[release/9.0.1xx] Don't fail metadata updates on missing assemblies (#40725)

src/BuiltInTools/DotNetDeltaApplier/HotReloadAgent.cs

@@ -82,7 +82,7 @@ private UpdateHandlerActions GetMetadataUpdateHandlerActions()
             var handlerActions = new UpdateHandlerActions();
             foreach (var assembly in sortedAssemblies)
             {
-                foreach (var attr in assembly.GetCustomAttributesData())
+                foreach (var attr in TryGetCustomAttributesData(assembly))
                 {
                     // Look up the attribute by name rather than by type. This would allow netstandard targeting libraries to
                     // define their own copy without having to cross-compile.
@@ -106,6 +106,25 @@ private UpdateHandlerActions GetMetadataUpdateHandlerActions()
             return handlerActions;
         }
 
+        private IList<CustomAttributeData> TryGetCustomAttributesData(Assembly assembly)
+        {
+            try
+            {
+                return assembly.GetCustomAttributesData();
+            }
+            catch (Exception e)
+            {
+                // In cross-platform scenarios, such as debugging in VS through WSL, Roslyn
+                // runs on Windows, and the agent runs on Linux. Assemblies accessible to Windows
+                // may not be available or loaded on linux (such as WPF's assemblies).
+                // In such case, we can ignore the assemblies and continue enumerating handlers for
+                // the rest of the assemblies of current domain.
+                _log($"'{assembly.FullName}' is not loaded ({e.Message})");
+
+                return new List<CustomAttributeData>();
+            }
+        }
+
         internal void GetHandlerActions(UpdateHandlerActions handlerActions, Type handlerType)
         {
             bool methodFound = false;

test/TestAssets/TestProjects/WatchAppMissingAssemblyFailure/App/App.csproj

@@ -0,0 +1,13 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <TargetFramework>$(CurrentTargetFramework)</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\Dep\Dep.csproj" />
+  </ItemGroup>
+
+</Project>

test/TestAssets/TestProjects/WatchAppMissingAssemblyFailure/App/Program.cs

@@ -0,0 +1,34 @@
+using System.Diagnostics;
+using System.Reflection.Metadata;
+
+[assembly: MetadataUpdateHandler(typeof(UpdateHandler))]
+
+// delete the dependency dll to cause load failure of DepSubType
+var depPath = Path.Combine(Path.GetDirectoryName(typeof(Program).Assembly.Location!)!, "Dep2.dll");
+File.Delete(depPath);
+Console.WriteLine($"File deleted: {depPath}");
+
+while (true)
+{
+    lock (UpdateHandler.Guard)
+    {
+        Printer.Print();
+        Dep.DepLib.F();
+    }
+
+    Thread.Sleep(100);
+}
+
+static class UpdateHandler
+{
+    // Lock to avoid the updated Print method executing concurrently with the update handler.
+    public static object Guard = new object();
+
+    public static void UpdateApplication(Type[] types)
+    {
+        lock (Guard)
+        {
+            Console.WriteLine($"Updated types: {(types == null ? "<null>" : types.Length == 0 ? "<empty>" : string.Join(",", types.Select(t => t.Name)))}");
+        }
+    }
+}

test/TestAssets/TestProjects/WatchAppMissingAssemblyFailure/App/Update.cs

@@ -0,0 +1,12 @@
+using System;
+
+public class DepType
+{
+    int F() => 1;
+}
+
+public class Printer
+{
+    public static void Print()
+        => Console.WriteLine("Hello!");
+}

test/TestAssets/TestProjects/WatchAppMissingAssemblyFailure/Dep/Dep.cs

@@ -0,0 +1,13 @@
+// This attribute is not causing Dep.dll to be loaded, but enough
+// to cause the HotReloadAgent to fail on getting custom attributes.
+[assembly: Dep2.Test()]
+
+namespace Dep;
+
+public class DepLib
+{
+    public static void F()
+    {
+        Console.WriteLine(1);
+    }
+}

test/TestAssets/TestProjects/WatchAppMissingAssemblyFailure/Dep/Dep.csproj

@@ -0,0 +1,12 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>$(CurrentTargetFramework)</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\Dep2\Dep2.csproj" />
+  </ItemGroup>
+
+</Project>

test/TestAssets/TestProjects/WatchAppMissingAssemblyFailure/Dep2/Dep2.cs

@@ -0,0 +1,9 @@
+namespace Dep2;
+
+public class Dep2Lib
+{
+    void F()
+    {
+        Console.WriteLine(1);
+    }
+}

test/TestAssets/TestProjects/WatchAppMissingAssemblyFailure/Dep2/Dep2.csproj

@@ -0,0 +1,8 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <TargetFramework>$(CurrentTargetFramework)</TargetFramework>
+    <ImplicitUsings>enable</ImplicitUsings>
+  </PropertyGroup>
+
+</Project>

test/TestAssets/TestProjects/WatchAppMissingAssemblyFailure/Dep2/TestAttribute.cs

@@ -0,0 +1,8 @@
+using System;
+
+namespace Dep2;
+
+[AttributeUsage(AttributeTargets.Assembly)]
+public class TestAttribute : Attribute
+{
+}

test/dotnet-watch.Tests/HotReload/ApplyDeltaTests.cs

@@ -120,5 +120,42 @@ public async Task BlazorWasm()
             //UpdateSourceFile(Path.Combine(testAsset.Path, "Pages", "Index.razor"), newSource);
             //await App.AssertOutputLineStartsWith(MessageDescriptor.HotReloadSucceeded);
         }
+
+        // Test is timing out on .NET Framework: https://github.com/dotnet/sdk/issues/41669
+        [CoreMSBuildOnlyFact]
+        public async Task HandleMissingAssemblyFailure()
+        {
+            var testAsset = TestAssets.CopyTestAsset("WatchAppMissingAssemblyFailure")
+                .WithSource();
+
+            App.Start(testAsset, [], "App");
+
+            await App.AssertWaitingForChanges();
+
+            var newSrc = /* lang=c#-test */"""
+                using System;
+
+                public class DepType
+                {
+                    int F() => 1;
+                }
+
+                public class Printer
+                {
+                    public static void Print()
+                        => Console.WriteLine("Updated!");
+                }
+                """;
+
+            // Delete all files in testAsset.Path named Dep.dll
+            foreach (var depDll in Directory.GetFiles(testAsset.Path, "Dep2.dll", SearchOption.AllDirectories))
+            {
+                File.Delete(depDll);
+            }
+
+            File.WriteAllText(Path.Combine(testAsset.Path, "App", "Update.cs"), newSrc);
+
+            await App.AssertOutputLineStartsWith("Updated types: Printer");
+        }
     }
 }