The Diagnostic That Changed Everything: Isolating an Intermittent Proof Failure in the FFI Stack

The Message

The subject message ([msg 1744]) is deceptively simple — a single bash command execution and its output:

``bash CGO_LDFLAGS="-L$(pwd)/extern/filecoin-ffi" go test -v -run "TestRepeatedC2SameSector" -count=1 -timeout 600s ./lib/proof/ 2>&1 ``

>

`` === RUN TestRepeatedC2SameSector === RUN TestRepeatedC2SameSector/raw-rust-json-repeated porep_vproof_test.go:533: iteration 0: raw Rust JSON C2+verify PASSED porep_vproof_test.go:516: iteration 1: SealCommitPhase2 with raw Rust JSON failed: post seal aggregation verifies === RUN TestRepeatedC2SameSector/go-roundtripped-json-repeated porep_vproof_test.go:558: iteration 0: Go-roundtripped JSON C2+verify PASSED porep_vproof_test.go:558: iteration 1: Go-roundtripped JSON C2+verify PASSED ``

This output, spanning just a few lines, represents a pivotal moment in a multi-day investigation into an intermittent production bug. The message is the culmination of a carefully designed experiment that would definitively reframe the entire debugging effort.

The Context: A Production Mystery

The investigation began with a troubling production issue: the CuZK proving engine was intermittently producing invalid PoRep (Proof of Replication) proofs, causing "porep failed to validate" errors in the ProofShare challenge protocol. The errors were sporadic — sometimes a proof would pass, sometimes it wouldn't — with no obvious pattern. This kind of intermittent failure is among the most difficult bugs to diagnose because it resists simple reproduction and suggests either a race condition, a state corruption, or a subtle data dependency.

The team had been pursuing two main hypotheses. The first was that the Go JSON serialization round-trip was corrupting the proof data. The PSProve pipeline involves serializing proof data from Rust into JSON, deserializing it in Go, then re-serializing it before sending it to the CuZK proving server. If any field was lost, truncated, or misinterpreted during this double round-trip, the proof would be invalid. The second hypothesis was that the CuZK GPU proving pipeline itself was producing bad proofs — perhaps due to a race condition in the partition assembly logic or a bug in the self-check gate.

Earlier tests had already produced tantalizing clues. The Go JSON round-trip tests (TestRoundtripPorepVproof) had passed consistently when run individually, but showed an intermittent "post seal aggregation verifies" error when run in sequence with other tests. This error message came from deep inside the Rust FFI library — specifically from filecoin-proofs/src/api/seal.rs at line 641, where the Rust code performs a self-verification after generating the SNARK proof. The error meant that the Rust proving stack itself was sometimes producing proofs that failed its own internal verification.

Why This Message Was Written

The message was written to execute a critical diagnostic experiment. The assistant needed to answer a fundamental question: Is the intermittent failure data-dependent or state-dependent?

The previous test runs had shown that TestCuzkWrapperRoundtrip — which seals a fresh sector each time it runs — would fail intermittently even when run in isolation with -count=5. Some iterations passed, some failed. But the failure pattern was opaque: was it because the random sector data was sometimes "unlucky" (a genuinely hard instance for the SNARK prover), or was it because some global state in the FFI was being corrupted between calls?

The assistant designed TestRepeatedC2SameSector specifically to distinguish between these two possibilities. The test seals one sector, then calls SealCommitPhase2 multiple times with the same data. If the failure is data-dependent, all iterations should either pass or fail consistently. If it's state-dependent, later iterations might fail even though the first succeeded.

This design choice reveals a sophisticated debugging methodology: instead of adding more logging or staring at code, the assistant created a controlled experiment with a clear binary outcome. The test name itself — TestRepeatedC2SameSector — encodes the experimental design: repeat the C2 phase on the same sector data.

The Assumptions Behind the Experiment

The experiment rested on several key assumptions:

First, that the Rust FFI's SealCommitPhase2 function is deterministic for identical inputs. If the proving algorithm is inherently randomized (e.g., using different blinding factors each call), then repeated calls with the same data could legitimately produce different results. The assistant implicitly assumed that the proof generation is deterministic — or at least that any randomness is captured in the input parameters (like the seed), which were held constant.

Second, that the Go JSON round-trip was not the primary cause. The test included both a "raw Rust JSON" subtest and a "go-roundtripped JSON" subtest. If the round-trip were the problem, the raw Rust JSON subtest should always pass while the go-roundtripped subtest might fail. The assistant was hedging — testing both paths simultaneously to gather comparative data.

Third, that the test environment was stable enough to produce meaningful results. Running GPU-accelerated proof generation in a test environment introduces variables like GPU memory fragmentation, driver state, and thermal conditions. The assistant assumed these were not the dominant factors.

The Result: A Paradigm Shift

The output was devastatingly clear. In the raw-rust-json-repeated subtest, iteration 0 passed, but iteration 1 failed with "post seal aggregation verifies". The same sector data, processed by the same Rust FFI function, produced a valid proof on the first call and an invalid proof on the second call.

