issue_comments

The only test failure is Black. I'm going to merge this and then reformat.

Yes, let's move ahead with getting this into an alpha.

If this is a concept ACK then I will move onto fixing the tests (adding new ones) and updating the documentation for the new plugin hook.

A few tests failed - Black, the test that checks the docs mention the new hook - the most interesting failing test looks like this one: ``` updated_metadata["databases"]["fixtures"]["queries"]["magic_parameters"][ "allow" ] = (allow if "query" in permissions else deny)

      cascade_app_client.ds._metadata = updated_metadata

E AttributeError: can't set attribute ``` From https://github.com/simonw/datasette/blob/0a7621f96f8ad14da17e7172e8a7bce24ef78966/tests/test_permissions.py#L439-L467

This test is directly manipulating _metadata purely for the purposes of simulating different permissions - I think updating it to manipulate _local_metadata instead would fix that.

Note that if we went with a "update_metadata" hook, the hook signature would look something like this (it would return nothing):

update_metadata( datasette=self, metadata=metadata, key=key, database=database, table=table, fallback=fallback )

The Datasette function _metadata_recursive_update(self, orig, updated) would disappear into the plugins. Doing this, though, we'd lose the easy ability to make the local metadata.yaml immutable (since we'd no longer have the recursive update).