[Libguestfs] [nbdkit] About the Rust bindings

[Richard W.M. Jones]

[To continue the conversation from
https://github.com/libguestfs/nbdkit/issues/5]

> Currently there's no way for the Rust plugin to report an error. The
> idiomatic way to do it would be for each callback to return a Result
> object, much like how the Go plugin currently does it.

I'm sure it's not idiomatic for Rust, but I just wanted to say that
there is a way to return an error: return -1.  But a bigger issue
seems to be how to pass back the correct errno (assuming that Rust
even collects this from low-level syscalls?).

For historical reasons nbdkit plugins written in C set errno on error
and nbdkit server reads the errno and may translate it into NBD errors
on the wire (so it's generally important to get it right).

However your plugin may set .errno_is_preserved = 0, then you can call
nbdkit_set_error() whenever convenient before returning an error to
set the errno that nbdkit will use.  In OCaml plugins we have to go
through some hoops to convert OCaml errno (which use a different
numbering) to system errno:

https://github.com/libguestfs/nbdkit/blob/904c4e5fdca557d6d25a60e6fb12fa058263481e/plugins/ocaml/NBDKit.ml#L250

The other issue (from golang bindings) was threads.  Does Rust use
native system pthreads?  If it does then there shouldn't be an issue.
golang implements its own green threads and that was a problem.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
Read my programming and virtualization blog: http://rwmj.wordpress.com
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