

View reviewed changes

Aaronontheweb mentioned this pull request Apr 23, 2026

fix: wrap outer SynchronizationContext in ActorCellKeepingSynchronizationContext #8182

Merged

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Aaronontheweb closed this May 7, 2026

		@@ -1,6 +1,4 @@
		<Project Sdk="Microsoft.NET.Sdk">
		<Import Project="../../../xunitSettings.props" />

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Arkatufus commented Apr 22, 2026

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Aaronontheweb commented Apr 23, 2026

Related: Wrapping SynchronizationContext for Hosting Compatibility

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Aaronontheweb commented May 7, 2026

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