Tracing the Supervisor Loop: How a Missing Port Caused a Silent Failure

In the middle of a high-stakes debugging session for a distributed GPU proving network, the assistant issued a single read command to inspect a shell script. Message [msg 1416] appears unremarkable at first glance — just another file read in a long chain of investigation. But this message represents a critical pivot point: the moment when the assistant stopped treating the symptom (curio failed to start) as the full story and began questioning whether the system's own monitoring was lying to it.

The Message

The assistant read lines 300–314 of /tmp/czk/docker/cuzk/entrypoint.sh:

300:         sleep 1
301:     done
302: 
303:     if [ "$cuzk_ready" != true ]; then
304:         log "cuzk failed to start, restarting in 5s..."
305:         kill "$CUZK_PID" 2>/dev/null || true
306:         CUZK_PID=""
307:         sleep 5
308:         continue
309:     fi
310:     log "cuzk ready"
311: 
312:     # Start curio
313:     log "Starting curio (listen=$CURIO_LISTEN)..."
314:     curio run --listen "$CURIO_LISTEN" > "$CURIO_LOG" 2>&1 &

This is the supervisor loop — the heart of the worker instance's lifecycle management. It controls the startup sequence for the two main processes: cuzk-daemon (the GPU proving engine) and curio (the Lotus-based coordination layer). The code reveals a critical asymmetry: cuzk gets a restart loop with a continue statement that jumps back to retry, but curio gets launched once in the background with no failure handling whatsoever.

Why This Message Was Written

The context is essential. The user had reported ([msg 1408]) that curio failed to start on a newly deployed instance (an RTX PRO 4000, label C.32729742), with a clear error: "Not able to establish connection to node with addr: ws://127.0.0.1:1234/rpc/v1". The assistant had already identified the root cause — the portavailc tunnel on the worker was not forwarding port 1234, which hosts the Lotus API. The fix was applied in [msg 1413] by adding -L 1234 to the tunnel command.

But then the assistant noticed something deeply puzzling. The dashboard showed this same instance as running ([msg 1415]). How could curio have failed to start, yet the instance be in a running state? Either the dashboard was wrong, or the failure was transient and curio had since recovered, or — most concerningly — the state machine was lying.

This is the moment the assistant realized that understanding the supervisor loop's behavior was not optional. The fix to the port tunnel might be correct, but if the supervisor loop silently swallowed failures, the system could be in a degraded state without anyone noticing. The assistant needed to trace the exact control flow to answer a specific question: what happens after curio fails?

The Thinking Process Revealed

The assistant's reasoning in the preceding messages shows a methodical, almost forensic approach. Let me reconstruct the chain:

  1. Symptom recognition ([msg 1408]): The user reports curio can't connect to 127.0.0.1:1234. The assistant immediately recognizes this as a tunnel problem because the architecture uses port forwarding through portavaild (controller) and portavailc (worker).
  2. Root cause identification (<msg id=1409–1413>): The assistant checks portavaild on the controller — it's forwarding port 1234. Then checks the entrypoint's portavailc invocation — it only forwards 1235, 5433, and 9042. The fix is obvious: add -L 1234 to the tunnel. This is applied in [msg 1413].
  3. The contradiction ([msg 1415]): After fixing the tunnel, the assistant checks the dashboard to see if the fix will help future instances. It notices instance C.32729742 (the one that reported the error) shows as running. The assistant explicitly flags this: "The running one (32729742 RTX PRO 4000) must have gotten past the lotus connection somehow, or it was from before this bug." Then, with a dawning realization: "Actually wait — the error message says ...742 — that IS the RTX PRO 4000 instance. It says 'try to fix the instance before restarting'. So curio failed to start on that one, but the state shows running..."
  4. Deep investigation ([msg 1416]): The assistant reads the supervisor loop code to understand the discrepancy. It needs to see the exact lines that handle curio startup and failure. This is not a casual read. The assistant is deliberately seeking lines 300–314 because those contain the boundary between the cuzk restart loop and the curio launch. The grep command in [msg 1411] had already found that curio run appears at line 314. Now the assistant wants the full context around that line.

What the Code Reveals

The supervisor loop structure is now laid bare:

Lines 300–301: The closing brace of an inner wait loop (probably waiting for cuzk to signal readiness via a health check or port availability).

Lines 303–308: The cuzk failure branch. If cuzk is not ready, the script logs the failure, kills the cuzk process, clears the PID variable, sleeps 5 seconds, and hits continue — which jumps back to the top of the while loop to retry. This is robust: cuzk gets infinite retries with a 5-second cooldown.

Lines 310: The success path. If cuzk is ready, log it.

