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 directoryThis 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.
Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones Read my programming and virtualization blog: http://rwmj.wordpress.com virt-builder quickly builds VMs from scratch http://libguestfs.org/virt-builder.1.html
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