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, 23 insertions, 1 deletions

diff --git a/actionpack/lib/action_dispatch/middleware/debug_exceptions.rb b/actionpack/lib/action_dispatch/middleware/debug_exceptions.rb
index 511306eb0e..33edad8bd9 100644
--- a/actionpack/lib/action_dispatch/middleware/debug_exceptions.rb
+++ b/actionpack/lib/action_dispatch/middleware/debug_exceptions.rb
@@ -50,10 +50,18 @@ module ActionDispatch
       end
     end
 
-    def initialize(app, routes_app = nil, response_format = :default)
+    cattr_reader :interceptors, instance_accessor: false, default: []
+
+    def self.register_interceptor(object = nil, &block)
+      interceptor = object || block
+      interceptors << interceptor
+    end
+
+    def initialize(app, routes_app = nil, response_format = :default, interceptors = self.class.interceptors)
       @app             = app
       @routes_app      = routes_app
       @response_format = response_format
+      @interceptors    = interceptors
     end
 
     def call(env)
@@ -67,12 +75,26 @@ module ActionDispatch
 
       response
     rescue Exception => exception
+      invoke_interceptors(request, exception)
       raise exception unless request.show_exceptions?
       render_exception(request, exception)
     end
 
     private
 
+      def invoke_interceptors(request, exception)
+        backtrace_cleaner = request.get_header("action_dispatch.backtrace_cleaner")
+        wrapper = ExceptionWrapper.new(backtrace_cleaner, exception)
+
+        @interceptors.each do |interceptor|
+          begin
+            interceptor.call(request, exception)
+          rescue Exception
+            log_error(request, wrapper)
+          end
+        end
+      end
+
       def render_exception(request, exception)
         backtrace_cleaner = request.get_header("action_dispatch.backtrace_cleaner")
         wrapper = ExceptionWrapper.new(backtrace_cleaner, exception)