[Libguestfs] [PATCH 9/9] filters: Move rdelay/wdelay from file plugin to new delay filter.

[Richard W.M. Jones]

On Wed, Jan 17, 2018 at 08:01:35PM -0600, Eric Blake wrote:
> Actually, thinking about this more:
> 
> When I added zero support, I documented in commit ac3f294a that for the
> file plugin, wdelay is indeed doubled on systems lacking efficient zero
> support.  But the fallback for handling EOPNOTSUPP is currently done at
> the plugins.c level (ie. it is part of next->zero()) - so a filter
> should never see a plugin returning EOPNOTSUPP.  Or put another way,
> your movement of wdelay from being part of the file plugin to now being
> a separate filter actually FIXES the double delay.  On the other hand, a
> filter can itself return EOPNOTSUPP (rather than calling next->zero), in
> which case we still need to support the fallback to .pwrite  higher up
> in the call stack

OK, so ignore my previous email.

However, I'm wary of doing fallbacks in general in the filter layer,
for a couple of reasons:

(1) If you run some filters twice (eg. offset filter) then you end up
doubling offsets which will cause data corruption.  This doesn't apply
in the EOPNOTSUPP case, but it does apply in the .zero fallback case
(where it falls back to .write).

(2) Filters are a new thing and we can just declare that they cannot
use EOPNOTSUPP.

> Along the same lines, with my FUA patches, if we keep the fallback of
> calling .flush in connections.c, then the flush will pass through the
> entire filter chain; but I had proposed moving the fallback flush into
> plugins.c, at which point the fallback flush is done at the layer that
> lacks FUA support.  And maybe we want a flush delay to mirror the
> existing rdelay and wdelay (simulating a long flush time can indeed be
> useful in tests); where it definitely matters whether the flush delay is
> triggered as part of FUA fallback, or only triggered on an actual
> NBD_CMD_FLUSH from the client.
> 
> I'm also thinking ahead to expansions - for example, there are proposals
> to add resize and block status commands to NBD.  In your implementation,
> if a filter does not define .can_flush or .flush, then its .can_flush
> implementation merely returns whatever the underlying backend's version
> would report.  But as we add new callbacks, such as a new .can_resize,
> we need to make sure we have sane defaults.  For example, the offset
> filter should probably not allow resizes (even if the underlying plugin
> does).  If the offset filter compiled in a version of nbdkit that lacks
> the .can_resize hook is run by nbdkit that has added the hook, we
> therefore want to default .can_resize to false as part of loading the
> smaller filter struct into memory (and NOT default to falling through to
> the plugin's .can_resize); a default based on fall-through should only
> happen if the filter had a size large enough to admit that the user
> explicitly omitted the callback in the filter, rather than it being NULL
> because of version mismatch.

We may have to say that the ABI guarantee only applies to plugins,
which might not be a bad thing since filters are much harder to write
than plugins and are going to be quite closely tied to both the
version and precise implementation of nbdkit.

Rich.

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