I've found some buggy behavior when caching & uncaching an array of
ActiveRecord objects with ActiveStorage attachments. Here's a truncated
code snippet demonstrating the bug:
class User < ActiveRecord::Base
has_one_attached :avatar
end
users = User.first 5
users.each { |user| user.avatar }
Rails.cache.write "key", users
uncached_users = Rails.cache.read "key"
users[1]
# => :@attachment_changes
users[2]
# => {}
In the above code, every element of the array users is an instance of User.
After writing to and reading from the cache, every element of uncached_users
should
still be an instance of User. However, after reading from the cache, the
second and third elements of uncached_users are a symbol and empty hash.
Here's a full executable test case:
https://gist.github.com/alipman88/12928dfd2f86afacd1af51aeb8ae5194
After some digging, I've determined the underlying cause is a bug in Ruby
itself, allowing instance variables to be added or removed during
marshallization. (Many of Rails' cache stores use Marshal.load &
Marshal.dump to serialize & deserialize objects.) While I'll skip over the
specifics, the bug will be corrected in future implementations of Ruby.
Here's the ticket on Ruby's issue tracker, for anyone seeking more detail:
https://bugs.ruby-lang.org/issues/15968
Although future versions of Ruby will fix the underlying issue by raising
a RuntimeError, this won't completely solve the problem: Rails developers
may still encounter a somewhat confounding error without an immediately
obvious cause.
I've submitted a pull request that fixes this issue by patching the
delegate_missing_to extension through which this behavior arises:
https://github.com/rails/rails/pull/36623
It's been a couple weeks since I submitted my pull request. Per advice in
the Contributing to Ruby on Rails guide, I'm hoping to nudge things along
and attract some code reviewers by posting here. I respect that Rails is a
volunteer project, and appreciate any feedback when folks are able to
provide it.
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