Re: [Libguestfs] [nbdkit PATCH] plugins: Add .can_zero callback

Originally, I thought that since the plugin always emulates .zero with a fallback to .pwrite, we didn't need to expose the backend's .can_zero to plugins. But there is another consideration, as shown at least in the nbd plugin: a plugin may implement a .zero callback that depends on support from a remote endpoint. If the remote endpoint doesn't support zeroes, then the plugin HAS to fail .zero with EOPNOTSUPP to get the automatic fallback to .pwrite; this is slightly wasteful to just telling nbdkit to not use the .zero callback when it won't work. At the same time, we still want to advertise zero support to the client, even if we won't be calling .zero, since handling NBD_CMD_WRITE_ZEROES allows for less network traffic; if it is ever truly necessary to avoid advertising to the guest, the nozero filter can accomplish that task. So, this patch exposes a .can_zero callback for plugins, which can usually be omitted, but which can be used to short- circuit calls to .zero, with the nbd plugin as the first user; while plugins.c continues to report true (well, the same result as .can_write) to the backend regardless of the plugin's answer, unless the plugin hits an error. Testing of the nbd changes (and thus of the general nbdkit handling of plugin .can_zero) is best done as part of the nozero filter: using the filter on the server side means the nbd plugin won't see a zero advertisement, and thus the client side does not call nbd's .zero, but still advertises zero to the client. Now that the set of feature callbacks is the same for plugins and filters, we can consolidate the documentation for filters and merely focus on the slight differences. This patch breaks ABI from the earlier exposure of .can_fua to plugins a few patches ago, but that is okay as there has not been a release in the meantime (the logical grouping was nicer this way). Had a release happened, .can_zero would have to be placed after .zero in struct nbdkit_plugin. Also, in plugins.c, rearrange get_error() to a more logical location. Signed-off-by: Eric Blake <eblake(a)redhat.com&gt;