Re: [Libguestfs] [nbdkit PATCH 1/2] sh, eval: Cache .can_zero and .can_flush

Thinking about this more, maybe the real problem is that all language bindings that have to wrap scripts (OCaml and Rust are exceptions as they directly call into the C code; but lua, perl, python, ruby, tcl, and sh all fit into the category I'm describing) MUST define .can_FOO at the C level because they don't know a priori whether the script they will be loading will also provide a .can_FOO callback, so we have a lot of duplicated code in each language binding. Life might be easier if we change the C contract for the .can_FOO callbacks to support the special return value of -2 to behave the same as if .can_FOO were missing. In a way, it would make it easier for the remaining languages to behave more like sh's special MISSING return value (sh is an oddball because we can't even probe whether .FOO is missing so .can_FOO is mandatory in the script; but in eval, python, etc, it's easy to implement a C binding for .can_FOO that introspects the script). I'll spin up a v2 patch along those lines, so we can compare the difference.