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

On Sat, Sep 21, 2019 at 03:54:11PM +0100, Richard W.M. Jones wrote:
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.

Is it usable without any plugins?  Does the null plugin take much space?  I
wouldn't think so.  Would it be too messy to just ship the null plugin
unconditionally, even if just for this particular purpose?

On the other hand any program that relies on such probing to work might depend
not only on nbdkit, but also on the null plugin.


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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