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

[Richard W.M. Jones]

On Thu, Mar 22, 2018 at 05:35:17PM -0500, Eric Blake wrote:
> 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 at redhat.com>

Yup, looks good, ACK.

Rich.

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