Only attempt to read the previous hostname file if it exists.

- Instead of always reading the previous hostname file even if it
  did not exist lets only read it if it is a valid variable and is
  actually a existent file instead of just attempting to read it
  always.
- Also update the logging that is done when a previous file does not exist.

cloudinit/distros/__init__.py

@@ -146,17 +146,37 @@ class Distro(object):
 
     def update_hostname(self, hostname, fqdn, prev_hostname_fn):
         applying_hostname = hostname
+
+        # Determine what the actual written hostname should be
         hostname = self._select_hostname(hostname, fqdn)
-        prev_hostname = self._read_hostname(prev_hostname_fn)
+
+        # If the previous hostname file exists lets see if we
+        # can get a hostname from it
+        if prev_hostname_fn and os.path.exists(prev_hostname_fn):
+            prev_hostname = self._read_hostname(prev_hostname_fn)
+        else:
+            prev_hostname = None
+
+        # Lets get where we should write the system hostname
+        # and what the system hostname is
         (sys_fn, sys_hostname) = self._read_system_hostname()
         update_files = []
+
+        # If there is no previous hostname or it differs
+        # from what we want, lets update it or create the
+        # file in the first place
         if not prev_hostname or prev_hostname != hostname:
             update_files.append(prev_hostname_fn)
 
+        # If the system hostname is different than the previous
+        # one or the desired one lets update it as well
         if (not sys_hostname) or (sys_hostname == prev_hostname
                                   and sys_hostname != hostname):
             update_files.append(sys_fn)
 
+        # Remove duplicates (incase the previous config filename)
+        # is the same as the system config filename, don't bother
+        # doing it twice
         update_files = set([f for f in update_files if f])
         LOG.debug("Attempting to update hostname to %s in %s files",
                   hostname, len(update_files))
@@ -173,6 +193,8 @@ class Distro(object):
             LOG.debug("%s differs from %s, assuming user maintained hostname.",
                        prev_hostname_fn, sys_fn)
 
+        # If the system hostname file name was provided set the
+        # non-fqdn as the transient hostname.
         if sys_fn in update_files:
             self._apply_hostname(applying_hostname)
 

cloudinit/distros/debian.py

@@ -88,37 +88,37 @@ class Distro(distros.Distro):
         return hostname
 
     def _write_hostname(self, your_hostname, out_fn):
-        conf = self._read_hostname_conf(out_fn)
+        conf = None
+        try:
+            # Try to update the previous one
+            # so lets see if we can read it first.
+            conf = self._read_hostname_conf(out_fn)
+        except IOError:
+            pass
         if not conf:
             conf = HostnameConf('')
-            conf.parse()
         conf.set_hostname(your_hostname)
         util.write_file(out_fn, str(conf), 0644)
 
     def _read_system_hostname(self):
-        conf = self._read_hostname_conf(self.hostname_conf_fn)
-        if conf:
-            sys_hostname = conf.hostname
-        else:
-            sys_hostname = None
+        sys_hostname = self._read_hostname(self.hostname_conf_fn)
         return (self.hostname_conf_fn, sys_hostname)
 
     def _read_hostname_conf(self, filename):
-        try:
-            conf = HostnameConf(util.load_file(filename))
-            conf.parse()
-            return conf
-        except IOError:
-            util.logexc(LOG, "Error reading hostname from %s", filename)
-            return None
+        conf = HostnameConf(util.load_file(filename))
+        conf.parse()
+        return conf
 
     def _read_hostname(self, filename, default=None):
-        conf = self._read_hostname_conf(filename)
-        if not conf:
+        hostname = None
+        try:
+            conf = self._read_hostname_conf(filename)
+            hostname = conf.hostname
+        except IOError:
+            pass
+        if not hostname:
             return default
-        if not conf.hostname:
-            return default
-        return conf.hostname
+        return hostname
 
     def _get_localhost_ip(self):
         # Note: http://www.leonardoborda.com/blog/127-0-1-1-ubuntu-debian/