Fix performance regression with empty template rendering

[pixeltrix]

Closes #1537.

The change in 80637fc introduced a performance regression because the template lookup cache was being lost every time an example was run.

This commit fixes the problem by caching the EmptyTemplateResolver instances between example runs in a hash indexed by the path string.

[JonRowe]

Can we add a benchmark for this? Before / After? That's what we'd normally do for this sort of thing... :)

[pixeltrix]

@JonRowe commit a benchmark test to the repo?

[JonRowe]

I meant to ./benchmarks but that might actually be a bit tricky, I'd be happy with a note referencing why this was necessary.

[pixeltrix]

NOTE: all tests carried out using the sample application in #1537

e52281c (v3.4.0)

$ rspec spec/controllers/badgers_controller_spec.rb
........................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................

Finished in 2.07 seconds (files took 2.98 seconds to load)
1000 examples, 0 failures

cf97d15 (v3.4.1)

$ rspec spec/controllers/badgers_controller_spec.rb
........................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................

Finished in 6.09 seconds (files took 4.25 seconds to load)
1000 examples, 0 failures

As you can see the tests take three times as long to run which is due to the extra file stats carried out whilst searching for the template. In 3.4.0 that template search is cached whereas 3.4.1 it isn't because we're creating a new resolver instance for every example run.

Caching the resolver instances gives the following improvement:

pixeltrix/rspec-rails@52522a9

$ rspec spec/controllers/badgers_controller_spec.rb
........................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................................

Finished in 1.77 seconds (files took 2.97 seconds to load)
1000 examples, 0 failures

This actually now gives a slightly improved performance over 3.4.0.

[JonRowe]

Thanks :)

[yujinakayama]

Will we cut a release for this?

[JonRowe]

Sure, why not, done.

[yujinakayama]

👍