The Smoking Gun: Tracing an Intermittent SNARK Failure to the FFI Layer

Introduction

In the course of a deep investigation into a PSProve PoRep (Proof of Replication) failure in the CuZK proving engine, a single message marked a decisive turning point. After days of methodical debugging—tracing enum mappings across Go, C, and Rust, examining JSON serialization round-trips, and ruling out seed masking as a cause—the assistant delivered a concise but powerful conclusion in message 1745: "This is the smoking gun." The message announced that the long-suspected culprit, the Go JSON round-trip, was definitively exonerated, and the true source of the intermittent failures was something far more fundamental: the FFI (Foreign Function Interface) layer's own proof generation was non-deterministic, sometimes producing invalid SNARKs even when called with identical inputs.

This article examines that message in depth: the reasoning that led to it, the assumptions it overturned, the knowledge it created, and the strategic pivot it triggered in the debugging effort.

The Context: A Long-Running Investigation

The assistant had been investigating why PSProve PoRep proofs intermittently failed with the error "porep failed to validate" in production. The investigation had already spanned multiple sessions and involved:

• Tracing RegisteredSealProof enum mappings across Go, C, and Rust to ensure structural parity
• Ruling out fr32 seed masking as the cause by tracing the complete seed flow through Rust proof crates and Filecoin chain actors
• Adding diagnostic logging to computePoRep in task_prove.go
• Extending the 2KiB roundtrip test suite to cover CuZK wrapper and FFI C2 verification paths
• Running extensive tests comparing raw Rust JSON paths against Go-roundtripped JSON paths The leading hypothesis had been that the Go JSON serialization/deserialization round-trip was corrupting proof data when it passed through the PSProve pipeline. This made intuitive sense: the PSProve flow involved Rust C1 output being unmarshaled into Go structs, then marshaled back to JSON for the CuZK wrapper, and if any field was lost or transformed incorrectly during this double round-trip, the proof would fail verification downstream.

The Critical Experiment

The message in question was triggered by the results of a carefully designed experiment: TestRepeatedC2SameSector. This test sealed a single 2KiB sector and then called ffi.SealCommitPhase2 multiple times on the same data, comparing raw Rust JSON against Go-roundtripped JSON. The logic was elegant: if the Go round-trip was corrupting data, then raw Rust JSON should pass consistently while Go-roundtripped JSON would fail intermittently. If both failed, the bug was upstream of the round-trip.

The test results were unambiguous:

iteration 0: raw Rust JSON C2+verify PASSED
iteration 1: SealCommitPhase2 with raw Rust JSON failed: post seal aggregation verifies
...
iteration 0: Go-roundtripped JSON C2+verify PASSED
iteration 1: Go-roundtripped JSON C2+verify PASSED
iteration 2: Go-roundtripped JSON C2+verify PASSED
iteration 3: SealCommitPhase2 with Go-roundtripped JSON failed: post seal aggregation verifies

Both raw Rust JSON and Go-roundtripped JSON failed intermittently. The raw path failed on the second call; the Go path failed on the fourth call. This pattern—identical inputs producing different outputs across repeated calls—pointed to a non-deterministic failure in the proof generation itself.

The Reasoning and Motivation

The message was written to communicate this breakthrough and to pivot the investigation. The assistant's reasoning, visible in the message's structure, follows a clear logical arc:

1. Assert the finding: "This is the smoking gun." — a definitive statement that the core question has been answered.
2. Interpret the evidence: "Both raw Rust JSON AND Go-roundtripped JSON fail intermittently. The issue is NOT the Go JSON round-trip at all."
3. Identify the true source: "It's ffi.SealCommitPhase2 itself producing invalid proofs intermittently when called multiple times in the same process."
4. Formulate a hypothesis: "This is a bellperson/FFI bug — the proof generation is non-deterministic and sometimes produces invalid SNARKs."
5. Propose the next question: "Let me check if this is a known issue with 2KiB test sectors specifically, or something more fundamental." The motivation was to redirect the investigation away from the Go JSON round-trip (which had consumed significant effort) and toward the actual root cause in the FFI/bellperson layer. The message also served to document the finding for the record, as the assistant was operating in a context where results needed to be captured and communicated.

Assumptions Made and Overturned

Several assumptions were tested and overturned by this message:

Assumption 1: The Go JSON round-trip was the likely cause. This was the dominant hypothesis for much of the investigation. The PSProve pipeline involved multiple serialization/deserialization steps, and it seemed plausible that one of them was corrupting data. The test results proved this assumption wrong: the raw Rust JSON path (which bypasses Go entirely) failed just as often.

Assumption 2: The FFI's SealCommitPhase2 was deterministic. The assistant had implicitly assumed that calling SealCommitPhase2 with the same inputs would produce the same output. The intermittent failures revealed that this was not the case—the proof generation had a non-deterministic component that sometimes produced invalid SNARKs.

