Split out the resource refresh decider logic into a separate function

So that we can unit test it

Author: cderici

juju/application.py

@@ -751,21 +751,8 @@ async def refresh(
         # Compute the difference btw resources needed and the existing resources
         resources_to_update = []
         for resource in charm_resources:
-            # should upgrade resource?
-            res_name = resource.get('Name', resource.get('name'))
-            # no, if it's upload
-            if existing_resources[res_name].origin == 'upload':
-                continue
-
-            # no, if we already have it (and upstream doesn't have a newer res available)
-            if res_name in existing_resources:
-                available_rev = resource.get('Revision', resource.get('revision', -1))
-                u_fields = existing_resources[res_name].unknown_fields
-                existing_rev = u_fields.get('Revision', resource.get('revision', -1))
-                if existing_rev >= available_rev:
-                    continue
-
-            resources_to_update.append(resource)
+            if utils.should_upgrade_resource(resource, existing_resources):
+                resources_to_update.append(resource)
 
         # Update the resources
         if resources_to_update:

juju/utils.py

@@ -523,3 +523,30 @@ def series_selector(series_arg='', charm_url=None, model_config=None, supported_
     # Charm hasn't specified a default (likely due to being a local charm
     # deployed by path). Last chance, best we can do is default to LTS.
     return DEFAULT_SUPPORTED_LTS
+
+
+def should_upgrade_resource(available_resource, existing_resources):
+    """Called in the context of upgrade_charm. Given a resource R, takes a look at the resources we
+    already have and decides if we need to refresh R.
+
+    :param dict[str] available_resource: The dict representing the client.Resource coming from the
+    charmhub api. We're considering if we need to refresh this during upgrade_charm.
+    :param dict[str] existing_resources: The dict coming from resources_facade.ListResources
+    representing the resources of the currently deployed charm.
+
+    :result bool: The decision to refresh the given resource
+    """
+    # should upgrade resource?
+    res_name = available_resource.get('Name', available_resource.get('name'))
+    # no, if it's upload
+    if existing_resources[res_name].origin == 'upload':
+        return False
+
+    # no, if we already have it (and upstream doesn't have a newer res available)
+    if res_name in existing_resources:
+        available_rev = available_resource.get('Revision', available_resource.get('revision', -1))
+        u_fields = existing_resources[res_name].unknown_fields
+        existing_rev = u_fields.get('Revision', u_fields.get('revision', -1))
+        if existing_rev >= available_rev:
+            return False
+    return True