Add explanatory comments for extension rolling upgrade

azurelinuxagent/common/protocol/wire.py

@@ -1315,6 +1315,7 @@ class ExtensionsConfig(object):
         logger.verbose("Load ExtensionsConfig.xml")
         self.ext_handlers = ExtHandlerList()
         self.vmagent_manifests = VMAgentManifestList()
+        # Default the rolling upgrade flag to False
         self.rolling_upgrade = False
         self.status_upload_blob = None
         self.status_upload_blob_type = None
@@ -1330,6 +1331,7 @@ class ExtensionsConfig(object):
 
         guest_agent = find(xml_doc, "GuestAgentExtension")
         ru_text = findtext(guest_agent, "ExtensionRollingUpgrade")
+        # The value of the flag defaults to False
         if ru_text and ru_text.lower() == "true":
             self.rolling_upgrade = True
 

azurelinuxagent/ga/exthandlers.py

@@ -301,10 +301,14 @@ class ExtHandlersHandler(object):
             state = ext_handler.properties.state
             # Valid states are enabled, disabled and uninstall
             if state == u"enabled":
+                # Return early to skip upgrading if the rolling_upgrade
+                # flag is True and the upgrade GUID is NOT new
                 if self.protocol.get_rolling_upgrade() and \
                         not self.is_new_guid(ext_handler):
                     return
             elif state == u"disabled" or state == u"uninstall":
+                # Remove the GUID from the dictionary in this case so that
+                # it is upgraded upon re-enabling/re-install
                 self.last_guids.pop(ext_handler.name, None)
             ext_handler_i.decide_version()
             if not ext_handler_i.is_upgrade and self.last_etag == etag: