On every request, Rails checks to see whether any ruby code has been changed on the filesystem. The default FileUpdateChecker does this by iterating over every file on the autoload_paths and comparing its modified-time.
In Discourse, our autoload path of `/app` includes the `/app/assets` directory, and therefore thousands of non-ruby files (e.g. node_modules). This makes the `Dir["/app"]` call very slow (>100ms in my case). On my machine, every Rails-handled request spends around 150-200ms in the FileUpdateChecker. This commit introduces a couple of changes to completely eliminate this wasted time:
- The `/app/assets` directory is excluded from the file watchers. For me, this cut the time spent in the file_watcher to around 50-100ms
- Switches our development config to use the `EventedFileUpdateChecker`, which makes use of the `listen` gem to subscribe to os-specific notifications of changes. This completely removes the `FileUpdateChecker` from the critical path
On my machine, topic_list requests now return in around 80ms (previously >200ms). Live code reload still works as it did before
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