Introduce ActionDispatch::DebugExceptions interceptors

Plugins interacting with the exceptions caught and displayed by
ActionDispatch::DebugExceptions currently have to monkey patch it to get
the much needed exception for their calculation.

With DebugExceptions.register_interceptor, plugin authors can hook into
DebugExceptions and process the exception, before being rendered. They
can store it into the request and process it on the way back of the
middleware chain execution or act on it straight in the interceptor.

The interceptors can be play blocks, procs, lambdas or any object that
responds to `#call`.

1 files changed, 24 insertions, 0 deletions

diff --git a/actionpack/test/dispatch/debug_exceptions_test.rb b/actionpack/test/dispatch/debug_exceptions_test.rb
index 60acba0616..045567ff83 100644
--- a/actionpack/test/dispatch/debug_exceptions_test.rb
+++ b/actionpack/test/dispatch/debug_exceptions_test.rb
@@ -3,6 +3,8 @@
 require "abstract_unit"
 
 class DebugExceptionsTest < ActionDispatch::IntegrationTest
+  InterceptedErrorInstance = StandardError.new
+
   class Boomer
     attr_accessor :closed
 
@@ -36,6 +38,8 @@ class DebugExceptionsTest < ActionDispatch::IntegrationTest
         raise RuntimeError
       when %r{/method_not_allowed}
         raise ActionController::MethodNotAllowed
+      when %r{/intercepted_error}
+        raise InterceptedErrorInstance
       when %r{/unknown_http_method}
         raise ActionController::UnknownHttpMethod
       when %r{/not_implemented}
@@ -76,9 +80,13 @@ class DebugExceptionsTest < ActionDispatch::IntegrationTest
     end
   end
 
+  Interceptor = proc { |request, exception| request.set_header("int", exception) }
+  BadInterceptor = proc { |request, exception| raise "bad" }
   RoutesApp = Struct.new(:routes).new(SharedTestRoutes)
   ProductionApp  = ActionDispatch::DebugExceptions.new(Boomer.new(false), RoutesApp)
   DevelopmentApp = ActionDispatch::DebugExceptions.new(Boomer.new(true), RoutesApp)
+  InterceptedApp = ActionDispatch::DebugExceptions.new(Boomer.new(true), RoutesApp, :default, [Interceptor])
+  BadInterceptedApp = ActionDispatch::DebugExceptions.new(Boomer.new(true), RoutesApp, :default, [BadInterceptor])
 
   test "skip diagnosis if not showing detailed exceptions" do
     @app = ProductionApp
@@ -499,4 +507,20 @@ class DebugExceptionsTest < ActionDispatch::IntegrationTest
       end
     end
   end
+
+  test "invoke interceptors before rendering" do
+    @app = InterceptedApp
+    get "/intercepted_error", headers: { "action_dispatch.show_exceptions" => true }
+
+    assert_equal InterceptedErrorInstance, request.get_header("int")
+  end
+
+  test "bad interceptors doesn't debug exceptions" do
+    @app = BadInterceptedApp
+
+    get "/puke", headers: { "action_dispatch.show_exceptions" => true }
+
+    assert_response 500
+    assert_match(/puke/, body)
+  end
 end