rails: ActionDispatch::Journey::Router NoMethodError (undefined method `names' for nil:NilClass)

When running a multithreaded rails server (JRuby + Puma), occasionally when the rails app starts and several incoming requests hit different routes at the same time, a route might become corrupted and throw this error.

Any subsequent requests to the corrupted route to fail with this error for the lifetime of the application. Only a restart of the application will fix this once it has been corrupted.

Root Cause

• The backtrace points to here which is because match_data is nil
• match_data is nil because the route.path.match returns nil
• route.path.match returns nil because the routes @re instance variable is set incorrectly
• @re is lazily evaluated, and in a multi-threaded application this is not safe

Replication

I created an app that replicates this issue here. I apologize for the difficult replication, but since this is a threading related bug it is naturally difficult. I spent several hours trying to get a simple-ish replication, but if we need a better one I can spend some more time on it.

Potential Fix

Here is a potential fix for this issue. I am happy to send a pull request if you want.

Backtrace Attached

rails_backtrace.txt

Thanks for looking at this issue 😃