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, 16 insertions, 0 deletions

diff --git a/activerecord/CHANGELOG.md b/activerecord/CHANGELOG.md
index c777af342f..d0dcff801b 100644
--- a/activerecord/CHANGELOG.md
+++ b/activerecord/CHANGELOG.md
@@ -1,3 +1,19 @@
+*   Add support for bidirectional destroy dependencies.
+
+    Fixes #13609.
+
+    Example:
+
+        class Content < ActiveRecord::Base
+          has_one :position, dependent: :destroy
+        end
+
+        class Position < ActiveRecord::Base
+          belongs_to :content, dependent: :destroy
+        end
+
+    *Seb Jacobs*
+
 *   `time` columns can now affected by `time_zone_aware_attributes`. If you have
     set `config.time_zone` to a value other than `'UTC'`, they will be treated
     as in that time zone by default in Rails 5.0. If this is not the desired