Queue and cancel Builds on one Machine #807

Parent

Build runner spec (#798)

What to build

Let concurrent clients wait for the selected Machine's single active Build slot without uploading source early or creating durable jobs. This extends the remote attempt already implemented; it is not a cluster queue or a Deploy scheduler.

Acceptance criteria

• Replace the remote slice's temporary busy refusal with one active Build and an in-memory FIFO of eight waiting Builds by default. Configure queue capacity, ten-minute queue timeout and thirty-minute active timeout through Machine-local policy, validating values at the input seam. A full queue rejects immediately and cannot be enlarged by a request.
• Keep each waiting client's capture and secrets on the client and signal admission before upload. Measure queue waiting separately from admitted execution, which includes upload/preparation/build/import/cleanup. Hold one slot across every solve of a multi-platform Build without making the queue depend on recipe-specific code.
• Remove a queued request on cancellation, disconnect or queue timeout. Active cancellation, observed disconnect or execution timeout requests bounded termination and cleanup; no downstream image publication or Deploy continues. If termination cannot be confirmed, report unknown and keep conflicting builder/cache ownership unavailable until safe.
• On daemon restart, discard the in-memory queue and fail waiting commands; never replay work or mark interrupted work successful. Handle owned abandoned resources through bounded lifecycle cleanup before unsafe reuse. Cleanup does not become a background build-recovery workflow.
• Show selected Machine, queue waiting versus execution, full/expired/cancelled/unknown outcomes, and completed versus unattempted work through the existing build progress/result interface. Add no public queue-management handshake, status polling, database, leases, automatic retries, or queue resource to the Cluster model.
• Use rung-1 policy tests with controlled time for FIFO order, bounds, expiry, disconnect and unreleased ownership after unknown termination. Use rung-2/3 transport/CLI checks for upload-after-admission and outcome rendering. Register a rung-4 multi-client exercise covering queued disconnect, active cancellation, upload timeout, daemon restart, and a subsequent safe build.

Blocked by

• #802 — Build a Dockerfile on an explicitly selected Machine