Merge OnException handlers for the same exception type into one catch clause

src/Testing/CoreTests/Acceptance/on_exception_same_type_in_two_middlewares.cs

@@ -0,0 +1,128 @@
+using JasperFx;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Hosting;
+using Shouldly;
+using Wolverine;
+using Wolverine.Tracking;
+using Xunit;
+
+namespace CoreTests.Acceptance;
+
+public class on_exception_same_type_in_two_middlewares
+{
+    [Fact]
+    public async Task both_handlers_run_in_registration_order()
+    {
+        var recorder = new SameTypeRecorder();
+        using var host = await Host.CreateDefaultBuilder()
+            .UseWolverine(opts =>
+            {
+                opts.Services.AddSingleton(recorder);
+                opts.Discovery.DisableConventionalDiscovery().IncludeType(typeof(SameTypeTriggerHandler));
+                opts.Policies.AddMiddleware(typeof(FirstSameTypeMiddleware));
+                opts.Policies.AddMiddleware(typeof(SecondSameTypeMiddleware));
+            }).StartAsync(TestContext.Current.CancellationToken);
+
+        await host.InvokeMessageAndWaitAsync(new SameTypeTrigger());
+
+        recorder.Calls.ShouldBe(["first", "second"]);
+    }
+
+    [Fact]
+    public async Task both_handlers_can_return_a_value()
+    {
+        var recorder = new SameTypeRecorder();
+        using var host = await Host.CreateDefaultBuilder()
+            .UseWolverine(opts =>
+            {
+                opts.Services.AddSingleton(recorder);
+                opts.Discovery.DisableConventionalDiscovery()
+                    .IncludeType(typeof(SameTypeTriggerHandler))
+                    .IncludeType(typeof(SameTypeCascadeHandler));
+                opts.Policies.AddMiddleware(typeof(FirstCascadingMiddleware));
+                opts.Policies.AddMiddleware(typeof(SecondCascadingMiddleware));
+            }).StartAsync(TestContext.Current.CancellationToken);
+
+        await host.TrackActivity()
+            .DoNotAssertOnExceptionsDetected()
+            .InvokeMessageAndWaitAsync(new SameTypeTrigger());
+
+        recorder.Calls.ShouldBe(["first", "second"], ignoreOrder: true);
+    }
+
+    [Fact]
+    public async Task both_handlers_can_return_a_continuation()
+    {
+        var recorder = new SameTypeRecorder();
+        using var host = await Host.CreateDefaultBuilder()
+            .UseWolverine(opts =>
+            {
+                opts.Services.AddSingleton(recorder);
+                opts.Discovery.DisableConventionalDiscovery().IncludeType(typeof(SameTypeTriggerHandler));
+                opts.Policies.AddMiddleware(typeof(FirstContinuationMiddleware));
+                opts.Policies.AddMiddleware(typeof(SecondContinuationMiddleware));
+            }).StartAsync(TestContext.Current.CancellationToken);
+
+        await host.InvokeMessageAndWaitAsync(new SameTypeTrigger());
+
+        recorder.Calls.ShouldBe(["first", "second"]);
+    }
+}
+
+public static class FirstContinuationMiddleware
+{
+    public static HandlerContinuation OnException(ConcurrencyException ex, SameTypeRecorder recorder)
+    {
+        recorder.Calls.Add("first");
+        return HandlerContinuation.Continue;
+    }
+}
+
+public static class SecondContinuationMiddleware
+{
+    public static HandlerContinuation OnException(ConcurrencyException ex, SameTypeRecorder recorder)
+    {
+        recorder.Calls.Add("second");
+        return HandlerContinuation.Continue;
+    }
+}
+
+public record SameTypeCascade(string From);
+
+public static class SameTypeCascadeHandler
+{
+    public static void Handle(SameTypeCascade message, SameTypeRecorder recorder) => recorder.Calls.Add(message.From);
+}
+
+public static class FirstCascadingMiddleware
+{
+    public static SameTypeCascade OnException(ConcurrencyException ex) => new("first");
+}
+
+public static class SecondCascadingMiddleware
+{
+    public static SameTypeCascade OnException(ConcurrencyException ex) => new("second");
+}
+
+public class SameTypeRecorder
+{
+    public ActionLog Calls { get; } = new();
+}
+
+public record SameTypeTrigger;
+
+public static class SameTypeTriggerHandler
+{
+    public static void Handle(SameTypeTrigger message) =>
+        throw new ConcurrencyException("conflict", typeof(SameTypeTrigger), Guid.NewGuid());
+}
+
+public static class FirstSameTypeMiddleware
+{
+    public static void OnException(ConcurrencyException ex, SameTypeRecorder recorder) => recorder.Calls.Add("first");
+}
+
+public static class SecondSameTypeMiddleware
+{
+    public static void OnException(ConcurrencyException ex, SameTypeRecorder recorder) => recorder.Calls.Add("second");
+}

src/Wolverine/Middleware/TryCatchFinallyFrame.cs

@@ -32,6 +32,28 @@ public TryCatchFinallyFrame() : base(true)
 
     public void AddCatchBlock(Type exceptionType, Frame[] frames)
     {
+        // C# rejects a second catch clause for the same type (CS0160).
+        var existing = _catchBlocks.FirstOrDefault(x => x.ExceptionType == exceptionType);
+        if (existing != null)
+        {
+            // The merged handlers share one scope, so a variable can't reuse an earlier handler's name (CS0128).
+            var used = existing.Frames.SelectMany(x => x.Creates).Select(x => x.Usage).ToHashSet();
+            foreach (var variable in frames.SelectMany(x => x.Creates).Where(x => !x.VariableType.IsValueTuple()))
+            {
+                var name = variable.Usage;
+                for (var i = 1; used.Contains(name); i++)
+                {
+                    name = $"{variable.Usage}{i}";
+                }
+
+                variable.OverrideName(name);
+                used.Add(name);
+            }
+
+            existing.Frames = [..existing.Frames, ..frames];
+            return;
+        }
+
         _catchBlocks.Add(new CatchBlock(exceptionType, frames));
         SortCatchBlocks();
     }
@@ -212,7 +234,7 @@ public CatchBlock(Type exceptionType, Frame[] frames)
     }
 
     public Type ExceptionType { get; }
-    public Frame[] Frames { get; }
+    public Frame[] Frames { get; internal set; }
     public Variable ExceptionVariable { get; }
 
     internal static string DefaultExceptionVariableName(Type exceptionType)