Write unit tests for the code herein

Important sections of this codebase are protected by `sync.Mutex`es or `sync.RWMutex`es, so special attention should be paid to writing concurrent tests (so that we can run `go test -race`)

Start by surveying the Go code protected by sync.Mutex and sync.RWMutex; the issue does not name specific files, tests, or entry points. Add unit coverage for the important sections, include concurrent tests, and run go test -race to verify the result.

Written by the indexing model from the issue text.