[Libguestfs] [nbdkit PATCH] Update filters to support FUA flags.

[Richard W.M. Jones]

On Fri, Jan 19, 2018 at 10:45:51AM -0600, Eric Blake wrote:
> I'm wondering if we're missing documentation here (and/or in the plugins
> document) that if .can_write returns true, the plugin must supply a
> .pwrite; likewise for .can_trim implying a .trim, and .can_flush
> implying a .flush.

It's a strange one.

For example if a plugin returns .can_pwrite == 1 but doesn't implement
.pwrite then the NBD connection will appear to be writable, but any
attempt to write will return EROFS.

On the other hand (a) plugins don't usually implement can_write
because the "autosensing" usually works, and (b) it doesn't crash or
do anything particularly bad.

can_trim/trim and can_flush/flush are similar.

However unfortunately src/plugins.c now assert-fails if the FUA flag
is present but plugin.flush does not exist.  Is this a bug?

Rich.

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