DataSourceEc2: only do dns check in mirror selection

This returns the check for an archive mirror in the DataSourceEc2 to
only do so by DNS resolution.  The 'rework' branch had made the check
wait and timeout on attempts to reach the mirror.   This resulted
in 120 seconds of waiting before failure.

For now, just go back to the old situation of checking by dns.

cloudinit/sources/DataSourceEc2.py

@@ -87,6 +87,7 @@ class DataSourceEc2(sources.DataSource):
         return self.get_mirror_from_availability_zone()
 
     def get_mirror_from_availability_zone(self, availability_zone=None):
+        # Return type None indicates there is no cloud specific mirror
         # Availability is like 'us-west-1b' or 'eu-west-1a'
         if availability_zone is None:
             availability_zone = self.get_availability_zone()
@@ -94,26 +95,26 @@ class DataSourceEc2(sources.DataSource):
         if self.is_vpc():
             return None
 
-        # Use the distro to get the mirror
         if not availability_zone:
             return None
 
-        mirror_tpl = self.distro.get_option('availability_zone_template')
-        if not mirror_tpl:
+        mirror_tpl = self.distro.get_option('package_mirror_ec2_template', None)
+
+        if mirror_tpl is None:
             return None
 
+        # in EC2, the 'region' is 'us-east-1' if 'zone' is 'us-east-1a'
         tpl_params = {
             'zone': availability_zone.strip(),
+            'region': availability_zone[:-1]
         }
         mirror_url = mirror_tpl % (tpl_params)
 
-        (max_wait, timeout) = self._get_url_settings()
-        worked = uhelp.wait_for_url([mirror_url], max_wait=max_wait,
-                                timeout=timeout, status_cb=LOG.warn)
-        if not worked:
-            return None
+        found = util.search_for_mirror([mirror_url])
+        if found is not None:
+            return mirror_url
 
-        return mirror_url
+        return None
 
     def _get_url_settings(self):
         mcfg = self.ds_cfg

cloudinit/util.py

@@ -805,7 +805,10 @@ def is_resolvable_url(url):
 
 
 def search_for_mirror(candidates):
-    """ Search through a list of mirror urls for one that works """
+    """
+    Search through a list of mirror urls for one that works
+    This needs to return quickly.
+    """
     for cand in candidates:
         try:
             if is_resolvable_url(cand):

config/cloud.cfg

@@ -70,5 +70,5 @@ system_info:
       templates_dir: /etc/cloud/templates/
       upstart_dir: /etc/init/
    package_mirror: http://archive.ubuntu.com/ubuntu
-   availability_zone_template: http://%(zone)s.ec2.archive.ubuntu.com/ubuntu/
+   package_mirror_ec2_template: http://%(region)s.ec2.archive.ubuntu.com/ubuntu/
    ssh_svcname: ssh