[Libguestfs] [nbdkit PATCH 4/5] api: Add .cleanup callback

[Richard W.M. Jones]

On Thu, May 06, 2021 at 08:59:39PM -0500, Eric Blake wrote:
> The existing .unload callback is currently called inner-to-outer,
> which is not suitable for use in late cleanups where a filter wants to
> make one final call into the underlying plugin.  We were careful to
> state that unload does not have a specific ordering, but switching
> that ordering may not be helpful.  Late cleanup may also want one
> final chance to open a context into the plugin, which implies one
> final access to the backend pointer from filters (in contrast to
> .unload taking no parameters).
> 
> So, I found it easier to introduce a new .cleanup callback,
> specifically stated to be the counterpart of .after_fork.  This leaves
> .load/.unload as a pair for handling things at the dlopen barrier
> (affecting only the local filter or plugin, but no interaction with
> other modules), and .after_fork/.cleanup for handling things related
> to the first and last points at which it is safe to interact with the
> underlying plugin.  The new callback has no return value, as by the
> time we are cleaning up, any failures are not going to change the fact
> we are shutting down anyway.
> ---
>  docs/nbdkit-filter.pod     | 22 +++++++++++++++++++---
>  docs/nbdkit-plugin.pod     | 33 ++++++++++++++++++++++++++++++---
>  include/nbdkit-filter.h    |  1 +
>  include/nbdkit-plugin.h    |  4 +++-
>  server/internal.h          |  2 ++
>  server/filters.c           | 13 +++++++++++++
>  server/main.c              |  1 +
>  server/plugins.c           | 13 +++++++++++++
>  tests/test-layers-filter.c |  8 ++++++++
>  tests/test-layers-plugin.c |  9 ++++++++-
>  tests/test-layers.c        | 12 ++++++++++++
>  11 files changed, 110 insertions(+), 8 deletions(-)

Yes this one is fine - it could be pushed immediately.

The difference between plugin.cleanup and plugin.unload
is quite subtle :-)

Rich.

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