Handler discovery should be decoupled from extension mechanism

src/Testing/CoreTests/Configuration/find_handlers_with_the_default_handler_discovery.cs

@@ -176,6 +176,20 @@ public void find_handlers_from_included_assembly()
         chainFor<Module2Message3>().ShouldNotBeNull();
         chainFor<Module2Message4>().ShouldNotBeNull();
     }
+
+    [Fact]
+    public void find_handlers_from_handler_module_assemblies()
+    {
+        with(opts =>
+        {
+            opts.Discovery.IncludeHandlerModules = true;
+        });
+
+        chainFor<Module2Message1>().ShouldNotBeNull();
+        chainFor<Module2Message2>().ShouldNotBeNull();
+        chainFor<Module2Message3>().ShouldNotBeNull();
+        chainFor<Module2Message4>().ShouldNotBeNull();
+    }
 }
 
 public interface IMovieSink
@@ -283,4 +297,4 @@ public class EventConsumer
     public void Consume(Event1 @event)
     {
     }
-}
+}

src/Testing/Module2/Module2.csproj

@@ -1,3 +1,5 @@
 <Project Sdk="Microsoft.NET.Sdk">
-
+  <ItemGroup>
+    <ProjectReference Include="..\..\Wolverine\Wolverine.csproj" />
+  </ItemGroup>
 </Project>

src/Testing/Module2/Properties/AssemblyInfo.cs

@@ -0,0 +1,3 @@
+using Wolverine.Attributes;
+
+[assembly: WolverineHandlerModule]

src/Wolverine/Attributes/WolverineHandlerModuleAttribute.cs

@@ -0,0 +1,10 @@
+using JasperFx;
+
+namespace Wolverine.Attributes;
+
+/// <summary>
+/// Assembly-level marker attribute that designates an assembly 
+/// as containing Wolverine message handlers for automatic discovery.
+/// </summary>
+[AttributeUsage(AttributeTargets.Assembly)]
+public class WolverineHandlerModuleAttribute : JasperFxAssemblyAttribute;

src/Wolverine/Configuration/HandlerDiscovery.cs

@@ -104,6 +104,11 @@ private static bool isNotPublicType(Type type)
         return true;
     }
 
+    /// <summary>
+    /// 
+    /// </summary>
+    public bool IncludeHandlerModules { get; set; } = false;
+
     /// <summary>
     ///  Customize the conventional filtering on the handler type discovery. This is *additive* to the
     /// built in conventional handler discovery. Disabling conventional discovery will negate anything
@@ -181,6 +186,22 @@ internal IEnumerable<Type> findAllMessages(HandlerGraph handlers)
             .Distinct()
             .SelectMany(actionsFromType).ToArray();
     }
+
+    /// <summary>
+    /// Discovers and includes all assemblies marked with [WolverineHandlerModule] attribute.
+    /// </summary>
+    internal HandlerDiscovery DiscoverHandlerModules()
+    {
+        var handlerModuleAssemblies = AssemblyFinder
+            .FindAssemblies(a => a.HasAttribute<WolverineHandlerModuleAttribute>())
+            .Concat(AppDomain.CurrentDomain.GetAssemblies())
+            .Distinct()
+            .Where(a => a.HasAttribute<WolverineHandlerModuleAttribute>())
+            .ToArray();
+
+        Assemblies.AddRange(handlerModuleAssemblies);
+        return this;
+    }
 
     private IEnumerable<(Type, MethodInfo)> actionsFromType(Type type)
     {

src/Wolverine/HostBuilderExtensions.cs

@@ -225,6 +225,11 @@ internal static IServiceCollection AddWolverine(this IServiceCollection services
 
         configure?.Invoke(options);
 
+        if (options.Discovery.IncludeHandlerModules)
+        {
+            options.HandlerGraph.Discovery.DiscoverHandlerModules();
+        }
+
         options.ApplyLazyConfiguration();
 
         return services;
@@ -476,4 +481,4 @@ public void Configure(WolverineOptions options)
             options.Durability.Mode = DurabilityMode.Solo;
         }
     }
-}
+}