users: Prevent mirror dummy users from being "reactivated."

These do not represent users who were ever active, so they should not
be able to be reactivated.

See also #26703.

zerver/actions/create_user.py

@@ -19,6 +19,7 @@ from zerver.lib.avatar import avatar_url
 from zerver.lib.create_user import create_user
 from zerver.lib.default_streams import get_slim_realm_default_streams
 from zerver.lib.email_notifications import enqueue_welcome_emails, send_account_registered_email
+from zerver.lib.exceptions import JsonableError
 from zerver.lib.mention import silent_mention_syntax_for_user
 from zerver.lib.send_email import clear_scheduled_invitation_emails
 from zerver.lib.stream_subscription import bulk_get_subscriber_peer_info
@@ -591,6 +592,10 @@ def do_activate_mirror_dummy_user(
 @transaction.atomic(savepoint=False)
 def do_reactivate_user(user_profile: UserProfile, *, acting_user: Optional[UserProfile]) -> None:
     """Reactivate a user that had previously been deactivated"""
+    if user_profile.is_mirror_dummy:
+        raise JsonableError(
+            _("Cannot activate a placeholder account; ask the user to sign up, instead.")
+        )
     change_user_is_active(user_profile, True)
 
     event_time = timezone_now()

zerver/tests/test_users.py

@@ -1567,6 +1567,20 @@ class ActivateTest(ZulipTestCase):
         result = self.client_post(f"/json/users/{invalid_user_id}/reactivate")
         self.assert_json_error(result, "No such user")
 
+    def test_api_with_mirrordummy_user(self) -> None:
+        self.login("iago")
+        desdemona = self.example_user("desdemona")
+        change_user_is_active(desdemona, False)
+
+        desdemona.is_mirror_dummy = True
+        desdemona.save(update_fields=["is_mirror_dummy"])
+
+        # Cannot deactivate a user which is marked as "mirror dummy" from importing
+        result = self.client_post(f"/json/users/{desdemona.id}/reactivate")
+        self.assert_json_error(
+            result, "Cannot activate a placeholder account; ask the user to sign up, instead."
+        )
+
     def test_api_with_insufficient_permissions(self) -> None:
         non_admin = self.example_user("othello")
         do_change_user_role(non_admin, UserProfile.ROLE_MEMBER, acting_user=None)