Fixed a bug where the debug view does not show the error page properly

There are two cases where the debug view does not show the error details
properly:

 * When the cause is mapped to an HTTP status code the last exception is
   unexpectedly uwrapped
 * When the last error is thrown from a view template the debug view is
   not using the `rescues/template_error.html.erb` to generate the view

Both the cases could be fixed by not unwrapping the exception. The only
case where the exception should be unwrapped is when the last error is
an `ActionView::Template::Error` object. In this case the HTTP status
code is determined based on the cause.

There are actually more wrapper exceptions that are intentionally
thrown. However, there is a consistent pattern of setting the original
message and original backtrace to the wrapper exception implemented, so
the debug view will not lose the information about what went wrong
eariler.

2 files changed, 15 insertions, 11 deletions

diff --git a/actionpack/lib/action_dispatch/middleware/exception_wrapper.rb b/actionpack/lib/action_dispatch/middleware/exception_wrapper.rb
index fb2b2bd3b0..1fb3e9db00 100644
--- a/actionpack/lib/action_dispatch/middleware/exception_wrapper.rb
+++ b/actionpack/lib/action_dispatch/middleware/exception_wrapper.rb
@@ -31,22 +31,34 @@ module ActionDispatch
       "ActionController::MissingExactTemplate" => "missing_exact_template",
     )
 
+    cattr_accessor :wrapper_exceptions, default: [
+      "ActionView::Template::Error"
+    ]
+
     attr_reader :backtrace_cleaner, :exception, :wrapped_causes, :line_number, :file
 
     def initialize(backtrace_cleaner, exception)
       @backtrace_cleaner = backtrace_cleaner
-      @exception = original_exception(exception)
+      @exception = exception
       @wrapped_causes = wrapped_causes_for(exception, backtrace_cleaner)
 
       expand_backtrace if exception.is_a?(SyntaxError) || exception.cause.is_a?(SyntaxError)
     end
 
+    def unwrapped_exception
+      if wrapper_exceptions.include?(exception.class.to_s)
+        exception.cause
+      else
+        exception
+      end
+    end
+
     def rescue_template
       @@rescue_templates[@exception.class.name]
     end
 
     def status_code
-      self.class.status_code_for_exception(@exception.class.name)
+      self.class.status_code_for_exception(unwrapped_exception.class.name)
     end
 
     def application_trace
@@ -122,14 +134,6 @@ module ActionDispatch
         Array(@exception.backtrace)
       end
 
-      def original_exception(exception)
-        if @@rescue_responses.has_key?(exception.cause.class.name)
-          exception.cause
-        else
-          exception
-        end
-      end
-
       def causes_for(exception)
         return enum_for(__method__, exception) unless block_given?
 
diff --git a/actionpack/lib/action_dispatch/middleware/show_exceptions.rb b/actionpack/lib/action_dispatch/middleware/show_exceptions.rb
index 3c88afd4d3..767143a368 100644
--- a/actionpack/lib/action_dispatch/middleware/show_exceptions.rb
+++ b/actionpack/lib/action_dispatch/middleware/show_exceptions.rb
@@ -45,7 +45,7 @@ module ActionDispatch
         backtrace_cleaner = request.get_header "action_dispatch.backtrace_cleaner"
         wrapper = ExceptionWrapper.new(backtrace_cleaner, exception)
         status  = wrapper.status_code
-        request.set_header "action_dispatch.exception", wrapper.exception
+        request.set_header "action_dispatch.exception", wrapper.unwrapped_exception
         request.set_header "action_dispatch.original_path", request.path_info
         request.path_info = "/#{status}"
         response = @exceptions_app.call(request.env)