In the uWSGI project we faced the same issue described here:
basically the &block, if mapped to a c function, is never called/used.
We bypassed the problem with monkeypatching:
simply defining the 'each' method
Note: in the clogger project, they stated 1.9.3 is not affected by that behaviour but we got confirmation it is still present.
So, just adding each will fix that?
Rack::BodyProxy#each, fixes rack/rack#434