[Libguestfs] [nbdkit PATCH] RFC: noextents: Add extentmode=data config parameter

[Richard W.M. Jones]

On Tue, Feb 08, 2022 at 02:29:48PM -0600, Eric Blake wrote:
> On Tue, Feb 08, 2022 at 12:39:02PM +0100, Laszlo Ersek wrote:
> > (The same question applies to "plugin-args" in the synopsys, more or
> > less...)
> 
> Hmm, we aren't always consistent, but I agree that it can be pruned
> without loss.

Yes, there's quite a lot of inconsistency across man pages.  I've
tried to make the use of =item, B<>, C<>, I<>, F<> consistent as far
as possible, but there are many other things to fix.  One day ...

> The code in server/ guarantees that we cannot call into a filter or
> plugin with an extents request that would read out of bounds; ie. on
> input, offset+count will never exceed what next->get_size() would tell
> us anyways.  Conversely, nbdkit_add_extent() already permits us to
> pass in redundant information prior to offset (as long as we make
> forward progress by eventually using nbdkit_add_extent for at least
> one byte at offset before returning), as well as to provide more
> information than needed (the upper layer can set a clamp, such as when
> FLAG_REQ_ONE is in use by the client, or at the 32-bit boundary
> condition, where our additions beyond that clamp are merely ignored).
> So we could just as easily write
> 
>   return nbdkit_add_extent (ret_extents, 0, INT64_MAX, 0);

Just to add to this point:

A key aim is to make nbdkit _plugins_ as easy to write as possible.
So plugins always receive bounds checked parameters, and the APIs that
plugins call are supposed to be easy to use / have no sharp edges.
Also of course the plugin API and ABI is stable.

This doesn't apply so much to filters.  There's no stable API/ABI for
filters, they're all in-tree, and it's quite complicated to write them
correctly.

However in this case the API (nbdkit_add_extent) is used by both
plugins and filters, so it is meant to be easy to use.

Rich.

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