Re: [Qemu-devel] [PATCH v4 RFC 9/8] nbd: Minimal structured read for client

[Eric Blake <eblake@redhat.com>]

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On 10/17/2017 04:17 PM, Eric Blake wrote:
> On 10/17/2017 07:57 AM, Vladimir Sementsov-Ogievskiy wrote:
>> Minimal implementation: for structured error only error_report error
>> message.
>>
>> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
>> ---

> But in the client, I then perform 'w 0 0' (a zero-byte write, which
> should fail because the server is read-only).  I see:
> 
> C: 19481@1508268433.381446:nbd_send_request Sending request to server: {
> .from = 0, .len = 0, .handle = 93997172956880, .flags = 0x1, .type = 1
> (write) }
> S: 19479@1508268433.381516:nbd_receive_request Got request: { magic =
> 0x25609513, .flags = 0x1, .type = 0x1, from = 0, len = 0 }
> S: 19479@1508268433.381527:nbd_co_receive_request_decode_type Decoding
> type: handle = 93997172956880, type = 1 (write)
> S: 19479@1508268433.381540:nbd_co_receive_request_payload_received
> Payload received: handle = 93997172956880, len = 0
> S: 19479@1508268433.381564:nbd_co_send_structured_error Send structured
> error reply: handle = 93997172956880, error = 1 (EPERM), msg = ''
> C: 19481@1508268433.381622:nbd_receive_structured_reply_chunk Got
> structured reply chunk: { flags = 0x1, type = 32769, handle =
> 93997172956880, length = 6 }
> C: wrote 0/0 bytes at offset 0
> C: 0 bytes, 1 ops; 0.0002 sec (0 bytes/sec and 4291.8455 ops/sec)
> 
> Oops - the client claimed success, even though the server replied with
> EPERM.  And the server didn't do a good job of including details on the
> error message.  So there's still some tweaks needed.

The server not sending details is a separate pre-existing issue; in
server.c, we set errp for NBD_CMD_WRITE_ZEROES but not for
NBD_CMD_WRITE.  You can get the server to cough up a message by using 'w
-z 0 1' instead.

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org


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