[Fleet installer] Add additional pre-verification check for IIS variables

tracer/src/Datadog.FleetInstaller/AppHostHelper.cs

@@ -27,6 +27,53 @@ public static bool RemoveAllEnvironmentVariables(ILogger log)
         return ModifyEnvironmentVariablesWithRetry(log, envVars, RemoveEnvVars);
     }
 
+    public static bool GetAppPoolEnvironmentVariable(ILogger log, string environmentVariable, out string? value)
+    {
+        using var serverManager = new ServerManager();
+        var appPoolsSection = GetApplicationPoolsSection(log, serverManager);
+        if (appPoolsSection is null)
+        {
+            value = null;
+            return false;
+        }
+
+        var (applicationPoolDefaults, applicationPoolsCollection) = appPoolsSection.Value;
+
+        // Check defaults
+        log.WriteInfo($"Checking applicationPoolDefaults for environment variable: {environmentVariable}");
+        if (TryGetEnvVar(applicationPoolDefaults.GetCollection("environmentVariables"), environmentVariable) is { } envValue)
+        {
+            value = envValue;
+            return true;
+        }
+
+        foreach (var appPoolElement in applicationPoolsCollection)
+        {
+            if (string.Equals(appPoolElement.ElementTagName, "add", StringComparison.OrdinalIgnoreCase))
+            {
+                // An app pool element
+                var poolName = appPoolElement.GetAttributeValue("name") as string;
+                if (poolName is null)
+                {
+                    // poolName can never be null, if it is, weirdness is afoot, so bail out
+                    log.WriteInfo("Found app pool element without a name, skipping");
+                    continue;
+                }
+
+                log.WriteInfo($"Checking app pool '{poolName}' for environment variable: {environmentVariable}");
+                if (TryGetEnvVar(appPoolElement.GetCollection("environmentVariables"), environmentVariable) is { } poolEnvValue)
+                {
+                    value = poolEnvValue;
+                    return true;
+                }
+            }
+        }
+
+        log.WriteInfo($"{environmentVariable} variable not found in any app pools");
+        value = null;
+        return false;
+    }
+
     private static bool ModifyEnvironmentVariablesWithRetry(
         ILogger log,
         ReadOnlyDictionary<string, string> envVars,
@@ -275,4 +322,23 @@ private static void RemoveEnvVars(ConfigurationElementCollection envVars, ReadOn
             envVars.Remove(element);
         }
     }
+
+    private static string? TryGetEnvVar(ConfigurationElementCollection envVars, string variable)
+    {
+        foreach (var envVarEle in envVars)
+        {
+            if (!string.Equals(envVarEle.ElementTagName, "add", StringComparison.OrdinalIgnoreCase))
+            {
+                continue;
+            }
+
+            if (envVarEle.GetAttributeValue("name") is string key
+             && key.Equals(variable, StringComparison.OrdinalIgnoreCase))
+            {
+                return envVarEle["value"] as string;
+            }
+        }
+
+        return null;
+    }
 }

tracer/src/Datadog.FleetInstaller/Commands/InstallCommand.cs

@@ -3,6 +3,7 @@
 // This product includes software developed at Datadog (https://www.datadoghq.com/). Copyright 2017 Datadog, Inc.
 // </copyright>
 
+using System;
 using System.CommandLine;
 using System.CommandLine.Invocation;
 using System.CommandLine.Parsing;
@@ -55,6 +56,37 @@ internal static ReturnCode Execute(
     {
         log.WriteInfo("Installing .NET tracer");
 
+        bool tryIisRollback;
+
+        try
+        {
+            log.WriteInfo("Checking IIS app pools for pre-existing instrumentation variable");
+            if (AppHostHelper.GetAppPoolEnvironmentVariable(log, Defaults.InstrumentationInstallTypeKey, out var value))
+            {
+                var expectedValue = Defaults.InstrumentationInstallTypeValue;
+                if (expectedValue.Equals(value, StringComparison.Ordinal))
+                {
+                    log.WriteInfo($"Found existing instrumentation install type with value {expectedValue}. Won't rollback IIS instrumentation if install fails");
+                    tryIisRollback = false;
+                }
+                else
+                {
+                    log.WriteInfo($"Found instrumentation install type {value}, but did not have expected value {expectedValue}. Will rollback IIS instrumentation if install fails");
+                    tryIisRollback = true;
+                }
+            }
+            else
+            {
+                log.WriteInfo("No existing fleet installer instrumentation install type found. Will rollback IIS instrumentation if install fails");
+                tryIisRollback = true;
+            }
+        }
+        catch (Exception ex)
+        {
+            log.WriteError(ex, "Error reading IIS app pools, installation failed");
+            return ReturnCode.ErrorReadingIisConfiguration;
+        }
+
         if (!FileHelper.CreateLogDirectory(log, tracerLogDirectory))
         {
             // This probably isn't a reason to bail out
@@ -69,6 +101,14 @@ internal static ReturnCode Execute(
         if (!AppHostHelper.SetAllEnvironmentVariables(log, tracerValues))
         {
             // hard to be sure exactly of the state at this point
+            if (tryIisRollback)
+            {
+                log.WriteInfo("Attempting IIS variable rollback");
+
+                // We ignore failures here
+                AppHostHelper.RemoveAllEnvironmentVariables(log);
+            }
+
             return ReturnCode.ErrorSettingAppPoolVariables;
         }
 

tracer/src/Datadog.FleetInstaller/Defaults.cs

@@ -11,6 +11,8 @@ namespace Datadog.FleetInstaller;
 internal class Defaults
 {
     public const string CrashTrackingRegistryKey = @"Software\Microsoft\Windows\Windows Error Reporting\RuntimeExceptionHelperModules";
+    public const string InstrumentationInstallTypeKey = "DD_INSTRUMENTATION_INSTALL_TYPE";
+    public const string InstrumentationInstallTypeValue = "windows_fleet_installer";
 
     public static string TracerLogDirectory
         => Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.CommonApplicationData), "Datadog .NET Tracer", "logs");

tracer/src/Datadog.FleetInstaller/ReturnCode.cs

@@ -17,4 +17,5 @@ internal enum ReturnCode
     ErrorRemovingAppPoolVariables,
     ErrorRemovingNativeLoaderFiles,
     ErrorRemovingCrashTrackerKey,
+    ErrorReadingIisConfiguration,
 }

tracer/src/Datadog.FleetInstaller/TracerValues.cs

@@ -27,7 +27,7 @@ public TracerValues(string tracerHomeDirectory)
             { "DD_DOTNET_TRACER_HOME", TracerHomeDirectory },
             { "COR_ENABLE_PROFILING", "1" },
             { "CORECLR_ENABLE_PROFILING", "1" },
-            { "DD_INSTRUMENTATION_INSTALL_TYPE", "windows_fleet_installer" },
+            { Defaults.InstrumentationInstallTypeKey, Defaults.InstrumentationInstallTypeValue },
         });
         FilesToAddToGac =
         [