Add support for bidirectional destroy dependencies

Prior to this commit if you defined a bidirectional relationship
between two models with destroy dependencies on both sides, a call to
`destroy` would result in an infinite callback loop.

Take the following relationship.

    class Content < ActiveRecord::Base
      has_one :content_position, dependent: :destroy
    end

    class ContentPosition < ActiveRecord::Base
      belongs_to :content, dependent: :destroy
    end

Calling `Content#destroy` or `ContentPosition#destroy` would result in
an infinite callback loop.

This commit changes the behaviour of `ActiveRecord::Callbacks#destroy`
so that it guards against subsequent callbacks.

Thanks to @zetter for demonstrating the issue with failing tests[1].

[1] rails#13609

1 files changed, 8 insertions, 1 deletions

diff --git a/activerecord/lib/active_record/callbacks.rb b/activerecord/lib/active_record/callbacks.rb
index f44e5af5de..07769e8952 100644
--- a/activerecord/lib/active_record/callbacks.rb
+++ b/activerecord/lib/active_record/callbacks.rb
@@ -289,7 +289,14 @@ module ActiveRecord
     end
 
     def destroy #:nodoc:
-      _run_destroy_callbacks { super }
+      begin
+        @_destroy_callback_already_called ||= false
+        return if @_destroy_callback_already_called
+        @_destroy_callback_already_called = true
+        _run_destroy_callbacks { super }
+      ensure
+        @_destroy_callback_already_called = false
+      end
     end
 
     def touch(*) #:nodoc: