[release/10.0] Fix restore of contexts in async task returning thunks #119893
+67
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The previous IL generated approximately implements the following:
```csharp
ExecutionAndSyncBlockStore store = default;
store.Push();
Continuation cont = null;
T result = default;
try
{
try
{
result = Inner(args);
cont = StubHelpers.AsyncCallContinuation();
}
catch (Exception ex)
{
return Task.FromException(ex);
}
}
finally
{
store.Pop();
}
if (cont == null)
return Task.FromResult(result);
return FinalizeTaskReturningThunk(cont);
```
However, since `FinalizeTaskReturningThunk` is where we call
`OnCompleted`, this means that user code gets called after we have
restored contexts.
To fix the issue change the thunks to look approximately like
```csharp
ExecutionAndSyncBlockStore store = default;
store.Push();
try
{
Continuation cont;
try
{
T result = Inner(args);
cont = StubHelpers.AsyncCallContinuation();
if (cont == null)
return Task.FromResult(result);
}
catch (Exception ex)
{
return Task.FromException(ex);
}
return FinalizeTaskReturningThunk(cont);
}
finally
{
store.Pop();
}
```
which ensures that the contexts do not get restored until after
`OnCompleted` has been called.
This reverts commit 9c461c5.
VSadov
approved these changes
Sep 19, 2025
JulieLeeMSFT
approved these changes
Sep 19, 2025
JulieLeeMSFT
added
Servicing-approved
|
Tagging subscribers to this area: @mangod9 |
|
We can merge when tests pass. |
4 tasks
|
/ba-g Failures are known |
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Backport of #119818 to release/10.0
/cc @jakobbotsch
Customer Impact
Runtime async does not properly propagate synchronization contexts around in some cases; in particular the synchronization context is sometimes restored too early, leading to custom awaiters not observing a synchronization context that was explicitly set by an async method.
Regression
This is new code with runtime async.
Testing
The fix was verified on existing libraries tests that caught the issue.
Risk
Low, change to new code that is only enabled with runtime async.