Clarify intentions around method redefinitions

Don't use remove_method or remove_possible_method just before a new
definition: at best the purpose is unclear, and at worst it creates a
race condition.

Instead, prefer redefine_method when practical, and
silence_redefinition_of_method otherwise.

2 files changed, 43 insertions, 23 deletions

diff --git a/activesupport/lib/active_support/core_ext/module/redefine_method.rb b/activesupport/lib/active_support/core_ext/module/redefine_method.rb
new file mode 100644
index 0000000000..5bd8e6e973
--- /dev/null
+++ b/activesupport/lib/active_support/core_ext/module/redefine_method.rb
@@ -0,0 +1,40 @@
+# frozen_string_literal: true
+
+class Module
+  # Marks the named method as intended to be redefined, if it exists.
+  # Suppresses the Ruby method redefinition warning. Prefer
+  # #redefine_method where possible.
+  def silence_redefinition_of_method(method)
+    if method_defined?(method) || private_method_defined?(method)
+      # This suppresses the "method redefined" warning; the self-alias
+      # looks odd, but means we don't need to generate a unique name
+      alias_method method, method
+    end
+  end
+
+  # Replaces the existing method definition, if there is one, with the passed
+  # block as its body.
+  def redefine_method(method, &block)
+    visibility = method_visibility(method)
+    silence_redefinition_of_method(method)
+    define_method(method, &block)
+    send(visibility, method)
+  end
+
+  # Replaces the existing singleton method definition, if there is one, with
+  # the passed block as its body.
+  def redefine_singleton_method(method, &block)
+    singleton_class.redefine_method(method, &block)
+  end
+
+  def method_visibility(method) # :nodoc:
+    case
+    when private_method_defined?(method)
+      :private
+    when protected_method_defined?(method)
+      :protected
+    else
+      :public
+    end
+  end
+end
diff --git a/activesupport/lib/active_support/core_ext/module/remove_method.rb b/activesupport/lib/active_support/core_ext/module/remove_method.rb
index bf0d686e16..704f144bdd 100644
--- a/activesupport/lib/active_support/core_ext/module/remove_method.rb
+++ b/activesupport/lib/active_support/core_ext/module/remove_method.rb
@@ -1,5 +1,7 @@
 # frozen_string_literal: true
 
+require_relative "redefine_method"
+
 class Module
   # Removes the named method, if it exists.
   def remove_possible_method(method)
@@ -10,28 +12,6 @@ class Module
 
   # Removes the named singleton method, if it exists.
   def remove_possible_singleton_method(method)
-    singleton_class.instance_eval do
-      remove_possible_method(method)
-    end
-  end
-
-  # Replaces the existing method definition, if there is one, with the passed
-  # block as its body.
-  def redefine_method(method, &block)
-    visibility = method_visibility(method)
-    remove_possible_method(method)
-    define_method(method, &block)
-    send(visibility, method)
-  end
-
-  def method_visibility(method) # :nodoc:
-    case
-    when private_method_defined?(method)
-      :private
-    when protected_method_defined?(method)
-      :protected
-    else
-      :public
-    end
+    singleton_class.remove_possible_method(method)
   end
 end