Merged PR 654144: Stop flushing statsprf.json file upon cancellation

This file gets flushed slightly differently as part of the scheduler's shutdown process. That's good for typical builds, but when a build is cancelled due to timeout that makes it harder to tell what was going on during at the point that it was cancelled. So skip flushing the file if cancellation is requested to preserve its state.

Public/Src/Engine/Scheduler/Tracing/OperationTracker.cs

@@ -218,6 +218,13 @@ namespace BuildXL.Scheduler.Tracing
 
         internal void WriteCountersFile(PipExecutionContext context, ILoggingConfiguration loggingConfiguration, TimeSpan? refreshInterval = null)
         {
+            // Skip further updates to tracker counters file when cancellation is requested to preserve state prior
+            // to winding everything down. This is helpful for debugging the state of builds that time out.
+            if (context.CancellationToken.IsCancellationRequested)
+            {
+                return;
+            }
+
             bool includeOutstanding = refreshInterval != null;
             lock (m_counterFileLock)
             {