quickly check to see if the previous instance id is still valid

This adds a check in cloud-init to see if the existing (cached)
datasource is still valid.  It relies on support from the Datasource
to implement 'check_instance_id'.  That method should quickly determine
(if possible) if the instance id found in the datasource is still valid.

This means that we can still notice new instance ids without
depending on a network datasource on every boot.

I've also implemented check_instance_id for the superclass and for
sources:
  DataSourceAzure (check dmi data)
  DataSourceConfigDrive (check dmi data)
  DataSourceNocloud (check the seeded data or kernel command line)
  DataSourceOpenstack (check dmi data)

LP: #1553815

1 files changed, 8 insertions, 11 deletions

diff --git a/bin/cloud-init b/bin/cloud-init
index 7f665e7e..11cc0237 100755
--- a/bin/cloud-init
+++ b/bin/cloud-init
@@ -212,6 +212,7 @@ def main_init(name, args):
     # Stage 4
     path_helper = init.paths
     if not args.local:
+        existing = "trust"
         sys.stderr.write("%s\n" % (netinfo.debug_info()))
         LOG.debug(("Checking to see if files that we need already"
                    " exist from a previous run that would allow us"
@@ -236,21 +237,17 @@ def main_init(name, args):
             LOG.debug("Execution continuing, no previous run detected that"
                       " would allow us to stop early.")
     else:
-        # The cache is not instance specific, so it has to be purged
-        # but we want 'start' to benefit from a cache if
-        # a previous start-local populated one...
-        manual_clean = util.get_cfg_option_bool(init.cfg,
-                                                'manual_cache_clean', False)
-        if manual_clean:
-            LOG.debug("Not purging instance link, manual cleaning enabled")
-            init.purge_cache(False)
-        else:
-            init.purge_cache()
+        existing = "check"
+        if util.get_cfg_option_bool(init.cfg, 'manual_cache_clean', False):
+            existing = "trust"
+
+        init.purge_cache()
         # Delete the non-net file as well
         util.del_file(os.path.join(path_helper.get_cpath("data"), "no-net"))
+
     # Stage 5
     try:
-        init.fetch()
+        init.fetch(existing=existing)
     except sources.DataSourceNotFoundException:
         # In the case of 'cloud-init init' without '--local' it is a bit
         # more likely that the user would consider it failure if nothing was