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

[Richard W.M. Jones]

On Thu, Mar 19, 2020 at 07:09:32AM -0500, Eric Blake wrote:
> 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.

What problem are we actually trying to solve here?  If it calls the
can_* functions in the plugin more than once it's not a big deal.

Rich.

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