nbd/server: Ignore write errors when replying to NBD_OPT_ABORT

Commit 37ec36f6 intentionally ignores errors when trying to reply
to an NBD_OPT_ABORT request for plaintext clients, but did not make
the same change for a TLS server.  Since NBD_OPT_ABORT is
documented as being a potential for an EPIPE when the client hangs
up without waiting for our reply, we don't need to pollute the
server's output with that failure.
Signed-off-by: NEric Blake <eblake@redhat.com>
Message-Id: <20181117223221.2198751-1-eblake@redhat.com>
Reviewed-by: NDaniel P. Berrangé <berrange@redhat.com>

nbd/server.c

@@ -1134,12 +1134,16 @@ static int nbd_negotiate_options(NBDClient *client, uint16_t myflags,
                 return -EINVAL;
 
             default:
-                ret = nbd_opt_drop(client, NBD_REP_ERR_TLS_REQD, errp,
+                /* Let the client keep trying, unless they asked to
+                 * quit. Always try to give an error back to the
+                 * client; but when replying to OPT_ABORT, be aware
+                 * that the client may hang up before receiving the
+                 * error, in which case we are fine ignoring the
+                 * resulting EPIPE. */
+                ret = nbd_opt_drop(client, NBD_REP_ERR_TLS_REQD,
+                                   option == NBD_OPT_ABORT ? NULL : errp,
                                    "Option 0x%" PRIx32
                                    " not permitted before TLS", option);
-                /* Let the client keep trying, unless they asked to
-                 * quit. In this mode, we've already sent an error, so
-                 * we can't ack the abort.  */
                 if (option == NBD_OPT_ABORT) {
                     return 1;
                 }