Lines 312–314: Curio launch. This is a one-shot background launch with no retry logic, no health check, and no failure handling. The &amp; at the end means curio runs in the background, and the script immediately continues to whatever comes next (likely entering a monitoring/sleep loop).

The critical insight: if curio fails to start, the supervisor loop does nothing. It doesn't log the failure, doesn't retry, doesn't transition the instance state. The cuzk restart loop continues running, the supervisor process stays alive, and from the outside, the instance appears "running" because the SSH connection is up and the supervisor process is active. But curio is dead.

Assumptions and Their Implications

The assistant made several assumptions in pursuing this line of inquiry:

Assumption 1: The dashboard state reflects curio health. This turned out to be false. The running state in the dashboard is set by the vast-manager based on whether the instance has completed benchmarking and the supervisor is alive — it does not independently verify that curio is running. The assistant discovered this by tracing the code rather than trusting the UI.

Assumption 2: The supervisor loop would handle curio failures similarly to cuzk failures. This was also false. The code shows a stark asymmetry: cuzk gets a full restart loop, curio gets nothing. This design choice likely reflects the assumption that curio is more stable or that its failure modes are non-recoverable — but neither assumption is documented or verified.

Assumption 3: The port 1234 fix alone would resolve the issue. While the fix is necessary, the assistant implicitly recognized that even with the fix, existing instances like C.32729742 would remain broken because the supervisor loop wouldn't restart curio. The fix would only help newly deployed instances or instances where the supervisor loop happens to restart.

Input Knowledge Required

To understand this message, a reader needs to grasp several layers of the system architecture:

  1. The portavail tunneling system: portavaild runs on the controller host and listens on port 22222, forwarding specific ports (1234, 1235, 5433, 9042, 4701) to the controller's localhost. portavailc runs on each worker instance and connects back to the controller, creating reverse tunnels. Port 1234 hosts the Lotus JSON-RPC API, which curio needs to connect to for chain interactions.
  2. The instance lifecycle state machine: Instances progress through states: registered → params_done → bench_done → running. The vast-manager on the controller tracks these states and kills instances that underperform.
  3. The supervisor loop pattern: The entrypoint.sh runs an infinite while loop that starts cuzk, waits for it to be ready, starts curio, and then presumably enters a monitoring phase. This is a common pattern for containerized multi-process applications.
  4. The relationship between cuzk and curio: cuzk-daemon is the GPU proving engine that generates proofs. curio is the coordination layer that communicates with the Lotus blockchain node. Curio depends on Lotus (port 1234) being reachable, which in turn depends on the portavail tunnel being correctly configured.

Output Knowledge Created

This single read operation produced several valuable insights:

  1. Confirmation of the failure mode: The supervisor loop does not restart curio on failure. This means the port 1234 tunnel fix, while necessary, is not sufficient to recover already-broken instances. They would need to be manually restarted or the supervisor loop would need to be enhanced.
  2. A design vulnerability: The asymmetry between cuzk and curio error handling represents a latent bug. Any transient failure that prevents curio from starting (network blip, Lotus restart, port conflict) will leave the instance in a permanently degraded state — appearing "running" but actually non-functional.
  3. A monitoring blind spot: The vast-manager dashboard cannot distinguish between "supervisor is running" and "curio is running". This means the system could have multiple instances in a zombie state, consuming resources but producing no proofs.
  4. A path forward: The assistant now knows that the entrypoint needs a curio health check and restart loop, similar to the cuzk loop. Alternatively, the vast-manager could add a curio-specific health probe. Either way, the knowledge from this read directly informs the next engineering decision.

The Deeper Lesson

Message [msg 1416] exemplifies a pattern that recurs throughout complex system debugging: the most important questions are often about the system's own perception of itself. The assistant could have stopped after fixing the port tunnel — the root cause was identified, the fix was applied, problem solved. But the assistant noticed a contradiction between two sources of truth (the error log and the dashboard) and chose to investigate.

This is the difference between fixing a bug and understanding a system. The port tunnel fix addresses the immediate failure, but the supervisor loop investigation reveals a deeper architectural issue: the system has no mechanism to detect or recover from curio failures. A truly robust system would need both the tunnel fix and a curio health monitor. The assistant's read operation in [msg 1416] was the step that uncovered this second, deeper problem.

In distributed systems, the most dangerous failures are the silent ones — the ones that don't trigger alerts because the monitoring system itself is built on incorrect assumptions. The assistant's willingness to question the dashboard, to trace the code path, and to understand the exact failure semantics is what separates a superficial fix from a thorough one. This message, a simple file read, is the artifact of that investigative rigor.