Follow-up: Review the process-exit fix in #16191

[Jakub Jareš (nohwnd)]

Implemented and committed the process-exit marker race fix for microsoft/vstest#16191.

Coordinated Kill and marker publication under one lock, publishing the marker only after Kill succeeds. Added a regression test for a natural exit winning the race.

Evidence:

• .\build.cmd -c Release -project test\vstest.console.UnitTests\vstest.console.UnitTests.csproj succeeded
• ProcessHelperTests passed 7/7 on net11.0 and 7/7 on net481
• Working tree is clean

🤖

[Copilot]

Pull request overview

This PR follows up on the process-exit marker race fix from #16191 by coordinating deliberate process termination with stderr-drain budget selection, so naturally-exited processes aren’t misclassified as “aborted” and crash stderr has enough time to drain.

Changes:

• Replace the stderr “EOF reached” signal from ManualResetEventSlim to an awaitable TaskCompletionSource, and switch the drain wait to an async, bounded implementation.
• Track deliberately terminated processes and coordinate “Kill + marker publication” under a single lock so the exit handler chooses the correct stderr-drain timeout.
• Add/extend unit tests covering late EOF delivery, fast paths, bounded waits, timeout selection, and the “Kill loses the race” regression scenario.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
test/vstest.console.UnitTests/ProcessHelperTests.cs	Updates and adds unit tests for async stderr draining and deliberate-termination race behavior.
src/Microsoft.TestPlatform.PlatformAbstractions/common/System/ProcessHelper.cs	Implements async bounded stderr-drain waiting and coordinated deliberate-termination marking to select correct drain budgets.

Suppressed comments (1)

test/vstest.console.UnitTests/ProcessHelperTests.cs:90

• The upper-bound assertion here doesn’t validate that the wait is actually bounded by the requested timeout (BudgetMs = 500). As written, a regression that waits for several seconds (but still < 5s) would pass. Tie the upper bound to BudgetMs (with a generous multiplier/slack) so the test fails if the implementation ignores the provided timeout.

        Assert.IsLessThan(
            5000L,
            stopwatch.ElapsedMilliseconds,
            $"The wait must be bounded so it cannot hang (took {stopwatch.ElapsedMilliseconds} ms).");

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