fix: Fall back to cached local ds if no valid ds found (#4997)

Rebooting an instance which has finished VMware guest
customization with DataSourceVMware will load
DataSourceNone due to metadata is NOT available.

This is mostly a re-post of PR#229, few differences are:
1. Let ds decide if fallback is allowed, not always fall back
   to previous cached LOCAL ds.
2. No comparing instance-id of cached ds with previous instance-id
   due to I think they are always identical.

Fixes GH-3402

cloudinit/sources/DataSourceVMware.py

@@ -176,7 +176,7 @@ def _get_data(self):
                 break
 
         if not self.data_access_method:
-            LOG.error("failed to find a valid data access method")
+            LOG.debug("failed to find a valid data access method")
             return False
 
         LOG.info("using data access method %s", self._get_subplatform())
@@ -270,6 +270,18 @@ def get_instance_id(self):
             self.metadata["instance-id"] = str(id_file.read()).rstrip().lower()
             return self.metadata["instance-id"]
 
+    def check_if_fallback_is_allowed(self):
+        if (
+            self.data_access_method
+            and self.data_access_method == DATA_ACCESS_METHOD_IMC
+            and is_vmware_platform()
+        ):
+            LOG.debug(
+                "Cache fallback is allowed for : %s", self._get_subplatform()
+            )
+            return True
+        return False
+
     def get_public_ssh_keys(self):
         for key_name in (
             "public-keys-data",

cloudinit/sources/__init__.py

@@ -347,6 +347,9 @@ def _unpickle(self, ci_pkl_version: int) -> None:
             if not hasattr(self, key):
                 setattr(self, key, value)
 
+        if not hasattr(self, "check_if_fallback_is_allowed"):
+            setattr(self, "check_if_fallback_is_allowed", lambda: False)
+
         if hasattr(self, "userdata") and self.userdata is not None:
             # If userdata stores MIME data, on < python3.6 it will be
             # missing the 'policy' attribute that exists on >=python3.6.
@@ -926,6 +929,16 @@ def check_instance_id(self, sys_cfg):
         # quickly (local check only) if self.instance_id is still
         return False
 
+    def check_if_fallback_is_allowed(self):
+        """check_if_fallback_is_allowed()
+        Checks if a cached ds is allowed to be restored when no valid ds is
+        found in local mode by checking instance-id and searching valid data
+        through ds list.
+
+        @return True if a ds allows fallback, False otherwise.
+        """
+        return False
+
     @staticmethod
     def _determine_dsmode(candidates, default=None, valid=None):
         # return the first candidate that is non None, warn if not valid

cloudinit/stages.py

@@ -359,20 +359,32 @@ def _get_data_source(self, existing) -> sources.DataSource:
             LOG.debug(myrep.description)
 
         if not ds:
-            util.del_file(self.paths.instance_link)
-            (cfg_list, pkg_list) = self._get_datasources()
-            # Deep copy so that user-data handlers can not modify
-            # (which will affect user-data handlers down the line...)
-            (ds, dsname) = sources.find_source(
-                self.cfg,
-                self.distro,
-                self.paths,
-                copy.deepcopy(self.ds_deps),
-                cfg_list,
-                pkg_list,
-                self.reporter,
-            )
-            LOG.info("Loaded datasource %s - %s", dsname, ds)
+            try:
+                cfg_list, pkg_list = self._get_datasources()
+                # Deep copy so that user-data handlers can not modify
+                # (which will affect user-data handlers down the line...)
+                ds, dsname = sources.find_source(
+                    self.cfg,
+                    self.distro,
+                    self.paths,
+                    copy.deepcopy(self.ds_deps),
+                    cfg_list,
+                    pkg_list,
+                    self.reporter,
+                )
+                util.del_file(self.paths.instance_link)
+                LOG.info("Loaded datasource %s - %s", dsname, ds)
+            except sources.DataSourceNotFoundException as e:
+                if existing != "check":
+                    raise e
+                ds = self._restore_from_cache()
+                if ds and ds.check_if_fallback_is_allowed():
+                    LOG.info(
+                        "Restored fallback datasource from checked cache: %s",
+                        ds,
+                    )
+                else:
+                    raise e
         self.datasource = ds
         # Ensure we adjust our path members datasource
         # now that we have one (thus allowing ipath to be used)