iotests: Make 233 output more reliable

We have a race between the nbd server and the client both trying
to report errors at once which can make the test sometimes fail
if the output lines swap order under load.  Break the race by
collecting server messages into a file and then replaying that
at the end of the test.

We may yet want to fix the server to not output ANYTHING for a
client action except when -v was used (to avoid malicious clients
from being able to DoS a server by filling up its logs), but that
is saved for a future patch.

Signed-off-by: Eric Blake <eblake@redhat.com>
CC: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20190117193658.16413-2-eblake@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

tests/qemu-iotests/233

@@ -2,7 +2,7 @@
 #
 # Test NBD TLS certificate / authorization integration
 #
-# Copyright (C) 2018 Red Hat, Inc.
+# Copyright (C) 2018-2019 Red Hat, Inc.
 #
 # This program is free software; you can redistribute it and/or modify
 # it under the terms of the GNU General Public License as published by
@@ -30,6 +30,7 @@ _cleanup()
 {
     nbd_server_stop
     _cleanup_test_img
+    rm -f "$TEST_DIR/server.log"
     tls_x509_cleanup
 }
 trap "_cleanup; exit \$status" 0 1 2 3 15
@@ -66,7 +67,7 @@ $QEMU_IO -c 'w -P 0x11 1m 1m' "$TEST_IMG" | _filter_qemu_io
 
 echo
 echo "== check TLS client to plain server fails =="
-nbd_server_start_tcp_socket -f $IMGFMT "$TEST_IMG"
+nbd_server_start_tcp_socket -f $IMGFMT "$TEST_IMG" 2> "$TEST_DIR/server.log"
 
 $QEMU_IMG info --image-opts \
     --object tls-creds-x509,dir=${tls_dir}/client1,endpoint=client,id=tls0 \
@@ -81,7 +82,7 @@ echo "== check plain client to TLS server fails =="
 nbd_server_start_tcp_socket \
     --object tls-creds-x509,dir=${tls_dir}/server1,endpoint=server,id=tls0,verify-peer=yes \
     --tls-creds tls0 \
-    -f $IMGFMT "$TEST_IMG"
+    -f $IMGFMT "$TEST_IMG" 2>> "$TEST_DIR/server.log"
 
 $QEMU_IMG info nbd://localhost:$nbd_tcp_port 2>&1 | sed "s/$nbd_tcp_port/PORT/g"
 
@@ -109,6 +110,10 @@ $QEMU_IO -c 'r -P 0x11 1m 1m' -c 'w -P 0x22 1m 1m' --image-opts \
 
 $QEMU_IO -f $IMGFMT -r -U -c 'r -P 0x22 1m 1m' "$TEST_IMG" | _filter_qemu_io
 
+echo
+echo "== final server log =="
+cat "$TEST_DIR/server.log"
+
 # success, all done
 echo "*** done"
 rm -f $seq.full