Adding a unit test to track number of failed PGOs. (#6497)

* Adding a unit test to track number of failed PGOs.

This ensures we check the failures with every commit
and if numbers jump more than 1% then we need to regenrate PGO.

* Fail the test if PGO file doesn't exist.

* Using 1% threshold for failed PGOs

Author: safihamid

test/WebJobs.Script.Tests.Integration/WebHostEndToEnd/SpecializationE2ETests.cs

@@ -18,7 +18,9 @@
 using Microsoft.Azure.WebJobs.Script.Configuration;
 using Microsoft.Azure.WebJobs.Script.Description;
 using Microsoft.Azure.WebJobs.Script.WebHost;
+using Microsoft.Azure.WebJobs.Script.WebHost.Middleware;
 using Microsoft.Azure.WebJobs.Script.Workers.Rpc;
+using Microsoft.Diagnostics.JitTrace;
 using Microsoft.Extensions.Configuration;
 using Microsoft.Extensions.DependencyInjection;
 using Microsoft.Extensions.Hosting;
@@ -267,6 +269,23 @@ public async Task StartAsync_SetsCorrectActiveHost_RefreshesLanguageWorkerOption
             Assert.Equal("7", rpcChannelAfterSpecialization.Config.Description.DefaultRuntimeVersion);
         }
 
+        [Fact]
+        public void ColdStart_JitFailuresTest()
+        {
+            var path = Path.Combine(Path.GetDirectoryName(new Uri(typeof(HostWarmupMiddleware).Assembly.CodeBase).LocalPath), WarmUpConstants.PreJitFolderName, WarmUpConstants.JitTraceFileName);
+
+            var file = new FileInfo(path);
+
+            Assert.True(file.Exists, $"Expected PGO file '{file.FullName}' does not exist. The file was either renamed or deleted.");
+
+            JitTraceRuntime.Prepare(file, out int successfulPrepares, out int failedPrepares);
+
+            var failurePercentage = (double) failedPrepares / successfulPrepares * 100;
+            
+            // using 1% as approximate number of allowed failures before we need to regenrate a new PGO file.
+            Assert.True( failurePercentage < 1.0 , $"Number of failed PGOs are more than 1 percent! Current number of failures are {failedPrepares}. This will definitely impact cold start! Time to regenrate PGOs and update the {WarmUpConstants.JitTraceFileName} file!");
+        }
+
         private IWebHostBuilder CreateStandbyHostBuilder(params string[] functions)
         {
             string scriptRootConfigPath = ConfigurationPath.Combine(ConfigurationSectionNames.WebHost, nameof(ScriptApplicationHostOptions.ScriptPath));