Re: [Nbd] [PATCH] doc: Specific error if NBD_OPT_GO refused without NBD_OPT_BLOCK_SIZE

[Alex Bligh <alex@...872...>]

Eric,

Thanks. Applied.

I've reserved NBD_REP_ERR_BLOCK_SIZE_REQD in master, but in future
it would be best if when applying a patch to a branch that
requires a reservation in master, a patch for master was
supplied too.

Alex

On 18 Apr 2016, at 20:40, Eric Blake <eblake@...696...> wrote:

> While we documented that a server could refuse a client that
> did not mention that it would handle non-default block sizes,
> we didn't mention what error to use.  For NBD_OPT_EXPORT_NAME,
> we have no choice (a hard disconnect is the only way possible).
> But for NBD_OPT_GO (and similarly for NBD_OPT_INFO), the
> existing NBD_REP_ERR_POLICY and NBD_REP_ERR_INVALID feel rather
> weak, so it's better to invent a specific
> NBD_REP_ERR_BLOCK_SIZE_REQD.
> 
> Suggested-by: Alex Bligh <alex@...872...>
> Signed-off-by: Eric Blake <eblake@...696...>
> ---
> 
> Applies to the extension-info branch; although if we split
> block-size into its own branch, this would move there.
> 
> Also requires a patch to the master branch to reserve the
> new error code.  Do you want me to submit that patch, too?
> 
> Question: Should we rearrange the various errors, so that
> NBD_REP_ERR_UNKNOWN and NBD_REP_ERR_BLOCK_SIZE_REQD are
> adjacent (since they are, for now, in the same extension
> branch), by hoising NBD_REP_ERR_SHUTDOWN to 2^32 + 6?  We
> don't yet have any released versions that use
> NBD_REP_ERR_SHUTDOWN, although it was added as normative text
> without going through the usual extension work.
> 
> doc/proto.md | 28 ++++++++++++++++++++++------
> 1 file changed, 22 insertions(+), 6 deletions(-)
> 
> diff --git a/doc/proto.md b/doc/proto.md
> index 5e646cf..8ef339c 100644
> --- a/doc/proto.md
> +++ b/doc/proto.md
> @@ -657,12 +657,14 @@ a client SHOULD assume a default minimum block size of 1, a preferred
> block size of 2^12 (4,096), and a maximum block size of the smaller of
> the export size or 0xffffffff (effectively unlimited).  A server that
> wants to enforce block sizes other than the defaults specified here
> -MAY refuse to go into transmission phase with a client that uses `NBD_OPT_EXPORT_NAME`
> -or failed to use `NBD_OPT_BLOCK_SIZE`, although a server SHOULD permit
> -such clients if block sizes can be agreed on externally.  When
> -allowing such clients, the server MUST cleanly error commands that
> -fall outside block size parameters without corrupting data; even so,
> -this may limit interoperability.
> +MAY refuse to go into transmission phase with a client that uses
> +`NBD_OPT_EXPORT_NAME` (via a hard disconnect) or which fails to use
> +`NBD_OPT_BLOCK_SIZE` (where the server uses
> +`NBD_REP_ERR_BLOCK_SIZE_REQD` in response to `NBD_OPT_GO`), although a
> +server SHOULD permit such clients if block sizes can be agreed on
> +externally.  When allowing such clients, the server MUST cleanly error
> +commands that fall outside block size parameters without corrupting
> +data; even so, this may limit interoperability.
> 
> A client MAY choose to operate as if tighter block sizes had been
> specified (for example, even when the server advertises the default
> @@ -909,6 +911,13 @@ of the newstyle negotiation.
>       export.  In this case, a FORCEDTLS server MUST NOT send
>       `NBD_REP_INFO` replies, but a SELECTIVETLS server MAY do so if
>       this is a TLS-only export.
> +    - `NBD_REP_ERR_BLOCK_SIZE_REQD`: The server requires the client to
> +      negotiate `NBD_OPT_BLOCK_SIZE` prior to entering transmission
> +      phase, because the server will be using non-default block sizes.
> +      In this case, the server SHOULD first send at least an
> +      `NBD_REP_INFO` reply with `NBD_INFO_BLOCK_SIZE` data.  This
> +      error MUST NOT be used if the client has already negotiated
> +      `NBD_OPT_BLOCK_SIZE`.
> 
>     Additionally, if TLS has not been initiated, the server MAY reply
>     with `NBD_REP_ERR_TLS_REQD` (instead of `NBD_REP_ERR_UNKNOWN`) to
> @@ -1133,6 +1142,13 @@ case that data is an error message string suitable for display to the user.
>     The server is unwilling to continue negotiation as it is in the
>     process of being shut down.
> 
> +* `NBD_REP_ERR_BLOCK_SIZE_REQD` (2^32 + 8)
> +
> +    The server is unwilling to enter transmission phase for a given
> +    export unless the client first acknowledges (via
> +    `NBD_OPT_BLOCK_SIZE`) that it will obey non-default block sizing
> +    requirements.
> +
> ### Transmission phase
> 
> #### Command flags
> -- 
> 2.5.5
> 
> 
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-- 
Alex Bligh