What should be done?

ferretdb --test-enable-new-auth --setup-timeout=1m --setup-username=user --setup-password=pass ...

When --setup-username flag is passed, FerretDB should create the specified user with the given password (that may be empty). It should not be an error if a user with that name already exists. It should do so without starting the handler but by using backends directly. It should try to establish a working backend connection up --setup-timeout.

test-enable-new-auth's value must be true.

Let's split the implementation into multiple PRs to make reviews easier and faster:

• Implement dummy flags that only check that --test-enable-new-auth is true when --setup-username is non-empty.
• Extract user creation logic into a separate function in the internal/backends package. It should accept a backend, username, and password. Use that function in the createUser command handler.
• Use that function in the CLI implementation. Implement retries by using Backend.Status method and ctxutil.SleepWithJitter function.

Testing should be done manually.

Where?

• https://github.com/FerretDB/FerretDB/tree/main/cmd/ferretdb
• https://github.com/FerretDB/FerretDB/tree/main/internal/backends

Definition of Done

• handler updated;
• spot refactorings done.