Tracing the FFI Boundary: How a Single File Read Exposed the Root Cause of a PoRep PSProve Failure

Introduction

In the middle of a sprawling debugging session spanning platform hardening, UI development, and deep protocol-level investigation, a single file read operation stands as a pivotal moment of clarity. Message 1620 captures the assistant reading the implementation of ffi.SealCommitPhase1 in the filecoin-ffi Go wrapper. At first glance, this appears to be a routine code inspection — just another step in a long investigation. But this moment represents the culmination of a meticulous chain of reasoning, where the assistant's hypothesis about a JSON serialization round-trip bug finally meets the concrete evidence needed to confirm it.

The Message: A File Read at the Critical Moment

The subject message is deceptively simple. The assistant reads lines 389–401 of /tmp/czk/extern/filecoin-ffi/proofs.go, which shows the Go wrapper function SealCommitPhase1:

return cgo.SealCommitPhase1(
    sp,
    &commR,
    &commD,
    cgo.AsSliceRefUint8([]byte(cacheDirPath)),
    cgo.AsSliceRefUint8([]byte(sealedSectorPath)),
    uint64(sectorNum),
    &proverID,
    &ticketBytes,
    &seedBytes,
    cgo.AsSliceRefPublicPieceInfo(filPublicPieceInfos),
)

The function takes a registered proof type, commitment values, sector metadata, randomness seeds, and piece information, then delegates to the CGO layer. It returns []byte — raw bytes. The question that hangs over this entire investigation is: what format are those bytes in?

The Context: A Production Bug That Defied Easy Diagnosis

To understand why this file read matters, we must step back. The production system had a baffling failure mode: PSProve tasks (ProofShare challenge verification) worked perfectly for SnapDeals proofs but consistently failed for PoRep (Proof-of-Replication) proofs with the error "porep failed to validate". This was not a random crash — the SNARK proof was generated successfully (no error from CuZK or SealCommitPhase2), but the resulting proof failed verification.

The user's report in message 1601 framed the puzzle precisely: "Snap PSProve tasks before were fine, the validate error is on all porep challenge PSProves." This narrowed the problem to the intersection of two systems: PSProve (the task infrastructure for handling ProofShare challenges) and PoRep (the specific proof type). Normal CuZK PoRep C2 tasks worked. Normal PSProve Snap tasks worked. Only the combination of PSProve + PoRep failed.

Building the Hypothesis: The JSON Round-Trip Theory

The assistant's investigation had already traced through the entire code path by the time it reached message 1620. In messages 1603–1611, the assistant meticulously compared two paths:

1. The normal PoRep C2 path (cuzk_funcs.go): The vanilla proof bytes come directly from ffi.SealCommitPhase1() — raw bytes straight from the Rust FFI. These are wrapped by the shared wrapC1Output function and sent to CuZK.
2. The PSProve PoRep path (task_prove.go): The Commit1OutRaw struct arrives as JSON from the market, gets deserialized into Go structs via json.Unmarshal, and then re-serialized via json.Marshal before being wrapped and sent to CuZK. The critical insight emerged in message 1611: the PSProve path performs a JSON round-trip through Go structs, while the normal path passes raw Rust JSON bytes directly. If the Go serialization produced structurally different JSON than the original Rust output, the Rust CuZK engine might fail to deserialize the VanillaProof correctly, causing verification to fail.

Why This File Read Matters

This brings us to message 1620. The assistant had already identified the structural difference between the two paths, but needed to confirm one crucial detail: what exactly does ffi.SealCommitPhase1 return?

Reading the Go wrapper confirms that SealCommitPhase1 returns []byte — raw bytes from the Rust side, passed through CGO without any transformation. The Go wrapper does not parse, validate, or modify the output. It is a pure pass-through. This means the bytes returned by ffi.SealCommitPhase1 are whatever the Rust seal_commit_phase1() function produces, serialized through serde_json.

This confirmation is critical because it establishes the baseline: the normal PoRep path passes pristine Rust-generated JSON to CuZK. The PSProve path, by contrast, passes JSON that has been deserialized into Go types and re-serialized. Any discrepancy between the two — whether in field ordering, whitespace, number formatting, or type representation — could cause the Rust CuZK engine to misinterpret the data.

The Deeper Issue: Type System Mismatches

The assistant's earlier investigation (messages 1613–1617) had already uncovered a subtle type system issue. The Go types PoseidonDomain and Sha256Domain are defined as [32]byte with custom MarshalJSON methods that serialize as base64-encoded strings. However, the struct fields in MerkleProof use the type alias HasherDomain = any, which means the concrete type information is erased at compile time. When JSON is deserialized into these fields, Go's json.Unmarshal produces []interface{} (arrays of float64 values), not PoseidonDomain or Sha256Domain instances. Consequently, the custom MarshalJSON methods are never invoked on the round-trip — the values are re-serialized as JSON arrays of numbers, which happens to match the Rust format.

But this is where things get treacherous. The Rust side uses serde_json, which by default serializes [u8; 32] as a JSON array of integers (e.g., [1, 2, 3, ...]). Go's json.Marshal for []interface{} containing float64 values also produces a JSON array of numbers. On the surface, these look identical. But subtle differences in number formatting (e.g., 1.0 vs 1, or the handling of large integers) could cause serde_json to reject the input.

The Broader Investigation

Message 1620 is not the end of the story — it is a stepping stone. The assistant would go on to examine the C1OutputWrapper struct definitions, discovering a SectorNum type mismatch (int64 in Go vs u64 in Rust), and would trace through the CuZK Rust pipeline's deserialization logic. But this file read represents the moment when the JSON round-trip hypothesis moved from speculation to confirmed structural difference.

