[PATCH 3/3] nbd: Use shutdown(SHUT_WR) after last item sent

[Eric Blake <eblake@redhat.com>]

Although the remote end should always be tolerant of a socket being
arbitrarily closed, there are situations where it is a lot easier if
the remote end can be guaranteed to read EOF even before the socket
has closed.  In particular, when using gnutls, if we fail to inform
the remote end about an impending teardown, the remote end cannot
distinguish between our closing the socket as intended vs. a malicious
intermediary interrupting things, and may result in spurious error
messages.  Or, we can end up with a deadlock where both ends are stuck
on a read() from the other end but neither gets an EOF.  Thus, after
any time a client sends NBD_OPT_ABORT or NBD_CMD_DISC, or a server has
finished replying (where appropriate) to such a request, it is worth
informing the channel that we will not be transmitting anything else.

Signed-off-by: Eric Blake <eblake@redhat.com>
---
 block/nbd.c  | 1 +
 nbd/client.c | 3 ++-
 nbd/server.c | 4 ++++
 3 files changed, 7 insertions(+), 1 deletion(-)

diff --git a/block/nbd.c b/block/nbd.c
index 2160859f6499..2906484390f9 100644
--- a/block/nbd.c
+++ b/block/nbd.c
@@ -1402,6 +1402,7 @@ static void nbd_client_close(BlockDriverState *bs)

     if (s->ioc) {
         nbd_send_request(s->ioc, &request);
+        qio_channel_shutdown(s->ioc, QIO_CHANNEL_SHUTDOWN_WRITE, NULL);
     }

     nbd_teardown_connection(bs);
diff --git a/nbd/client.c b/nbd/client.c
index ba173108baab..1b8b3a9ae3bd 100644
--- a/nbd/client.c
+++ b/nbd/client.c
@@ -1,5 +1,5 @@
 /*
- *  Copyright (C) 2016-2019 Red Hat, Inc.
+ *  Copyright (C) 2016-2020 Red Hat, Inc.
  *  Copyright (C) 2005  Anthony Liguori <anthony@codemonkey.ws>
  *
  *  Network Block Device Client Side
@@ -103,6 +103,7 @@ static void nbd_send_opt_abort(QIOChannel *ioc)
      * even care if the request makes it to the server, let alone
      * waiting around for whether the server replies. */
     nbd_send_option_request(ioc, NBD_OPT_ABORT, 0, NULL, NULL);
+    qio_channel_shutdown(ioc, QIO_CHANNEL_SHUTDOWN_WRITE, NULL);
 }


diff --git a/nbd/server.c b/nbd/server.c
index 02b1ed080145..e21a1f662cc2 100644
--- a/nbd/server.c
+++ b/nbd/server.c
@@ -1168,6 +1168,8 @@ static int nbd_negotiate_options(NBDClient *client, Error **errp)
                                    "Option 0x%" PRIx32
                                    " not permitted before TLS", option);
                 if (option == NBD_OPT_ABORT) {
+                    qio_channel_shutdown(client->ioc,
+                                         QIO_CHANNEL_SHUTDOWN_WRITE, NULL);
                     return 1;
                 }
                 break;
@@ -1187,6 +1189,8 @@ static int nbd_negotiate_options(NBDClient *client, Error **errp)
                  * disconnecting, but that we must also tolerate
                  * guests that don't wait for our reply. */
                 nbd_negotiate_send_rep(client, NBD_REP_ACK, NULL);
+                qio_channel_shutdown(client->ioc,
+                                     QIO_CHANNEL_SHUTDOWN_WRITE, NULL);
                 return 1;

             case NBD_OPT_EXPORT_NAME:
-- 
2.26.0.rc2