Private-shard runner managers stop polling for exactly 1h
🤖 *beep boop — clanker comment.*
## Summary
On the `private` shard (the only one with `FF_USE_JOB_ROUTER`), 4 to 8 of the 108 gprd runner managers are at any time stuck in a `GetJob` gRPC call that returns after exactly 3600 s with HAProxy's 504 error page. ~145 episodes a day. While stuck, the manager polls nothing and runs nothing; the pod is `Running`, no restarts, no health-check failures, no log line until the hour is up.
Four things line up to produce this:
1. **KAS drains a connection.** `GracefulStop` on pod shutdown, or `MaxConnectionAge` (2 h), so it happens continuously, with bursts on rollouts. grpc-go sends GOAWAY in two steps: first with last-stream-id 2³¹−1, a PING, then GOAWAY with the real last id N.
2. **grpc-go discards late streams silently.** Any stream HAProxy sent before it processed the first GOAWAY has id > N. grpc-go v1.83.2 `internal/transport/http2_server.go:546` drops it (`state != reachable` → `s.cancel(); return nil`) without a RST_STREAM. RFC 7540 §6.8 allows that and puts the retry on the client.
3. **HAProxy never errors those streams.** HAProxy 2.8.16 (`src/mux_h2.c` `h2c_handle_goaway`) keeps only the first GOAWAY's last-stream-id (`if (h2c->last_sid < 0) h2c->last_sid = last`). On the second GOAWAY it wakes streams above N, but `h2s_wake_one_stream` only errors a stream when `id > h2c->last_sid` = 2³¹−1, so nothing happens. The stream waits for `timeout server`, which is `1h` on the `kas_grpc` backend (set for the agent tunnels), then HAProxy returns its 504 errorfile. Same code in HAProxy v3.4.0.
4. **The runner has no deadline and one request slot.** `router/client.go` calls `GetJob` with a context that is only cancelled on shutdown. gRPC keepalive (`Time: 55s`) does not help, PINGs are answered hop-by-hop by Cloudflare and HAProxy. `FF_USE_ADAPTIVE_REQUEST_CONCURRENCY` (default on) multiplies the limit by 0.95 on every `no_job`, so on this mostly idle shard 73 of 117 managers sit at an effective `request_concurrency` of 1 (configured: 4). One lost stream stops all polling on that manager for the hour.
## Evidence
Runner side (`mimir-gitlab-gprd`, gprd, 108 private managers, 24 h to 2026-09-11 09:30Z):
- Stuck managers: zero `gitlab_runner_job_router_get_job_duration_seconds_count` increase over 10 m with `gitlab_runner_request_concurrency = 1`, and `gitlab_runner_request_concurrency_exceeded_total` climbing at ~74/5 min (the 4 s `check_interval` loop finding the slot taken). `gitlab_runner_request_concurrency_hard_limit` = 4, `_adaptive_limit` = `_used_limit` = 1.
- On recovery the histogram `_sum` jumps by the hang length: 124 events of ~3600 s with `result="error"`, 11 `no_job`, 3 `job`.
- Log line at recovery (113 in gprd, 44 in gstg):
```
Error requesting a job rpc error: code = Unavailable desc = unexpected HTTP status code received from server: 504 (Gateway Timeout); transport: received unexpected content-type "text/html" data: "<!DOCTYPE html> ... <title>504 Server Error</title> ...
Using job router at grpcs://kas.gitlab.com
```
- Hang starts (504 time − 3600 s) cluster after KAS pod churn: 13 in 05:40–05:50Z after the 05:36Z KAS rollout, 5 at 10:40Z, 3 at 14:30Z, 4 at 17:50Z, each within minutes of new `gitlab-kas` pods. The rest fall on single-pod HPA churn and connection age-out.
KAS side: `job_router_get_job_duration_seconds` (job `gitlab-kas`, gprd) has ~6 observations over 60 s in the same 24 h against ~145 on the runner side; per-result totals agree to 0.1 %. KAS never ran a handler for the hung streams. Its 30-minute `longPollResponseHeaderTimeout` toward Workhorse never fires.