This single result demolished the Go JSON round-trip hypothesis. If the raw Rust JSON path — which never touches Go serialization — also fails intermittently, then the bug cannot be in the Go JSON marshaling. The failure must be in the Rust FFI layer itself, or in the underlying proving stack (bellperson/supraseal).

The go-roundtripped-json-repeated subtest showed both iterations passing, which might seem contradictory. But this is consistent with the intermittent nature of the bug: sometimes the second call fails, sometimes it doesn't. The raw Rust path happened to hit the failure this time; the Go round-trip path might hit it on a subsequent run.

Input Knowledge Required

To understand this message, one needs knowledge of several layers of the system architecture:

  1. The PoRep proving pipeline: Proof of Replication in Filecoin involves two phases — Phase 1 (seal setup, generating CommR and other commitments) and Phase 2 (SNARK proof generation). The C2 step is SealCommitPhase2, which produces the final compact proof.
  2. The FFI boundary: Go code calls into Rust via CGo FFI bindings. The filecoin-ffi library wraps the Rust filecoin-proofs crate. The "post seal aggregation verifies" error originates in Rust's own self-check after proof generation.
  3. The CuZK architecture: CuZK is a GPU-accelerated proving system that replaces the standard CPU-based bellperson prover. It has its own pipeline with partition workers and slot-based processing, and includes a diagnostic self-check that was found to be non-fatal (returning proofs even when the self-check failed).
  4. The Go testing framework: The test uses Go's testing package with subtests (t.Run), and the -count and -run flags control test execution.

Output Knowledge Created

This message created several critical pieces of knowledge:

Definitive ruling out of the Go JSON round-trip hypothesis: The raw Rust JSON path failing proves that the bug exists downstream of the serialization layer. This saved the team from pursuing a dead end.

Confirmation of FFI/Rust layer instability: The Rust seal_commit_phase2 function is producing invalid proofs on repeated calls with identical inputs. This points to a state corruption issue — perhaps a global cache, a reused buffer that isn't properly cleared, or a GPU memory leak.

A reproducible test case: The TestRepeatedC2SameSector test provides a reliable way to trigger the bug, which is essential for debugging and verification of fixes.

Redirection of investigative effort: The team can now focus on the Rust FFI layer, the bellperson proving stack, and the supraseal GPU backend, rather than the Go serialization code.

The Thinking Process Visible in the Message

The assistant's reasoning is visible in the sequence of actions leading up to this message. In the preceding messages ([msg 1723] through [msg 1743]), the assistant systematically:

  1. Ran the existing test suite to establish a baseline
  2. Observed the intermittent "post seal aggregation verifies" error
  3. Traced the error to its source in the Rust code
  4. Recognized that the error occurred even without CuZK involvement (pure FFI path)
  5. Designed the TestRepeatedC2SameSector experiment to isolate data-dependence vs. state-dependence
  6. Wrote and compiled the test (with a minor fix for an fmt import)
  7. Executed the test and captured the results The message itself is the execution of step 7 — the moment of truth. The assistant chose to run the test with -count=1 (a single run) rather than -count=5 or higher, suggesting confidence that even one run would be revealing. The 600-second timeout indicates awareness that proof generation can be slow, even for 2KiB sectors.

Mistakes and Incorrect Assumptions

The assistant's implicit assumption that the Rust FFI is deterministic for identical inputs may be incorrect. If the proving algorithm uses internal randomness (e.g., for blinding or zero-knowledge properties), repeated calls with the same inputs could legitimately produce different proofs. However, the fact that the Rust code performs a self-verification and reports failure suggests that the proof is genuinely invalid, not merely different.

Another subtle assumption is that the test environment's GPU state is clean between iterations. If the FFI library initializes GPU resources lazily and fails to properly reinitialize for the second call, that could explain the pattern. The assistant did not attempt to reset GPU state between iterations.

The test also assumes that the sector sealing process (sealTestSector) is deterministic. If the Phase 1 output contains randomness, then even with the same sector data, the C1 output (and thus the C2 input) could differ between calls. However, the test seals once and reuses the C1 output, so this is controlled.

Broader Significance

This message represents a textbook example of the scientific method applied to debugging. The assistant formulated two competing hypotheses, designed an experiment with a clear falsification criterion, executed it, and used the results to eliminate one hypothesis. The one-line output — "SealCommitPhase2 with raw Rust JSON failed: post seal aggregation verifies" — was worth more than a thousand lines of code review.

The investigation would now pivot to the Rust FFI layer. The next steps would involve examining the bellperson/supraseal GPU proving backend for state corruption issues, adding retry logic for transient proving failures, or both. The fix that eventually emerged — making the CuZK self-check mandatory across all pipeline paths — addressed the symptom (invalid proofs reaching the caller) but the deeper issue of GPU proving instability remained an open concern.

In the broader narrative of the coding session, this message is the turning point where the team stopped chasing a serialization ghost and started confronting a genuine proving engine instability. The diagnostic test the assistant designed became a permanent part of the test suite, serving as a canary for FFI-level issues in future development.