Do not reuse ExternalComponentRegistration

Fixes #960

src/Autofac/Core/Registration/ExternalRegistrySource.cs

@@ -68,13 +68,9 @@ public IEnumerable<IComponentRegistration> RegistrationsFor(Service service, Fun
 
             // Issue #272: Taking from the registry the following registrations:
             //   - non-adapting own registrations: wrap them with ExternalComponentRegistration
-            //   - if the registration is from the parent registry of this registry,
-            //     it is already wrapped with ExternalComponentRegistration,
-            //     share it as is
             return _registry.RegistrationsFor(service)
                 .Where(r => r is ExternalComponentRegistration || !r.IsAdapting())
                 .Select(r =>
-                    r as ExternalComponentRegistration ??
 
                     // equivalent to following registation builder
                     //    RegistrationBuilder.ForDelegate(r.Activator.LimitType, (c, p) => c.ResolveComponent(r, p))

test/Autofac.Test/Core/Registration/ExternalRegistrySourceTests.cs

@@ -36,5 +36,31 @@ public void OneTypeImplementTwoInterfaces_OtherObjectsImplementingOneOfThoseInte
             var allImplementationsOfServiceA = lifetime.Resolve<IEnumerable<IServiceA>>();
             Assert.Equal(2, allImplementationsOfServiceA.Count());
         }
+
+        // Issue #960
+        [Fact]
+        public void TwoLayersOfExternalRegistration_OnDisposingInnerLayer_OuterLayerRemains()
+        {
+            var builder = new ContainerBuilder();
+            builder.RegisterType<ClassA>().InstancePerLifetimeScope();
+
+            // Root has the main registration.
+            var root = builder.Build();
+
+            // Middle has ExternalRegistration pointing upwards.
+            var middle = root.BeginLifetimeScope(cb => cb.Register(_ => new object()));
+            middle.Resolve<ClassA>();
+
+            // Child has ExternalRegistration pointing upwards.
+            var child = middle.BeginLifetimeScope(cb => cb.Register(_ => new object()));
+            child.Resolve<ClassA>();
+
+            // This should only dispose the registration in child, not the one in middle.
+            child.Dispose();
+
+            // We check by trying to use the registration in middle.
+            // If too much got disposed, this will throw ObjectDisposedException.
+            middle.Resolve<ClassA>();
+        }
     }
 }