HAProxy side (BigQuery `gitlab-production.haproxy_logs`, `kas~` frontend lines in `jsonPayload.message`, format `%Tq/%Tw/%Tc/%Tr/%Tt`):
- 156 × `kas_grpc/kas-gke ... 504 ... sH--` with `~4000/0/0/-1/3604000`: request received, then server-side timeout waiting for response headers after exactly 3,600,000 ms. 115 `JobRouter/GetJob`, 30 `AgentConfiguration/GetConfiguration`, 11 `ReconcileProjects`, so agentk RPCs hit the same thing.
- 97 × `GetJob 200 ----` with `3600829/0/0/125/3600954`, each paired with an `sH` on the same LB from the same client `ip:port`, started one `check_interval` (4 s) or one long-poll (52 s) later, released within a second of the 504: a manager at effective concurrency 2 sent both and the second was held behind the first.
- Runner-side (113 log lines) and HAProxy-side (115 `sH` GetJob) counts agree.
## What would help
- **Runner: deadline on `GetJob`** (`router/client.go`). `DeadlineExceeded` is already a breaker failure in `isRouterFailure`. Bounds the loss to one deadline instead of an hour, for every cause. The direct HTTP path already has a 10 m `ResponseHeaderTimeout`; the router path has nothing.
- **Runner: keep the adaptive limit from reaching 1**, or turn `FF_USE_ADAPTIVE_REQUEST_CONCURRENCY` off on the private shard in `argocd/apps` (config change). With 4 slots a lost stream costs a quarter of a manager's polling instead of all of it. The stuck request also counts as `no_job` on release and lowers the limit further.
- **HAProxy: report upstream** that `h2c_handle_goaway` ignores a later GOAWAY with a smaller last-stream-id, so streams above it are never errored; with that fixed, `retry-on` could re-send them. Separately, route `/gitlab.agent.job_router.rpc.JobRouter/` to its own backend with a `timeout server` of a couple of minutes; the 1 h exists for the agent tunnels.
- **KAS: nothing to change in the discard** (spec-conformant), but for the planned push model: on a proxied h2 stream only application-level messages prove liveness, so a long-lived job stream needs periodic server messages and a client-side "no message for N seconds" cutoff.
- **Alert** (works on live data):
```promql
(sum by (cluster, shard, pod) (increase(gitlab_runner_job_router_get_job_duration_seconds_count{cluster=~"runner-managers-.*"}[10m])) == 0)
and on (pod) (gitlab_runner_request_concurrency{cluster=~"runner-managers-.*"} >= 1)
```
## Open
- Staging shows the same 3600 s hangs plus a mode of exactly 600 s (23 in 24 h, also 504); the 600 s value is not in the shared HAProxy attributes.
- The 63 × `ReverseTunnel/Connect 400 cH--` at 3600 s are agentk tunnels hitting `timeout client 1h`; probably expected, not checked.
- Confirming the GOAWAY race directly needs `show sess` / h2 traces on an LB during a live hang, or a reproducer (HAProxy 2.8 → grpc-go server with `MaxConnectionAge` in the seconds range, a client polling at a few requests per second).
## Related
- gitlab-org/gitlab-runner#39726 (409 disables a manager for an hour), gitlab-org/gitlab-runner!7299
- grpc-go GOAWAY background: https://github.com/grpc/grpc-go/issues/6830 (referenced in KAS `internal/cmd/kas/agent_server.go`)
- Runner-side code: `router/client.go`, `router/client_conn_factory.go`, `commands/builds_helper.go` (AIMD), `commands/multi.go` `doJobRequest`
- KAS: `internal/module/job_router/server/`, `internal/cmd/kas/agent_server.go`
- HAProxy config: `gitlab-cookbooks/gitlab-haproxy/attributes/default.rb` (`kas_grpc` `timeout server 1h`), `templates/default/frontends/kas.erb`
/cc @vtak @timofurrer @ash2k @tmaczukin @rsarangadharan @jarv @cmcfarland
issue
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