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

[Richard W.M. Jones]

On Fri, Sep 04, 2020 at 04:18:41PM -0500, Eric Blake wrote:
> 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.
> 

Patch looks fine, ACK.

I've no particular opinion on whether this should be one or two flags.
Whatever you think is best.

Rich.

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