Re: [Libguestfs] [libnbd PATCH 0/3] Improve type-safety of ocaml/golang getters

On Sat, Sep 05, 2020 at 08:40:57PM -0500, Eric Blake wrote: > Natural fallout after my recent testsuite additions that fixed a > couple of ocaml bugs in the setters. However, on at least the OCaml > code, I'm not sure what we should do if a newer libnbd ever returns a > bit that an older NBD.mli was not expecting at the time the OCaml > compiler ran (see below). I'm also not sure if there is a more > efficient way to avoid outputting Val_FOO() converters for types not > referenced in REnum/RFlags than my hack of __attribute__((unused)).

> Maybe we want > the generator to produce a full list of 31 variants per 'flags' type, > using placeholders for all bits not currently in use, to make it > easier to receive an unknown bit from the getter and turn around to > re-supply it to the setter? In such a setup, libnbd would still be > rejecting input of out-of-range bits in relation to what libnbd knew > at its compilation time. While placeholders could be used, a more natural way is to extend the flags type (in OCaml) to: type t = | FIXED_NEWSTYLE | NO_ZEROES | UNKNOWN_FLAG of int where the unknown (at the time of compilation) flags would be encoded as UNKNOWN_FLAG + the bit index of the unknown flag. eg: [ FIXED_NEWSTYLE; UNKNOWN_FLAG 2 ] => 1 | (1<<2) (1<<3) | 2 => [ UNKNOWN_FLAG 3; NO_ZEROES ] Even though we distribute the OCaml bindings with libnbd it's conceivable this could be useful because OCaml bindings are generally statically linked into the final binary so you could get a newer libnbd.so.0 / old binary with old OCaml bindings scenario.