Re: [Libguestfs] [libnbd PATCH 1/2] api: Add nbd_set_strict_mode

Right now, libnbd has refused to issue a command not advertised as supported by a server, including unknown flags, mainly because the NBD protocol does not guarantee what the server will do, and libnbd would rather stay in sync with the server than drop the connection. However, for integration purposes, it can be handy to coerce libnbd into sending something to see how the server will react (whether it be an extension libnbd has not yet learned, or an intentional bad request to test server error handling). Time to make this something the user can control, by adding a new strictness mode. A later patch will add another knob to the mode. --- Question: Should we split this into two knobs? Sending unknown flags is dangerous (in the future, a flag might be defined to alter the on-wire layout expected by either the client or server, and get things out-of-sync or even cause data corruption), while sending known commands contrary to advertised flags is less risky (so far, NBD_CMD_WRITE is the only command that alters what the client sends, so a server can easily reject unknown commands by assuming they match the layout of NBD_CMD_READ; and I just fixed a long-standing bug where we were sending trim and zero requests contrary to server advertisement). Trying to provoke a write to a read-only server is different than trying to experiment with an unknown flag, which is why it might make sense to have two separate knobs for the gating done in this patch.