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

2 files changed, 6 insertions, 0 deletions

diff --git a/activerecord/test/fixtures/content.yml b/activerecord/test/fixtures/content.yml
new file mode 100644
index 0000000000..0d12ee03dc
--- /dev/null
+++ b/activerecord/test/fixtures/content.yml
@@ -0,0 +1,3 @@
+content:
+  id: 1
+  title: How to use Rails
diff --git a/activerecord/test/fixtures/content_positions.yml b/activerecord/test/fixtures/content_positions.yml
new file mode 100644
index 0000000000..9e85773f8e
--- /dev/null
+++ b/activerecord/test/fixtures/content_positions.yml
@@ -0,0 +1,3 @@
+content_positions:
+  id: 1
+  content_id: 1