Federating with a live Mastodon

The differential harness asks whether the two servers answer a client the same way. This one asks whether they can talk to each other: eunha's other federation tests run eunha against eunha, where both sides share eunha's reading of ActivityPub, so a misreading is invisible. This builds a pair that shares nothing but the specification.

scripts/federation_test.sh
scripts/federation_test.sh --keep      # leave both up to poke at

Twenty-four checks, both directions: resolve the account, follow, deliver a status, favourite it, boost it, delete it, and see each land on the other side.

Both servers run inside the container network. eunha used to run on the host behind a host-side Caddy, and that cannot work: mastodon.test is a network alias, so a host process cannot resolve it — eunha could not fetch an actor's public key and rejected every inbound activity with a 401. In the network they resolve each other by name, and the harness needs no /etc/hosts entry, no certificate trusted on the host, and no eunha built for this machine. The certificates are a throwaway CA made with openssl, trusted by both sides through SSL_CERT_FILE and by nothing else; the script drives both servers over plain published ports, which is why nothing on the host has to trust them.

Two things that cost a while, both recorded in the script:

• Rails refuses a request whose Host is not its LOCAL_DOMAIN — 403 on every endpoint, including those needing no authentication. Since mastodon.test does not resolve on the host, the only way in is the published port with the name supplied by hand. Without it the readiness loop spins forever against a Mastodon that is up and answering.
• A follow commits at different moments on the two sides. The sender listing the receiver as a follower is what makes it deliver; the receiver having committed the follow is what makes it keep what arrives, because Mastodon drops an activity from an account no local account follows yet. Waiting on the sender alone leaves a gap in which a status is delivered, accepted with a 2xx, and silently discarded — one run missed by 2.9ms, and it read as eunha failing to deliver.

The run also exercises what nothing else did: eunha delivers to Mastodon's shared inbox (https://mastodon.test/inbox), not its per-actor one. Mastodon delivers to eunha's per-actor inbox, which is its own choice when there is a single recipient on the far side.