Block update change to labels mode (Azure#8927)

Author: Tratcher

src/containerapp/HISTORY.rst

@@ -4,6 +4,7 @@ Release History
 ===============
 upcoming
 ++++++
+* 'az containerapp update/up': Disallow changing `--revisions-mode` to Labels.
 * 'az containerapp session code-interpreter': Fix `--path` in examples
 * 'az containerapp env premium-ingress': Deprecate `--min-replicas` and `--max-replicas` parameters, use workload profile scale instead.
 

src/containerapp/azext_containerapp/containerapp_decorator.py

@@ -143,6 +143,14 @@ def validate_arguments(self):
         if not self.containerapp_def:
             raise ResourceNotFoundError("The containerapp '{}' does not exist".format(self.get_argument_name()))
 
+        # Transitioning into labels mode is complicated and we don't want to combine it with other updates.
+        # If the app was not previously in labels mode throw an error saying to use Set-Mode instead.
+        if (self.get_argument_revisions_mode()
+                and self.get_argument_revisions_mode().lower() == "labels"
+                and safe_get(self.containerapp_def, "properties", "configuration", "activeRevisionsMode").lower() != "labels"):
+            raise ArgumentUsageError("The containerapp '{}' is not in labels mode. Please use `az containerapp revision set-mode` to switch to labels mode first.".format(
+                self.get_argument_name()))
+
         validate_revision_suffix(self.get_argument_revision_suffix())
         # Validate that max_replicas is set to 0-1000
         if self.get_argument_max_replicas() is not None: