Stream build progress

The same record as GET builds/{id}/progress, as text/event-stream, for as long as the build runs, up to 50 minutes. A build cannot run that long, so a stream still open then ends with an error event rather than closing quietly.

Three event types. progress carries a BuildProgress and is sent only when something actually moved, so every one of them is news. done carries the final BuildProgress and is the last event of a build that finished — including one that FAILED, which is a done whose status says failed rather than an error. error is the last event of a stream that could not go on, and says nothing about the build. Its data is {request_id, error} when the hypervisor running the build stopped answering or the stream reached its 50 minutes; poll for the outcome. When your authority went while the stream was open — the key deleted, the membership removed, the account suspended — it also carries status (401 or 403) and reason: "revoked", the refusal the same request would get now; polling is refused the same way, so do not poll with that credential.

Attaching to a build that has already finished is not an error: you get one progress and one done immediately. Lines beginning : are keepalives and can be ignored.

An account may hold eight of these open at once; the ninth is refused with a 429 naming the limit. It is a bound on concurrency rather than on price, because a stream is charged once and then polls for as long as the build runs. There is one build per hypervisor, so this is more streams than a fleet has builds to watch.

Needs an account-wide key: one confined to a workspace is a 403.

Requires the viewer role or higher.