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

[Richard W.M. Jones]

On Thu, Mar 08, 2018 at 05:03:11PM -0600, Eric Blake wrote:
> 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

I do wonder if this patch is really worthwhile.  Old nbdkit / new
plugin is probably going to go untested in general, and therefore will
just cause problems/complexity.

Rich.

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