Reduce calls to get path info when finding routes

[jasonhl]

Summary

👋 I work for Shopify. Recently, I was doing some profiling of production requests in order to get a better understanding of where time goes for a subset of cache hits to our main application. These typically take 40-50ms. In particular, I was interested in what happens in our middleware stack and all the things we do on every request prior to doing the actual "work".

That led me to this (from a profile viewed in Speedscope):

The calls to Rack::Request::Helpers#path_info were somewhat surprising. Taking a look at the code, that same function is called repeatedly for no good reason. So, I changed the code to simply keep it in a local variable and reuse the value.

I didn't write a benchmark because this change is pretty simple and straightforward. I also suspect it should have basically no performance effect on applications which have a small number of routes (it will save some allocations, however). I suspect this is noticeable at Shopify scale because of the sheer number of routes that need to be iterated over.

[kaspth]

I’d still like to see a benchmark to know the impact, but I’ll leave that call to @rafaelfranca

[rafaelfranca]

Speedscope shows it will shave almost 2ms. That is 5% of the request time for those actions and this code not only is faster but it make more sense to me.

[kaspth]

Perfect! 🙌