tests: check if qemu-nbd is still alive before waiting

If the qemu-nbd UNIX socket has not shown up, the tests will sleep a bit
and then check again repeatedly for up to 30 seconds. This is pointless
if the qemu-nbd process has quit due to an error, so check whether the
pid is still alive before waiting and retrying.
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>
Message-Id: <20181116155325.22428-5-berrange@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Signed-off-by: NEric Blake <eblake@redhat.com>

tests/qemu-iotests/common.nbd

@@ -37,11 +37,19 @@ function nbd_server_stop()
 
 function nbd_server_wait_for_unix_socket()
 {
+    pid=$1
+
     for ((i = 0; i < 300; i++))
     do
         if [ -r "$nbd_unix_socket" ]; then
             return
         fi
+        kill -s 0 $pid 2>/dev/null
+        if test $? != 0
+        then
+            echo "qemu-nbd unexpectedly quit"
+            exit 1
+        fi
         sleep 0.1
     done
     echo "Failed in check of unix socket created by qemu-nbd"
@@ -52,5 +60,5 @@ function nbd_server_start_unix_socket()
 {
     nbd_server_stop
     $QEMU_NBD -v -t -k "$nbd_unix_socket" "$@" &
-    nbd_server_wait_for_unix_socket
+    nbd_server_wait_for_unix_socket $!
 }