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Re: [Libguestfs] [PATCH v4 07/12] v2v: nbdkit: Add the readahead filter unconditionally if it is available.



On Fri, Sep 20, 2019 at 09:33:06AM -0500, Eric Blake wrote:
> Checking for file existence for filters is somewhat less fragile than
> for plugins, because all filters are built in-tree (we've specifically
> documented that we don't provide ABI guarantees for filters, so the only
> sane way to use a filter is to compile it at the same time/version as
> the nbdkit binary that will load it).  But you do have a point that
> checking for files is still more fragile than just asking nbdkit whether
> a given filter exists:
> 
> $ nbdkit --dump-plugin --filter=nosuch null
> nbdkit: error: cannot open filter 'nosuch':
> /usr/lib64/nbdkit/filters/nbdkit-nosuch-filter.so: cannot open shared
> object file: No such file or directory

This is definitely the best we can do now, and I've posted a patch
suggesting this change for the nbdkit-probing(1) man page.

However it's not without a subtle problem: It requires the null plugin
to be present.  It's possible to ship nbdkit on its own.  On Fedora
try installing just the nbdkit-server package.  The null plugin won't
be available so this command will always fail.

So we do also need to change nbdkit to allow easier probing for
plugins or filters in the long term.

Rich.

-- 
Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
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