Fix .NET Framework startup deadlock from configBuilder sync-over-async

tracer/src/Datadog.Trace.ClrProfiler.Managed.Loader/ManagedProfilerAssemblyResolver.NetFramework.cs

@@ -0,0 +1,81 @@
+// <copyright file="ManagedProfilerAssemblyResolver.NetFramework.cs" company="Datadog">
+// Unless explicitly stated otherwise all files in this repository are licensed under the Apache 2 License.
+// This product includes software developed at Datadog (https://www.datadoghq.com/). Copyright 2017 Datadog, Inc.
+// </copyright>
+
+#if NETFRAMEWORK
+
+#nullable enable
+
+using System;
+using System.IO;
+using System.Reflection;
+
+namespace Datadog.Trace.ClrProfiler.Managed.Loader;
+
+// Owns the AppDomain.AssemblyResolve callback on .NET Framework. Kept on a
+// separate class from Startup so the handler can be dispatched without
+// forcing Startup's type-initializer to have finished. If the handler were
+// a method on Startup, a configBuilder attached to <appSettings> that
+// issues sync-over-async work during Startup..cctor could deadlock: the
+// main thread would be blocked inside the .cctor waiting for a Task, whose
+// continuation runs on a ThreadPool thread that probes Type.GetType and
+// fires AssemblyResolve, whose handler would then wait on Startup..cctor.
+internal static class ManagedProfilerAssemblyResolver
+{
+    // Seeded by Startup..cctor before the handler is subscribed.
+    internal static string? ManagedProfilerDirectory { get; set; }
+
+    internal static Assembly? OnAssemblyResolve(object sender, ResolveEventArgs args)
+    {
+        try
+        {
+            return ResolveAssembly(args.Name);
+        }
+        catch (Exception ex)
+        {
+            StartupLogger.Log(ex, "Error resolving assembly: {0}", args.Name);
+        }
+
+        return null;
+    }
+
+    internal static Assembly? ResolveAssembly(string name)
+    {
+        var assemblyName = new AssemblyName(name);
+
+        // On .NET Framework, having a non-US locale can cause mscorlib
+        // to enter the AssemblyResolve event when searching for resources
+        // in its satellite assemblies. Exit early so we don't cause
+        // infinite recursion.
+        if (string.Equals(assemblyName.Name, "mscorlib.resources", StringComparison.OrdinalIgnoreCase) ||
+            string.Equals(assemblyName.Name, "System.Net.Http", StringComparison.OrdinalIgnoreCase) ||
+            string.Equals(assemblyName.Name, "vstest.console.resources", StringComparison.OrdinalIgnoreCase))
+        {
+            return null;
+        }
+
+        // WARNING: Logs must not be added _before_ we check for the above bail-out conditions
+        var path = string.IsNullOrEmpty(ManagedProfilerDirectory) ? $"{assemblyName.Name}.dll" : Path.Combine(ManagedProfilerDirectory, $"{assemblyName.Name}.dll");
+        StartupLogger.Debug("Assembly Resolve event received for: {0}. Looking for: {1}", name, path);
+
+        if (File.Exists(path))
+        {
+            if (name.StartsWith("Datadog.Trace, Version=", StringComparison.Ordinal) && name != Startup.AssemblyName)
+            {
+                StartupLogger.Debug("  Trying to load '{0}' which does not match the expected version ('{1}'). [Path={2}]", name, Startup.AssemblyName, path);
+                return null;
+            }
+
+            StartupLogger.Debug("Calling Assembly.LoadFrom(\"{0}\")", path);
+            var assembly = Assembly.LoadFrom(path);
+            StartupLogger.Debug("Assembly loaded: {0}", assembly.FullName);
+            return assembly;
+        }
+
+        StartupLogger.Debug("Assembly not found in path: {0}", path);
+        return null;
+    }
+}
+
+#endif

tracer/src/Datadog.Trace.ClrProfiler.Managed.Loader/Startup.NetFramework.cs

@@ -7,9 +7,7 @@
 
 #nullable enable
 
-using System;
 using System.IO;
-using System.Reflection;
 
