gRPC builder stream silently dies, builder never reconnects

## Summary The gRPC bidirectional stream between `hydra-builder` and the queue runner silently dies. When this happens: 1. The **builder process keeps running** but stops receiving new work — no child `nix` processes, idle CPU 2. The **queue runner** still considers the builder's job slots as occupied (`currentJobs: 4`) and never reclaims them 3. `sinceLastPing` on the runner side grows indefinitely (observed 51,696s / ~14h and 39,156s / ~10.9h) 4. The builder's `--ping-interval 10` does not detect the dead stream or trigger reconnection The only fix is to manually restart the builder service (`launchctl kickstart -k`), after which it immediately reconnects and starts receiving builds. ## Observed pattern From the builder logs, the sequence before going silent is typically: ``` INFO Finished building <drv> INFO Start uploading paths to queue runner directly INFO Finished uploading paths to queue runner directly. elapsed=138ms INFO Successfully completed build process for <drv> INFO Building <next-drv> ← receives new build warning: file ... does not exist in binary cache ... ← fetching inputs ... (silence — no more log entries for 10+ hours) ``` The builder appears to get stuck during the input-fetching phase of a build. Meanwhile the gRPC stream dies — possibly because the long input-fetch blocks the ping response, or a network interruption goes undetected. From the queue runner's `/status` endpoint: ```json { "hostname": "builder-A", "sinceLastPing": 51696, "currentJobs": 4, "failedBuilds": 0, "succeededBuilds": 161 } ``` Other builders on the same network show `sinceLastPing: 2-6` and are healthy. ## Expected behavior - The builder should detect the dead gRPC stream (via ping timeout or TCP keepalive) and reconnect automatically - The queue runner should have a configurable timeout for `sinceLastPing` — after which it marks the builder as disconnected, reclaims the job slots, and reschedules the builds on other machines ## Environment - Queue runner: `hydra-queue-runner 0.1.0-c1fe4808` - Builder: `hydra-builder 0.1.0-c1fe4808` - `--ping-interval 10` configured on all builders - 10 darwin builders on a LAN, connected to queue runner via IPv6 - Observed across multiple builders, recurring after restarts ## Workaround Restart the builder service. It reconnects immediately and starts receiving builds within seconds.