The contract of readpartial is that it will return up to the amount requested and no more. However the implementation of readpartial in WEBrick's HTTPRequest does not honor this contract:

The assumption described here may hold in CRuby, but in JRuby's implementation of copy_stream we may use an intermediate buffer to hold data on its way, and if readpartial returns more data than requested we will overflow that buffer. I will be fixing JRuby to raise a more Ruby-friendly error, but it's still going to be an error if we readpartial N bytes and get N+1.

https://github.com/jruby/jruby/blob/a612f900243926e45a3d3465bc8ae5d9e5258cce/core/src/main/java/org/jruby/RubyIO.java#L4433-L4454

This is the root cause of a failure in test_big_bodies from test_httpproxy.rb as described in jruby/jruby#6246. The copy_stream call deep inside the proxy test fails due to this buffer overflow, which causes the request's piped content to be prematurely closed and the proxy test to error out while trying to write request data.