Currently, all errors in pre_save_hooks are caught and logged, so they cannot cancel the save in progress. This patch will allow you to raise HTTPErrors to cancel saves with a message that will be shown to the user.
Until you can require that, though, you need to implement the quota enforcement by overriding ContentsManager.save instead of a hook:
from tornado.web import HTTPError
from jupyter_server.services.contents.largefilemanager import LargeFileManager
class QuotaFileManager(LargeFileManager):
def _check_quota(self):
return True # whatever you want here
def save(self, model, path):
# this bit can move to a hook following https://github.com/jupyter-server/jupyter_server/pull/456
if not self._check_quota():
# raising here means cancelling the save
# the user will see this message in a dialog
raise HTTPError(400, "Quota exceeded")
return super().save(model, path)
# use our contents manager subclass
c.ServerApp.contents_manager_class = QuotaFileManager