if we got an exception when reading the displayfd, we could end up ha…

…ving 'buf' undefined, causing a bug - just bail out sooner if we can't read the displayfd

git-svn-id: https://xpra.org/svn/Xpra/trunk@21079 3bb7dfac-3a0b-4e04-842a-767bc560f471

src/xpra/x11/vfb_util.py

@@ -228,6 +228,8 @@ def pathexpand(s):
     xvfb = None
     try:
         if use_display_fd:
+            def displayfd_err(msg):
+                raise InitException("%s: %s" % (xvfb_executable, msg))
             r_pipe, w_pipe = os.pipe()
             try:
                 if PYTHON3:
@@ -251,13 +253,10 @@ def preexec():
                     buf = read_displayfd(r_pipe)
                 except Exception as e:
                     log("read_displayfd(%s)", r_pipe, exc_info=True)
-                    log.error("Error reading displayfd pipe %s:", r_pipe)
-                    log.error(" %s", e)
+                    displayfd_err("failed to read displayfd pipe %s: %s" % (r_pipe, e))
             finally:
                 osclose(r_pipe)
                 osclose(w_pipe)
-            def displayfd_err(msg):
-                raise InitException("%s: %s" % (xvfb_executable, msg))
             n = parse_displayfd(buf, displayfd_err)
             new_display_name = ":%s" % n
             log("Using display number provided by %s: %s", xvfb_executable, new_display_name)