Re: [Libguestfs] [nbdkit PATCH v2] main: Add option to disable SR advertisement

On 8/20/19 2:28 PM, Eric Blake wrote: > When we added support for .extents, we had nbdkit unconditionally > support structured replies if the client requests them, and the > plugin's .can_extents has no impact on what the server advertises. > However, while the plugin API doesn't care whether the client > requested SR, there are still integration situations where not > advertising SR can be useful (such as comparison on what a client does > with no block status vs. a block status that always reports > allocated). We already have the command line options -o/-n for > tweaking core server functionality; add --no-sr to the mix. > > In particular, doing this found that 'qemu-nbd --list' from qemu 4.2 > is rather picky: it hangs up on a server that replies with > NBD_REP_ERR_POLICY, rather than silently proceeding without SR support > (at least libnbd is more tolerant). > > Signed-off-by: Eric Blake <eblake(a)redhat.com&gt; > --- > > Looks much different as a command line option instead of a hack to > the noextents filter, but I like the result a lot better. > > I'm open to bike-shed opinions on the option name, or even if we want > to emulate a tri-state option --protocol=[old|no-sr|new] (with -o and > -n remaining synonyms for 2 of the 3 options for back-compat). I > purposefully did not burn a short-option letter on the new option, as > it is unlikely to be commonly needed. > > docs/nbdkit-plugin.pod | 5 +++- > docs/nbdkit-protocol.pod | 21 ++++++++++++---- > docs/nbdkit.pod | 12 ++++++++-- > docs/synopsis.txt | 2 +- > server/internal.h | 1 + > server/options.h | 4 +++- > server/main.c | 5 ++++ > server/protocol-handshake-newstyle.c | 14 +++++++++++ > tests/test-eflags.sh | 36 ++++++++++++++++++++++++++++ > 9 files changed, 91 insertions(+), 9 deletions(-) Oh, and I missed squashing this in: diff --git i/filters/noextents/nbdkit-noextents-filter.pod w/filters/noextents/nbdkit-noextents-filter.pod index 47223928..9cc586ff 100644 --- i/filters/noextents/nbdkit-noextents-filter.pod +++ w/filters/noextents/nbdkit-noextents-filter.pod @@ -11,7 +11,11 @@ nbdkit-noextents-filter - disable extents in the underlying plugin “Extents” are a feature of the NBD protocol / nbdkit which allow the client to detect sparse regions of the underlying disk. C<nbdkit-noextents-filter> disables this so that the plugin appears to -be fully allocated. +be fully allocated, at least to a client that requests structured +replies. It is also possible to use the I<--no-sr> option to nbdkit +to prevent the client from using structured replies, which among its +other side effects will prevent the client from querying extents at +all.