nbd/client: Trace all server option error messages

Not all servers send free-form text alongside option error replies, but
for servers that do (such as qemu), we pass the server's message as a
hint alongside our own error reporting.  However, it would also be
useful to trace such server messages, since we can't guarantee how the
hint may be consumed.

Signed-off-by: Eric Blake <eblake@redhat.com>
Message-Id: <20181218225714.284495-3-eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>

nbd/client.c

@@ -171,6 +171,8 @@ static int nbd_handle_reply_err(QIOChannel *ioc, NBDOptionReply *reply,
             goto cleanup;
         }
         msg[reply->length] = '\0';
+        trace_nbd_server_error_msg(reply->type,
+                                   nbd_reply_type_lookup(reply->type), msg);
     }
 
     switch (reply->type) {

nbd/trace-events

@@ -1,6 +1,7 @@
 # nbd/client.c
 nbd_send_option_request(uint32_t opt, const char *name, uint32_t len) "Sending option request %" PRIu32" (%s), len %" PRIu32
 nbd_receive_option_reply(uint32_t option, const char *optname, uint32_t type, const char *typename, uint32_t length) "Received option reply %" PRIu32" (%s), type %" PRIu32" (%s), len %" PRIu32
+nbd_server_error_msg(uint32_t err, const char *type, const char *msg) "server reported error 0x%" PRIx32 " (%s) with additional message: %s"
 nbd_reply_err_unsup(uint32_t option, const char *name) "server doesn't understand request %" PRIu32 " (%s), attempting fallback"
 nbd_opt_go_start(const char *name) "Attempting NBD_OPT_GO for export '%s'"
 nbd_opt_go_success(void) "Export is good to go"