Upstream NBD now documents[1] an extension that supports 64-bit effect
lengths in requests. As part of that extension, the size of the reply
headers will change in order to permit a 64-bit length in the reply
for symmetry[2]. Additionally, where the reply header is currently 16
bytes for simple reply, and 20 bytes for structured reply; with the
extension enabled, there will only be one extended reply header, of 32
bytes, with both structured and extended modes sending identical
payloads for chunked replies.
Since we are already wired up to use iovecs, it is easiest to allow
for this change in header size by splitting each structured reply
across multiple iovecs, one for the header (which will become wider in
a future patch according to client negotiation), and the other(s) for
the chunk payload, and removing the header from the payload struct
definitions. Rename the affected functions with s/structured/chunk/
to make it obvious that the code will be reused in extended mode.
Interestingly, the client side code never utilized the packed types,
so only the server code needs to be updated.
[1]https://github.com/NetworkBlockDevice/nbd/blob/extension-ext-header/doc/proto.md
as of NBD commit e6f3b94a934
[2] Note that on the surface, this is because some future server might
permit a 4G+ NBD_CMD_READ and need to reply with that much data in one
transaction. But even though the extended reply length is widened to
64 bits, for now the NBD spec is clear that servers will not reply
with more than a maximum payload bounded by the 32-bit
NBD_INFO_BLOCK_SIZE field; allowing a client and server to mutually
agree to transactions larger than 4G would require yet another
extension.
Signed-off-by: Eric Blake<eblake@redhat.com>