Fix mixed synchronization for _runStartedClients in ParallelProxyExecutionManager

[azat-msft]

Addresses task 4 of #16195.

Problem

_runStartedClients in ParallelProxyExecutionManager was incremented via Interlocked.Increment from a Task.Run lambda outside any lock, but read inside lock (_executionStatusLockObject) in HandlePartialRunComplete. The lock does not fence the atomic write performed on a concurrent thread, so per the C# memory model this is formally a data race (an existing // BUG comment acknowledged the concern).

Fix

Increment _runStartedClients under _executionStatusLockObject, matching how the sibling counter _runCompletedClients is already handled. This guards the write and read with the same lock.

Validation

• dotnet build src/Microsoft.TestPlatform.CrossPlatEngine -c Release — 0 warnings, 0 errors.

Co-authored-by: Copilot 223556219+Copilot@users.noreply.github.com

[Copilot]

Pull request overview

This PR updates ParallelProxyExecutionManager in CrossPlatEngine to eliminate mixed synchronization on the _runStartedClients counter, aligning its update strategy with the lock-based reads performed during run-completion handling (Task 4 of #16195).

Changes:

• Moves _runStartedClients increment from an Interlocked.Increment performed outside a lock to a lock (_executionStatusLockObject)-guarded increment.
• Adds explanatory comments clarifying why the increment is now performed under the execution-status lock.

Comments suppressed due to low confidence (1)

src/Microsoft.TestPlatform.CrossPlatEngine/Client/Parallel/ParallelProxyExecutionManager.cs:419

• _runStartedClients is now incremented under _executionStatusLockObject, but the subsequent trace statements read _runStartedClients outside the lock. Since the motivation is to avoid mixed synchronization for this field, consider capturing a local snapshot under the lock (and use that for the trace lines) so all reads/writes in this block share the same synchronization pattern and the comment remains accurate.

                EqtTrace.Verbose("ParallelProxyExecutionManager.StartTestRunOnConcurrentManager: Initializing test run. Started clients: " + _runStartedClients);
                proxyExecutionManager.InitializeTestRun(testRunCriteria, eventHandler);

                EqtTrace.Verbose("ParallelProxyExecutionManager.StartTestRunOnConcurrentManager: Execution starting. Started clients: " + _runStartedClients);
                proxyExecutionManager.StartTestRun(testRunCriteria, eventHandler);

[nohwnd]

/azp run

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