Fix flaky AsyncContinuation cDAC dump test on osx-arm64 by using Task.Yield

[max-charlamb]

Note

This pull request was prepared with assistance from GitHub Copilot.

Problem

AsyncContinuationDumpTests.ThreadLocalContinuation_IsContinuation fails intermittently on osx-arm64 R2R with:

Could not find AsyncDispatcherInfo type in CoreLib

Failure tracked in #127774. Recently observed in builds 1413917 and 1411121.

Fixes #127774

Root cause

The test relies on the runtime having executed AsyncHelpers+RuntimeAsyncTask<T>.DispatchContinuations(), which writes AsyncDispatcherInfo.t_current and forces the JIT to load the AsyncDispatcherInfo MethodTable.

The current debuggee uses await Task.Delay(1) to force a suspension. On fast machines (in particular Apple Silicon CI machines) the 1ms timer can fire before the runtime-async Await helper checks IsCompleted, so the awaiter is observed as already-completed and the await runs straight through synchronously. With no suspension there is no dispatch, no t_current write, and the AsyncDispatcherInfo MT is never loaded.

I downloaded the failing osx-arm64 dump and walked the crashed thread:

[2-4] FailFast
[5]   InnerAsync
[6]   OuterAsync   (async2 body)
[7]   OuterAsync   (thunk)
[8]   Main
[9]   CallEntryPoint

There is no DispatchContinuations frame on the stack. OuterAsync calls InnerAsync directly, confirming the await did not suspend.

I also walked all 3 modules' TypeDefToMethodTable maps and confirmed AsyncDispatcherInfo MT is genuinely null everywhere (i.e. cDAC is reporting reality - the type never got loaded). Object, Task, and Continuation MTs are all loaded as expected.

Fix

Replace await Task.Delay(1) with await Task.Yield() in InnerAsync.

Task.Yield() returns a YieldAwaitable whose awaiter's IsCompleted always returns false, so the continuation is unconditionally posted back via DispatchContinuations. This guarantees AsyncDispatcherInfo.t_current is written before InnerAsync resumes and calls FailFast, regardless of machine timing.

Validation

Posting as a draft to run CI across all platforms; the osx-arm64 R2R leg is the one to watch.

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[Copilot]

Pull request overview

This PR updates the AsyncContinuation cDAC dump test debuggee to make async suspension deterministic, avoiding intermittent failures where the AsyncDispatcherInfo type hasn’t been forced to load on very fast machines.

Changes:

• Replace await Task.Delay(1) with await Task.Yield() to guarantee a suspension point (since YieldAwaiter.IsCompleted is always false).
• Add an explanatory comment documenting why Task.Delay(1) was timing-dependent and why Task.Yield() is used instead.

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.

[Copilot]

Pull request overview

Copilot reviewed 1 out of 1 changed files in this pull request and generated no new comments.