[Internal] Performance: Fixes exception serialization when tracing is…

… not enabled (#1999)

* ToString only if tracing is enabled

* tests

Microsoft.Azure.Cosmos/src/ResourceThrottleRetryPolicy.cs

@@ -117,17 +117,16 @@ private Task<ShouldRetryResult> ShouldRetryInternalAsync(TimeSpan? retryAfter)
             }
         }
 
-        private string GetExceptionMessage(Exception exception)
+        private object GetExceptionMessage(Exception exception)
         {
-            DocumentClientException dce = exception as DocumentClientException;
-            if (dce != null && dce.StatusCode != null && (int)dce.StatusCode < (int)StatusCodes.InternalServerError)
+            if (exception is DocumentClientException dce && dce.StatusCode != null && (int)dce.StatusCode < (int)StatusCodes.InternalServerError)
             {
                 // for client related errors, don't print out the whole call stack.
                 // simply return the message to prevent CPU overhead on ToString() 
                 return exception.Message;
             }
 
-            return exception.ToString();
+            return exception;
         }
 
         /// <summary>

Microsoft.Azure.Cosmos/tests/Microsoft.Azure.Cosmos.Tests/ResourceThrottleRetryPolicyTests.cs

@@ -0,0 +1,77 @@
+//------------------------------------------------------------
+// Copyright (c) Microsoft Corporation.  All rights reserved.
+//------------------------------------------------------------
+
+namespace Microsoft.Azure.Cosmos.Tests
+{
+    using System;
+    using System.Collections.Generic;
+    using System.Diagnostics;
+    using System.Threading.Tasks;
+    using Microsoft.Azure.Cosmos.Core.Trace;
+    using Microsoft.VisualStudio.TestTools.UnitTesting;
+
+    [TestClass]
+    public class ResourceThrottleRetryPolicyTests
+    {
+        private readonly List<TraceListener> existingListener = new List<TraceListener>();
+        private SourceSwitch existingSourceSwitch;
+
+        [TestInitialize]
+        public void CaptureCurrentTraceConfiguration()
+        {
+            foreach (TraceListener listener in DefaultTrace.TraceSource.Listeners)
+            {
+                this.existingListener.Add(listener);
+            }
+
+            DefaultTrace.TraceSource.Listeners.Clear();
+            this.existingSourceSwitch = DefaultTrace.TraceSource.Switch;
+        }
+
+        [TestCleanup]
+        public void ResetTraceConfiguration()
+        {
+            DefaultTrace.TraceSource.Listeners.Clear();
+            foreach (TraceListener listener in this.existingListener)
+            {
+                DefaultTrace.TraceSource.Listeners.Add(listener);
+            }
+
+            DefaultTrace.TraceSource.Switch = this.existingSourceSwitch;
+        }
+
+        [TestMethod]
+        public async Task DoesNotSerializeExceptionOnTracingDisabled()
+        {
+            // No listeners
+            ResourceThrottleRetryPolicy policy = new ResourceThrottleRetryPolicy(0);
+            CustomException exception = new CustomException();
+            await policy.ShouldRetryAsync(exception, default);
+            Assert.AreEqual(0, exception.ToStringCount, "Exception was serialized");
+        }
+
+        [TestMethod]
+        public async Task DoesSerializeExceptionOnTracingEnabled()
+        {
+            // Let the default trace listener
+            DefaultTrace.TraceSource.Switch = new SourceSwitch("ClientSwitch", "Error");
+            DefaultTrace.TraceSource.Listeners.Add(new DefaultTraceListener());
+            ResourceThrottleRetryPolicy policy = new ResourceThrottleRetryPolicy(0);
+            CustomException exception = new CustomException();
+            await policy.ShouldRetryAsync(exception, default);
+            Assert.AreEqual(1, exception.ToStringCount, "Exception was not serialized");
+        }
+
+        private class CustomException : Exception
+        {
+            public int ToStringCount { get; private set; } = 0;
+
+            public override string ToString()
+            {
+                ++this.ToStringCount;
+                return string.Empty;
+            }
+        }
+    }
+}