[Libguestfs] [PATCH nbdkit 0/7] server: Implement NBD_FLAG_CAN_MULTI_CONN.

First thing to say is that I need to do a *lot* more testing on this, so this is just an early peek. In particular, although it passed ‘make check && make check-valgrind’ I have *not* tested it against a multi-conn-aware client such as the Linux kernel >= 4.9. This implements NBD_FLAG_CAN_MULTI_CONN, described in the protocol doc as: "NBD_FLAG_CAN_MULTI_CONN: Indicates that the server operates entirely without cache, or that the cache it uses is shared among all connections to the given device. In particular, if this flag is present, then the effects of NBD_CMD_FLUSH and NBD_CMD_FLAG_FUA MUST be visible across all connections when the server sends its reply to that command to the client. In the absence of this flag, clients SHOULD NOT multiplex their commands over more than one connection to the export." This is necessary to support the Linux nbd client -C option. The only plugin which sets the flag so far is file. The ocaml, sh and nbd plugins allow the flag to be controlled or passed through. Notable also is that the blocksize filter has to filter out this flag, because I'm not convinced that the bounce buffer is safe. However I believe the other filters *are* safe (although not really certain about the fua filter, and the new cache filter is tricky too). My feeling is that we should set the flag unconditionally for all readonly connections, but I can think of nasty corner cases where it might not work. We should most probably set it in all plugins that are readonly (eg. nbdkit-pattern-plugin). Rich.