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, 0 deletions

diff --git a/activerecord/test/schema/schema.rb b/activerecord/test/schema/schema.rb
index 21b23d8e0c..477afe6feb 100644
--- a/activerecord/test/schema/schema.rb
+++ b/activerecord/test/schema/schema.rb
@@ -216,6 +216,14 @@ ActiveRecord::Schema.define do
   add_index :companies, [:firm_id, :type], name: "company_partial_index", where: "rating > 10"
   add_index :companies, :name, name: 'company_name_index', using: :btree
 
+  create_table :content, force: true do |t|
+    t.string :title
+  end
+
+  create_table :content_positions, force: true do |t|
+    t.integer :content_id
+  end
+
   create_table :vegetables, force: true do |t|
     t.string :name
     t.integer :seller_id