Cherry-picking Specific Active Support Behavior

The Active Support library has always (or, close enough to always) allowed us to cherry-pick specific extensions/behaviors, to load only strictly needed dependencies. That is, rather than loading the entirety of the library, we can load just the bits and pieces we need. This helps keep the amount of things loaded in memory smaller, and faster by doing less work. It can also aide in understanding by making more explicit the dependencies some code relies on.

I’ve been using this technique for years, and it’s been solid. Until today, while working on a Rails 7 code base, when I started seeing an error:

uninitialized constant ActiveSupport::IsolatedExecutionState (NameError)

As it happens, what I’d been doing for years worked, but more by accident than design. Active Support 7 has fixed the glitch.

How it used to “work”

Let’s say we want to use Active Support’s Time extensions for Numeric types. For example, that’s what that allows us to express some past date/time like this:

5.days.ago #=> 2022-09-30 09:08:47.983955 -0400

We could cherry-pick in just those extensions, right in the file where we’re using them, like so:

require "active_support/core_ext/numeric/time"

5.days.ago #=> 2022-09-30 09:08:47.983955 -0400

But as of Active Support 7, that doesn’t quite work. It raises a NameError because the ActiveSupport::IsolatedExecutionState constant cannot be found.

require "active_support/core_ext/numeric/time"

5.days.ago #=>
# NameError: uninitialized constant ActiveSupport::IsolatedExecutionState
#       ::ActiveSupport::IsolatedExecutionState[:time_zone] || zone_default
#                      ^^^^^^^^^^^^^^^^^^^^^^^^
# from .../ruby/gems/3.1.0/gems/activesupport-7.0.4/lib/active_support/core_ext/time/zones.rb:15:in `zone'

What changed?

Active Support 7 added a new abstraction to better handle different concurrency mechanisms (think, Threads vs Fibers). In doing so, everywhere in Active Support that used to directly reference Thread.current, was changed to leverage ::ActiveSupport::IsolatedExecutionState.

That means attempting to directly cherry-pick a bit of Active Support, as had been possible for years, we might also need the IsolatedExecutionState constant loaded. So, the direct cherry-picking feature is… broken?

Not exactly. It turns out, Active Support never intended us to directly cherry-pick by only requiring the one file we need. The intent, and guides, have long told us to first load the library itself, and then cherry-pick any specific bits we want. That is, we should have been doing this all along:

require "active_support"
require "active_support/core_ext/numeric/time"

5.days.ago #=> 2022-09-30 09:08:47.983955 -0400

So really, nothing changed. Before, things just kinda worked by accident.

Does that mean all of Active Support is loaded?

I’d always assumed that requiring the library would also load the whole library. Though, I should have known better. mea culpa.

Instead, requiring the library via require "active_support" only loads the bare minimum, while setting up all of the Ruby autoload-ing needed to reference other bits of the library later. Huzzah!

If we do want or need to eagerly load the whole library, that’s done like so:

require "active_support/all"

Phew, glad to have gotten to the bottom of that one! 🎉

And a shout-out to @gektin for helping me dive into this one.