iotests: Do not rely on unavailable domains in 162

There are some (mostly ISP-specific) name servers who will redirect
non-existing domains to special hosts. In this case, we will get a
different error message when trying to connect to such a host, which
breaks test 162.

162 needed this specific error message so it can confirm that qemu was
indeed trying to connect to the user-specified port. However, we can
also confirm this by setting up a local NBD server on exactly that port;
so we can fix the issue by doing just that.

Reported-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

tests/qemu-iotests/162

@@ -43,10 +43,21 @@ echo '=== NBD ==='
 $QEMU_IMG info 'json:{"driver": "nbd", "host": 42}'
 
 # And this should not treat @port as if it had not been specified
-# (We cannot use localhost with an invalid port here, but we need to use a
-#  non-existing domain, because otherwise the error message will not contain
-#  the port)
-$QEMU_IMG info 'json:{"driver": "nbd", "host": "does.not.exist.example.com", "port": 42}'
+# (We need to set up a server here, because the error message for "Connection
+#  refused" does not contain the destination port)
+
+# Launching qemu-nbd is done in a loop: We try to set up an NBD server on some
+# random port and continue until success, i.e. until we have found a port that
+# is not in use yet.
+while true; do
+    port=$((RANDOM + 32768))
+    if $QEMU_NBD -p $port -f raw --fork null-co:// 2> /dev/null; then
+        break
+    fi
+done
+
+$QEMU_IMG info "json:{'driver': 'nbd', 'host': 'localhost', 'port': $port}" \
+    | grep '^image' | sed -e "s/$port/PORT/"
 
 # This is a test for NBD's bdrv_refresh_filename() implementation: It expects
 # either host or path to be set, but it must not assume that they are set to

tests/qemu-iotests/162.out

@@ -2,7 +2,7 @@ QA output created by 162
 
 === NBD ===
 qemu-img: Could not open 'json:{"driver": "nbd", "host": 42}': Failed to connect socket: Invalid argument
-qemu-img: Could not open 'json:{"driver": "nbd", "host": "does.not.exist.example.com", "port": 42}': address resolution failed for does.not.exist.example.com:42: Name or service not known
+image: nbd://localhost:PORT
 image: nbd+unix://?socket=42
 
 === SSH ===