I think protobuf and CFFI structure their operations in a very similar way - process-op is analogous to proto-to-lisp, it takes a "specification" file and generates a lisp file and some other files. protobuf generates lisp(fasl) files only, so it does not need to do anything special to support bundle operations. CFFI's process-op generates some .o and .so files that a bundle operation may need. The current implementation adds .o and .so files to outputs of compile-op and it causes the problem I described.

I do not know what methods need to be defined on process-op to make bundle operations to pick up its output files. From my very limited understanding of ASDF I do not think there is a way to do it. Method "component-depends-on ((o gather-operation) (s system))" determines input-files of a bundle-op. The method returns dependencies of one operation only (e.g. compile-op), but in case of CFFI's wrapper-file we need output files of two operations: process-op and compile-op.