"Environment data not found in the schema"

[gisborne]

I've somehow gotten into a state where parallel specs cause this:

rake aborted!
ActiveRecord::NoEnvironmentInSchemaError: 

Environment data not found in the schema. To resolve this issue, run: 

        bin/rails db:environment:set RAILS_ENV=test

It also reports some of the most recent specs have not been run, even though they have.

When I dig into the source of the error, the process that is throwing the exception sees an empty ar_internal_metadata table, although if I look in that table in the test database myself, the environment value is there before and after trying to run specs.

A similar issue was reported in Rails 5.