Only raise DelegationError if it's is the source of the exception

This fixes situations where nested NoMethodError exceptions are masked
by delegations. This would cause confusion especially where there was a
problem in the Rails booting process because of a delegation in the
routes reloading code.

Fixes rails#10559

Author: pixeltrix

activesupport/CHANGELOG.md

@@ -1,3 +1,9 @@
+*   Only raise `Module::DelegationError` if it's the source of the exception.
+
+    Fixes #10559
+
+    *Andrew White*
+
 *   Make `Time.at_with_coercion` retain the second fraction and return local time.
 
     Fixes #11350

activesupport/lib/active_support/core_ext/module/delegation.rb

@@ -183,16 +183,17 @@ def #{method_prefix}#{method}(#{definition})        # def customer_name(*args, &
         exception = %(raise DelegationError, "#{self}##{method_prefix}#{method} delegated to #{to}.#{method}, but #{to} is nil: \#{self.inspect}")
 
         module_eval(<<-EOS, file, line - 2)
-          def #{method_prefix}#{method}(#{definition})        # def customer_name(*args, &block)
-            _ = #{to}                                         #   _ = client
-            _.#{method}(#{definition})                        #   _.name(*args, &block)
-          rescue NoMethodError                                # rescue NoMethodError
-            if _.nil?                                         #   if _.nil?
-              #{exception}                                    #     # add helpful message to the exception
-            else                                              #   else
-              raise                                           #     raise
-            end                                               #   end
-          end                                                 # end
+          def #{method_prefix}#{method}(#{definition})                                          # def customer_name(*args, &block)
+            _ = #{to}                                                                           #   _ = client
+            _.#{method}(#{definition})                                                          #   _.name(*args, &block)
+          rescue NoMethodError => e                                                             # rescue NoMethodError => e
+            location = "%s:%d:in `%s'" % [__FILE__, __LINE__ - 2, '#{method_prefix}#{method}']  #   location = "%s:%d:in `%s'" % [__FILE__, __LINE__ - 2, 'customer_name']
+            if _.nil? && e.backtrace.first == location                                          #   if _.nil? && e.backtrace.first == location
+              #{exception}                                                                      #     # add helpful message to the exception
+            else                                                                                #   else
+              raise                                                                             #     raise
+            end                                                                                 #   end
+          end                                                                                   # end
         EOS
       end
     end

activesupport/test/core_ext/module_test.rb

@@ -66,6 +66,23 @@ def self.table_name
   delegate :name, :to => :client, :prefix => false
 end
 
+Product = Struct.new(:name) do
+  delegate :name, :to => :manufacturer, :prefix => true
+  delegate :name, :to => :type, :prefix => true
+
+  def manufacturer
+    @manufacturer ||= begin
+      nil.unknown_method
+    end
+  end
+
+  def type
+    @type ||= begin
+      nil.type_name
+    end
+  end
+end
+
 class ParameterSet
   delegate :[], :[]=, :to => :@params
 
@@ -264,6 +281,16 @@ def test_delegation_invokes_the_target_exactly_once
     assert_equal [3], se.ints
   end
 
+  def test_delegation_doesnt_mask_nested_no_method_error_on_nil_receiver
+    product = Product.new('Widget')
+
+    # Nested NoMethodError is a different name from the delegation
+    assert_raise(NoMethodError) { product.manufacturer_name }
+
+    # Nested NoMethodError is the same name as the delegation
+    assert_raise(NoMethodError) { product.type_name }
+  end
+
   def test_parent
     assert_equal Yz::Zy, Yz::Zy::Cd.parent
     assert_equal Yz, Yz::Zy.parent