Assumption 3: The intermittent failures in production were cuzk-specific. Since the production failures occurred when using CuZK's pipeline proving modes (Phase 6 and Phase 7), the team had focused on cuzk's code. The discovery that the same failure occurred in the plain FFI path (without cuzk involvement) showed that the bug was more fundamental.

Input Knowledge Required

To fully understand this message, one needs knowledge of:

• The PSProve proof pipeline: How PoRep proofs flow from Rust C1 generation through Go serialization to CuZK C2 generation and verification.
• The FFI layer: How Go calls into Rust's filecoin-proofs library via CGO, and that SealCommitPhase2 is the Rust function that generates the final SNARK proof.
• The error message: "post seal aggregation verifies" — this is Rust's own self-verification check inside seal_commit_phase2, which generates a proof and immediately verifies it. If verification fails, it returns this error.
• The test infrastructure: The 2KiB sector tests that had been developed to isolate the issue, including TestRepeatedC2SameSector which sealed one sector and called C2 multiple times.
• The previous investigation: The enum mapping analysis, seed masking investigation, and JSON round-trip tests that had ruled out other potential causes.

Output Knowledge Created

This message created several important pieces of knowledge:

1. The Go JSON round-trip is not the cause of PSProve failures. This eliminated a major line of investigation and saved significant future effort.
2. The FFI's SealCommitPhase2 is non-deterministic. When called multiple times in the same process with identical inputs, it sometimes produces invalid proofs that fail its own internal verification.
3. The intermittent failure pattern matches production behavior. The same "post seal aggregation verifies" error that appeared in tests was likely the same error occurring in production when cuzk's pipeline modes produced invalid proofs.
4. The bug is in the bellperson/supraseal layer. Since the FFI calls into Rust's filecoin-proofs library, which uses bellperson for SNARK proving and supraseal for GPU acceleration, the non-determinism likely originates in one of these lower layers.
5. A new investigative direction is needed. The focus should shift from data integrity (JSON round-trips) to proof generation stability (GPU proving, parameter caching, or global state issues in the FFI).

The Thinking Process

The assistant's thinking process, visible in the message's progression, shows a methodical approach to hypothesis testing:

First, the assistant recognized the significance of the test results immediately. The phrase "This is the smoking gun" indicates that the test had been designed specifically to resolve the central question, and the results were conclusive.

Second, the assistant interpreted the pattern correctly. Both paths failing meant the issue was upstream of the JSON round-trip. The raw Rust JSON path failing was particularly telling because it eliminated all Go code from the equation.

Third, the assistant formulated a hypothesis about the nature of the bug: "bellperson/FFI bug — the proof generation is non-deterministic and sometimes produces invalid SNARKs." This was a precise characterization that would guide future investigation.

Fourth, the assistant immediately planned the next step: checking whether this was a 2KiB-specific issue or something more fundamental. This showed strategic thinking—if the bug only affected small test sectors, it might be a different problem than the production issue with full-size sectors.

The message also includes a second test run (TestMultipleSectorsC2) which confirmed the finding with a different test structure, strengthening the conclusion.

The Broader Impact

This message was pivotal because it fundamentally changed the trajectory of the investigation. Before it, the team was focused on data integrity and serialization correctness. After it, the focus shifted to the stability of the proof generation itself. The subsequent work involved:

• Fixing cuzk's pipeline modes to reject invalid proofs (making the self-check mandatory)
• Deploying the fix to production
• Proactively patching the same bug in two additional pipeline paths (batched multi-sector and single-sector)
• Hot-swapping the production daemon with a locally built binary The message also demonstrated the value of well-designed experiments. The TestRepeatedC2SameSector test was elegant because it controlled for the data variable (using the same sector data) and isolated the call repetition variable. This design made the non-determinism visible in a way that earlier tests (which sealed fresh sectors each time) had not.

Conclusion

Message 1745 represents a classic debugging breakthrough: the moment when a carefully designed experiment eliminates a leading hypothesis and reveals the true nature of a bug. The assistant's concise conclusion—"This is the smoking gun"—captures the significance of the finding. The Go JSON round-trip was not the problem; the FFI's own proof generation was non-deterministic. This discovery redirected the investigation toward the bellperson/supraseal layer and ultimately led to a fix that made cuzk's self-check mandatory, preventing invalid proofs from reaching the ProofShare protocol.

The message also exemplifies good debugging practice: formulate hypotheses, design experiments that can falsify them, interpret results objectively, and pivot when the evidence demands it. The TestRepeatedC2SameSector test was the key that unlocked the mystery, and message 1745 was the announcement that the lock had been opened.