[omid-io]

Description

Fixes #136268.

In Windows x86 CoreCLR CI runs, System.Threading.Tasks.Tests.TaskContinueWithTests.RunContinueWithStressTestsNoState was failing with System.OutOfMemoryException inside StringBuilder.ToString().

Root Cause

When waiting on a large number of tasks (e.g., 5,000–10,000+ canceled continuation tasks in stress tests), Task.WaitAllCore iterates over all tasks and calls Task.AddExceptionsForCompletedTask(ref exceptions, task).
Previously, this routed through ask.GetExceptions(true), which for each canceled task:

1. Created a new TaskCanceledException.
2. Called canceledException.SetCurrentStackTrace(), walking the call stack with
   ew StackTrace(fNeedFileInfo: true) and serializing the trace via StringBuilder allocations.
3. Wrapped it into an intermediate AggregateException, solely for AddExceptionsForCompletedTask to immediately extract ex.InternalInnerExceptions and discard the wrapper.

Under 32-bit x86 address space constraints (2GB user space), generating thousands of stack trace captures and strings concurrently caused address space exhaustion and OutOfMemoryException.

Fix

In Task.AddExceptionsForCompletedTask:

• If .ExceptionRecorded is true, continue retrieving exceptions via .GetExceptions(true) and copy them to the list.
• If .IsCanceled (with no other exceptions recorded), directly add
  ew TaskCanceledException(t) to the exception list without triggering expensive stack trace formatting or intermediate AggregateException allocations.
• Preserve .UpdateExceptionObservedStatus() to ensure attached child semantics remain intact.

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🟡 Changes recommended

The change loses diagnostic stack traces on inner TaskCanceledException instances from WaitAll, requiring trace preservation and a regression assertion for their StackTrace.

1 open issue

• Preserve stack traces for canceled tasks in WaitAll exceptions · New

What changed in this PR

This PR targets the Windows x86 stress-test OOM reported in #136268 by reducing canceled-task exception allocations in CoreLib’s Task.WaitAll path.

Changes:

• Adds canceled-task exceptions directly, avoiding per-task stack-trace capture and intermediate AggregateException allocations.
• Retains existing exception retrieval for recorded faults and preserves exception-observation bookkeeping.

File	Description
src/​libraries/​System.Private.CoreLib/​src/​System/​Threading/​Tasks/​Task.cs	Adds a cancellation-specific path in AddExceptionsForCompletedTask to reduce allocations.

_{Review effort: Balanced}

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