Re: [Libguestfs] [nbdkit PATCH v3 15/15] RFC: plugins: Add back-compat for new plugin with old nbdkit

If we bump NBDKIT_API_VERSION, we have forcefully made older nbdkit reject all plugins that opt to the newer API: $ nbdkit ./plugins/nbd/.libs/nbdkit-nbd-plugin.so --dump-plugin nbdkit: ./plugins/nbd/.libs/nbdkit-nbd-plugin.so: plugin is incompatible with this version of nbdkit (_api_version = 2) But with a bit of finesse, we can make opting in to FUA support orthogonal to NBDKIT_API_VERSION, by introducing a new witness level that the user controls, and by providing sane fallbacks so that newer plugins correctly populate the fields expected by older nbdkit. --- v3: rework entirely, add new variable instead of NBDKIT_API_VERSION This patch is still RFC; if you like it, it's probably better to rebase portions of this patch into the rest of the series, rather than churning NBDKIT_API_VERSION temporarily to 2 and now back to 1