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`.

3 files changed, 59 insertions, 1 deletions

diff --git a/actionpack/CHANGELOG.md b/actionpack/CHANGELOG.md
index a370c3fc99..79209e908f 100644
--- a/actionpack/CHANGELOG.md
+++ b/actionpack/CHANGELOG.md
@@ -1,3 +1,15 @@
+*   Introduce ActionDispatch::DebugExceptions.register_interceptor
+
+    Exception aware plugin authors can use the newly introduced
+    `.register_interceptor` method to get the processed exception, instead of
+    monkey patching DebugExceptions.
+
+        ActionDispatch::DebugExceptions.register_interceptor do |request, exception|
+          HypoteticalPlugin.capture_exception(request, exception)
+        end
+
+    *Genadi Samokovarov*
+
 *   Output only one Content-Security-Policy nonce header value per request.
 
     Fixes #32597.
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)
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