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

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(-)