Re: [PATCH v2] nbd/server: Suppress Broken pipe errors on abrupt disconnection

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

14.09.2021 19:32, Richard W.M. Jones wrote:
> On Tue, Sep 14, 2021 at 06:21:58PM +0300, Vladimir Sementsov-Ogievskiy wrote:
>> 14.09.2021 17:52, Richard W.M. Jones wrote:
>>> On the
>>> server side when the server receives NBD_CMD_DISC it must complete any
>>> in-flight requests, but there's no requirement for the server to
>>> commit anything to disk.  IOW you can still lose data even though you
>>> took the time to disconnect.
>>>
>>> So I don't think there's any reason for libnbd to always gracefully
>>
>> Hmm. Me go to NBD spec :)
>>
>> I think, there is a reason:
>>
>> "The client and the server MUST NOT initiate any form of disconnect other than in one of the above circumstances."
>>
>> And the only possibility for client to initiate a hard disconnect listed above is "if it detects a violation by the other party of a mandatory condition within this document".
>>
>> So at least, nbdsh violates NBD protocol. May be spec should be updated to satisfy your needs.
> 
> I would say the spec is at best contradictory, but if you read other
> parts of the spec, then it's pretty clear that we're allowed to drop
> the connection whenever we like.  This section says as much:
> 
> https://github.com/NetworkBlockDevice/nbd/blob/5805b25ad3da96e7c0b3160cda51ea19eb518d5b/doc/proto.md#terminating-the-transmission-phase

Hmm, that was exactly the section I read and quoted :)

> 
>    There are two methods of terminating the transmission phase:
>    ...
>    "The client or the server drops the TCP session (in which case it
>    SHOULD shut down the TLS session first). This is referred to as
>    'initiating a hard disconnect'."

Right. Here the method is defined, no word that client can do it at any time.

Next, in same section:

    Either side MAY initiate a hard disconnect if it detects a violation by the other party of a mandatory condition within this document.

Next
    
    The client MAY issue a soft disconnect at any time

And finally:

    The client and the server MUST NOT initiate any form of disconnect other than in one of the above circumstances.


Or do you mean some other spec section I missed?

> 
> Anyway we're dropping the TCP connection because sometimes we are just
> interrogating an NBD server eg to find out what it supports, and doing
> a graceful shutdown is a waste of time and internet.
> 
>>> shut down (especially in this case where there are no in-flight
>>> requests), and anyway it would break ABI to make that change and slow
>>> down the client in cases when there's nothing to clean up.
>>
>> Which ABI will it break?
> 
> Our contract with callers using nbd_close(3), if nbd_shutdown(3) is
> not called beforehand.
> https://libguestfs.org/nbd_shutdown.3.html
> https://libguestfs.org/nbd_create.3.html (really nbd_close ...)
> 
> Rich.
> 


-- 
Best regards,
Vladimir