After switching to Passenger 4 I started seeing some errors in my log:
Skipping compiling of passenger_native_support.so
Downloading precompiled passenger_native_support.so for the current Ruby interpreter...
(set PASSENGER_DOWNLOAD_NATIVE_SUPPORT_BINARY=0 to disable)
Continuing without passenger_native_support.so.
At the time I was debugging something else and I thought this may have been the cause of the problems I was having. I looked into it and it turns out this error is non fatal and your app will still work fine with it, but the only drawback to not dealing with it is a tiny performance decrease in benchmarks.
If you want to fix it you have to run the commands in each version of ruby installed on your system:
gem install passenger
passenger-config build-native-support
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