 namespace Datadog.Trace.ClrProfiler.Managed.Loader
 {
@@ -22,57 +20,6 @@ internal static string ComputeTfmDirectory(string tracerHomeDirectory)
         {
             return Path.Combine(Path.GetFullPath(tracerHomeDirectory), "net461");
         }
-
-        private static Assembly? AssemblyResolve_ManagedProfilerDependencies(object sender, ResolveEventArgs args)
-        {
-            try
-            {
-                return ResolveAssembly(args.Name);
-            }
-            catch (Exception ex)
-            {
-                StartupLogger.Log(ex, "Error resolving assembly: {0}", args.Name);
-            }
-
-            return null;
-        }
-
-        private static Assembly? ResolveAssembly(string name)
-        {
-            var assemblyName = new AssemblyName(name);
-
-            // On .NET Framework, having a non-US locale can cause mscorlib
-            // to enter the AssemblyResolve event when searching for resources
-            // in its satellite assemblies. Exit early so we don't cause
-            // infinite recursion.
-            if (string.Equals(assemblyName.Name, "mscorlib.resources", StringComparison.OrdinalIgnoreCase) ||
-                string.Equals(assemblyName.Name, "System.Net.Http", StringComparison.OrdinalIgnoreCase) ||
-                string.Equals(assemblyName.Name, "vstest.console.resources", StringComparison.OrdinalIgnoreCase))
-            {
-                return null;
-            }
-
-            // WARNING: Logs must not be added _before_ we check for the above bail-out conditions
-            var path = string.IsNullOrEmpty(ManagedProfilerDirectory) ? $"{assemblyName.Name}.dll" : Path.Combine(ManagedProfilerDirectory, $"{assemblyName.Name}.dll");
-            StartupLogger.Debug("Assembly Resolve event received for: {0}. Looking for: {1}", name, path);
-
-            if (File.Exists(path))
-            {
-                if (name.StartsWith("Datadog.Trace, Version=", StringComparison.Ordinal) && name != AssemblyName)
-                {
-                    StartupLogger.Debug("  Trying to load '{0}' which does not match the expected version ('{1}'). [Path={2}]", name, AssemblyName, path);
-                    return null;
-                }
-
-                StartupLogger.Debug("Calling Assembly.LoadFrom(\"{0}\")", path);
-                var assembly = Assembly.LoadFrom(path);
-                StartupLogger.Debug("Assembly loaded: {0}", assembly.FullName);
-                return assembly;
-            }
-
-            StartupLogger.Debug("Assembly not found in path: {0}", path);
-            return null;
-        }
     }
 }
 

tracer/src/Datadog.Trace.ClrProfiler.Managed.Loader/Startup.cs

@@ -17,7 +17,9 @@ namespace Datadog.Trace.ClrProfiler.Managed.Loader
     /// </summary>
     public sealed partial class Startup
     {
-        private const string AssemblyName = "Datadog.Trace, Version=3.43.0.0, Culture=neutral, PublicKeyToken=def86d061d0d2eeb";
+        // internal so ManagedProfilerAssemblyResolver can reference it. Safe because const strings are inlined at compile time.
+        // Do not add non-const static members on Startup that the resolver needs - that would re-introduce the .cctor deadlock.
+        internal const string AssemblyName = "Datadog.Trace, Version=3.43.0.0, Culture=neutral, PublicKeyToken=def86d061d0d2eeb";
         private const string AzureAppServicesSiteExtensionKey = "DD_AZURE_APP_SERVICES"; // only set when using the AAS site extension
         private const string TracerHomePathKey = "DD_DOTNET_TRACER_HOME";
 
@@ -78,7 +80,17 @@ static Startup()
 
                 try
                 {
+#if NETFRAMEWORK
+                    // On .NET Framework, route AssemblyResolve through a class other than Startup so
+                    // the handler doesn't require Startup..cctor to have finished. If a configBuilder
+                    // on <appSettings> issues sync-over-async work during Startup..cctor, the async
+                    // continuation may fire AssemblyResolve on a ThreadPool thread; a handler on
+                    // Startup itself would deadlock waiting on Startup..cctor.
+                    ManagedProfilerAssemblyResolver.ManagedProfilerDirectory = ManagedProfilerDirectory;
+                    AppDomain.CurrentDomain.AssemblyResolve += ManagedProfilerAssemblyResolver.OnAssemblyResolve;
+#else
                     AppDomain.CurrentDomain.AssemblyResolve += AssemblyResolve_ManagedProfilerDependencies;
+#endif
                 }
                 catch (Exception ex)
                 {
@@ -181,7 +193,11 @@ private static void TryInvokeManagedMethod(string typeName, string methodName, s
                 // We will try to resolve it manually as a last chance.
                 StartupLogger.Log(ex, "Error on assembly load: {0}, Trying to solve it manually...", assemblyString);
 
+#if NETFRAMEWORK
+                var assembly = ManagedProfilerAssemblyResolver.ResolveAssembly(assemblyString);
+#else
                 var assembly = ResolveAssembly(assemblyString);
+#endif
                 if (assembly is not null)
                 {
                     StartupLogger.Log("Assembly '{0}' was resolved manually.", assemblyString);