Fix a threading issue in ExceptionDiagnoser #1736 (#2182)

When a Benchmark spawns multiple threads that throw Exceptions,
FirstChanceException event handler is called concurrently. It must count
Exceptions in a thread-safe way.

Note: do not use lock keyword because that will influence
Monitor lock contention count of ThreadingDiagnoser.

Author: leonvandermeer

src/BenchmarkDotNet/Engines/ExceptionsStats.cs

@@ -1,13 +1,16 @@
 using System;
 using System.Runtime.ExceptionServices;
+using System.Threading;
 
 namespace BenchmarkDotNet.Engines
 {
     internal class ExceptionsStats
     {
         internal const string ResultsLinePrefix = "// Exceptions: ";
 
-        internal ulong ExceptionsCount { get; private set; }
+        private long exceptionsCount;
+
+        internal long ExceptionsCount { get => exceptionsCount; }
 
         public void StartListening()
         {
@@ -21,7 +24,7 @@ public void Stop()
 
         private void OnFirstChanceException(object sender, FirstChanceExceptionEventArgs e)
         {
-            ExceptionsCount++;
+            Interlocked.Increment(ref exceptionsCount);
         }
 
         public static string ToOutputLine(double exceptionCount) => $"{ResultsLinePrefix} {exceptionCount}";

tests/BenchmarkDotNet.IntegrationTests/ExceptionDiagnoserTests.cs

@@ -8,6 +8,7 @@
 using System;
 using System.Collections.Generic;
 using System.Linq;
+using System.Threading.Tasks;
 using Xunit;
 
 namespace BenchmarkDotNet.IntegrationTests
@@ -24,7 +25,8 @@ public void ExceptionCountIsAccurate()
             AssertStats(summary, new Dictionary<string, (string metricName, double expectedValue)>
             {
                 { nameof(ExceptionCount.DoNothing), ("ExceptionFrequency", 0.0) },
-                { nameof(ExceptionCount.ThrowOneException), ("ExceptionFrequency", 1.0) }
+                { nameof(ExceptionCount.ThrowOneException), ("ExceptionFrequency", 1.0) },
+                { nameof(ExceptionCount.ThrowFromMultipleThreads), ("ExceptionFrequency", 100.0) }
             });
         }
 
@@ -42,6 +44,22 @@ public void ThrowOneException()
 
             [Benchmark]
             public void DoNothing() { }
+
+            [Benchmark]
+            public async Task ThrowFromMultipleThreads()
+            {
+                void ThrowMultipleExceptions()
+                {
+                    for (int i = 0; i < 10; i++)
+                    {
+                        ThrowOneException();
+                    }
+                }
+
+                var tasks = Enumerable.Range(1, 10).Select(
+                    i => Task.Run(ThrowMultipleExceptions));
+                await Task.WhenAll(tasks);
+            }
         }
 
         private IConfig CreateConfig()