Coprocessors currently do not support generic interfaces because type erasure makes their generic parameters appear as Objects to Invocation.java.
This can be overcome by writing out the parameters using their own types (rather than the type parameters), and then separately writing the class names for the type parameters. While it would be ideal to implement this in Invocation.java, some other code seems to be relying on its write order and doing so breaks other RPC code. The modification can, however, be implemented in Exec.java instead.
The included patch modifies Invocation.java's fields to that they are protected scope, and fully implements the read and write methods for Exec rather than using the parent method for the parent fields. ExecResult is also modified to accommodate generic returns in the same way.
Integration test included in patch. Demonstrates generic interface using objects, arrays, and primitives, and checks that all primitive classes work as well.