The Three-Character Fix: How a Job ID Collision Nearly Broke Production ProofShare

The Message

[assistant] [edit] /tmp/czk/tasks/proofshare/task_prove.go
Edit applied successfully.

This is the entirety of message 1982 in the conversation. On its surface, it is a banal confirmation from an AI coding assistant that a file edit tool completed without error. Yet this three-line message represents the culmination of a debugging odyssey that spanned multiple days, crossed language boundaries between Go and Rust, traversed GPU proving pipelines, and ultimately fixed a production outage that was silently corrupting every single proof in a distributed Filecoin ProofShare system.

The Context: A Production System in Distress

To understand why this trivial confirmation matters, we must reconstruct the crisis that led to it. The ProofShare system is a distributed proving marketplace where GPU workers compete to generate proofs for Filecoin sectors. The user had deployed a new cuzk binary (the Rust-based GPU proving engine) that had passed all benchmarks successfully. Yet when the system ran at full capacity with multiple concurrent proof challenges, every single one of the ten PoRep partitions was producing invalid proofs. The verification log told a devastating story: "PER-PARTITION VERIFICATION: 0/10 valid."

The user had already eliminated numerous potential causes. They had ruled out a GPU flakiness hypothesis by running with proofshare_max_tasks=1 (serializing all work), which produced correct proofs. They had ruled out binary corruption by extracting a fresh build from a Docker container. They had ruled out data format mismatches through exhaustive enum mapping audits. The bug was exquisitely intermittent—it only manifested under parallelism.

The Reasoning: Tracing the Collision

Message 1978 contains the assistant's breakthrough reasoning. The key insight came from examining the job_id format string: "ps-porep-%d-%d", which interpolated only the miner ID and sector number. In the ProofShare system, all challenge sectors from the cusvc/powsrv service target the same artificial sector (miner=1000, sector=1) because the system uses a bench sector for all challenges. This means every concurrent PSProve task sends an identical job_id="ps-porep-1000-1" to the cuzk engine.

The assistant reasoned: "The job_id serves as a key in the engine's JobTracker.assemblers HashMap, so when concurrent proofs share the same job_id, their partition results collide and overwrite each other." This was confirmed by a Rust panic message: "partition 0 already inserted" at cuzk-core/src/pipeline.rs:1910:9. The partition assembler in the Rust engine was keying on job_id, so when two concurrent proofs both claimed "partition 0," the second insertion triggered a fatal panic.

The fix was conceptually simple: include the harmony task ID in the RequestId format string, changing it from "ps-porep-%d-%d" to "ps-porep-%d-%d-%d" (miner, sector, taskID). This would make each concurrent invocation's job_id unique, preventing the partition assembler from mixing results across different proofs.

The Execution: A Two-Edit Fix

Message 1980 was the assistant's first attempt. It edited task_prove.go to add the taskID parameter to the computePoRep call. However, this immediately triggered a compile error reported by the LSP: "too many arguments in call to computePoRep." The function signature didn't expect a task ID parameter.

Message 1981 was the second edit, presumably updating the computePoRep function definition to accept the new taskID parameter. The assistant had to thread the parameter through the call chain—from computeProof (which already received taskID but only passed it to computeSnap, not computePoRep), down to the actual RequestId formatting line.

Message 1982 confirms that this second edit was applied successfully. The LSP error was resolved. The code now compiled.

Assumptions and Knowledge

The assistant made several critical assumptions in this fix. First, it assumed that the Rust engine's JobTracker.assemblers HashMap was the only place where job_id collisions caused problems. This was a reasonable assumption given the "partition 0 already inserted" panic message, but it left open the possibility of other race conditions in the pipeline. Second, the assistant assumed that the non-proofshare cuzk path (in lib/ffi/cuzk_funcs.go) was safe because it used real sector IDs that are naturally unique per job. This was confirmed by grep: the normal PoRep path used "porep-%d-%d" with real miner/sector pairs that don't collide.

The input knowledge required to understand this message includes: the architecture of the ProofShare system (distributed proving marketplace), the role of cuzk (Rust GPU proving engine with a Go client), the concept of partition workers (splitting a proof into 10 partitions for parallel GPU processing), the job tracking system (HashMap-based assembler in the Rust engine), and the Go harmony task framework (which assigns unique task IDs to each invocation).

The Output Knowledge Created

This message produced a corrected source file that would, after rebuild and deployment, generate unique job_id values for every concurrent proof invocation. The output knowledge is the fix itself: the pattern of including a unique invocation identifier (taskID) in the request ID to prevent key collisions in distributed systems. This pattern is now documented in the codebase as a lesson learned.

Mistakes and Incorrect Assumptions

The assistant's initial fix in message 1980 was incomplete—it added the parameter to the call site without updating the function signature. This is a common oversight when making multi-site edits. The LSP error in message 1981 caught this immediately, and message 1982 represents the corrected version.

A more subtle assumption was that simply making the job_id unique would fix the proof validity issue. The assistant's reasoning implicitly assumed that the partition mixing was the sole cause of all 10 partitions being invalid. While the "partition 0 already inserted" panic and the 0/10 validity rate strongly supported this, there was a possibility that other issues (like the earlier cuzk binary problems) contributed. The user's experiment with proofshare_max_tasks=1 producing correct proofs was the key evidence that isolation was sufficient—proving that the collision was the root cause.

The Broader Significance

Message 1982 is a testament to the difficulty of debugging distributed proving systems. The bug crossed multiple abstraction layers: a Go task scheduler dispatched concurrent jobs, a Rust GPU engine tracked them by a string key, and a C++ CUDA backend executed the actual proving. The symptom (invalid proofs) was far removed from the cause (key collision). The fix itself was trivial—adding a single integer to a format string—but finding it required systematic elimination of hypotheses across language boundaries, careful reading of panic messages, and a controlled experiment with serialized execution.

This message also illustrates the iterative nature of AI-assisted coding. The assistant did not produce a perfect fix in one shot. It reasoned, read source files, made an edit, encountered a compile error, made a second edit, and confirmed success. Each cycle tightened the correctness of the change. The final message, with its mundane "Edit applied successfully," belies the complexity of the journey that produced it.