The assistant's thinking process is visible in the progression: first identifying the two code paths differ (msg 1611), then questioning whether the Go types have proper serialization support (msg 1613–1615), then checking for custom unmarshalers (msg 1616–1617), and finally reading the FFI boundary itself (msg 1620). Each step narrows the search space, eliminating possibilities and focusing on the most likely root cause.

Conclusion

Message 1620 exemplifies the methodical, evidence-driven debugging that characterizes the most effective engineering investigations. A single file read — seemingly mundane — represents the intersection of a well-formed hypothesis with empirical verification. The assistant did not guess at the root cause; it traced the data flow from production error through Go task infrastructure, through serialization boundaries, and finally to the FFI layer where Rust meets Go. This message captures the moment when the investigation pivoted from "what's different?" to "here is exactly where the difference causes failure." It is a testament to the power of systematic reasoning in complex distributed systems debugging.## Assumptions, Knowledge, and Potential Blind Spots

To fully appreciate the significance of message 1620, it is worth examining the assumptions the assistant was operating under and the knowledge required to interpret what was read.

Input Knowledge Required

A reader coming to this message cold would need substantial context. They would need to understand that ffi.SealCommitPhase1 is the Go entry point for the first phase of the Filecoin Proof-of-Replication protocol, implemented in Rust and bridged through CGO. They would need to know that the function returns a serialized SealCommitPhase1Output struct containing Merkle proofs at challenge positions, and that this output is consumed by a second phase (SealCommitPhase2) or by the CuZK accelerated proving engine. They would also need familiarity with the PSProve task system — a distributed task queue where workers receive challenge data from a market service, generate SNARK proofs, and submit them for verification.

Without this context, the file read would appear as just another function definition. The assistant's investigation had already established this context across the preceding 19 messages, making the read meaningful.

Assumptions Made

The assistant made several implicit assumptions in this message:

1. That the FFI boundary is a pure pass-through. The assistant assumed that cgo.SealCommitPhase1 returns the exact bytes produced by Rust's serde_json serialization, without any Go-side transformation. This is a reasonable assumption given the CGO pattern used, but it is not verified — the CGO layer could theoretically modify the output. The assistant did not read the CGO implementation to confirm.
2. That the JSON round-trip is the primary suspect. The assistant had already committed to the hypothesis that the PSProve failure was caused by a serialization fidelity issue. This file read was conducted to confirm a detail within that hypothesis, not to test alternative explanations. Other possible root causes — such as a mismatch in the RegisteredProof enum values, a difference in how the CuZK client constructs the gRPC request, or a bug in the verification logic itself — had been partially explored but were not the focus of this read.
3. That the normal PoRep C2 path is the "correct" baseline. The assistant assumed that because normal CuZK PoRep C2 tasks work, their data flow is the reference standard. This is reasonable, but it is worth noting that both paths could theoretically have bugs — the normal path might be accidentally correct despite a flawed implementation, or both paths might share a latent issue that only manifests under the specific conditions of PSProve.

Potential Mistakes

The most significant potential blind spot in this message is the failure to verify the CGO layer. The assistant reads the Go wrapper (proofs.go) but not the CGO implementation (cgo/proofs.go). If the CGO layer performs any JSON parsing, validation, or re-serialization, the assumption that the normal path passes "pristine Rust JSON" would be incorrect. In practice, the CGO layer likely passes bytes through without modification, but this is an unverified assumption.

Another subtle issue: the assistant focuses on the JSON format of the SealCommitPhase1Output struct, but the actual bytes passed to CuZK are wrapped in a C1OutputWrapper struct that adds additional fields like sector_num and miner_id. The wrapper's own JSON serialization could introduce its own fidelity issues independent of the inner vanilla proof bytes. The assistant would later investigate this wrapper, discovering the SectorNum type mismatch, but at the time of message 1620, the wrapper was not yet the focus.

The Output Knowledge Created

Message 1620 produced a concrete, verifiable fact: ffi.SealCommitPhase1 returns []byte from the Rust FFI without Go-side transformation. This fact anchors the entire investigation. It means that any difference between the normal PoRep path and the PSProve path must originate either:

• In how the PSProve path obtains its Commit1OutRaw data (from the market's JSON, not from ffi.SealCommitPhase1), or
• In how the PSProve path re-serializes that data (through Go's json.Marshal), or
• In how the CuZK Rust engine deserializes the wrapped proof. The first two possibilities point to the JSON round-trip. The third points to the wrapper structure. Both lines of investigation would prove fruitful in subsequent messages.

The Thinking Process in Action

What makes message 1620 particularly instructive is what it reveals about the assistant's reasoning strategy. The investigation follows a classic debugging pattern: trace the data flow from error back to source, comparing parallel paths at each step.

1. Observe the symptom: PSProve + PoRep fails, PSProve + Snap works, normal CuZK PoRep works.
2. Isolate the variable: The only difference is the combination of PSProve infrastructure with PoRep proof type.
3. Trace the PSProve PoRep path: Follow the code from task dispatch through proof computation to verification.
4. Trace the normal PoRep path: Follow the equivalent code for comparison.
5. Identify the divergence: The PSProve path round-trips through Go JSON serialization; the normal path does not.
6. Verify the baseline: Read ffi.SealCommitPhase1 to confirm the normal path passes raw Rust bytes. Each step eliminates possibilities and narrows the search. By message 1620, the assistant has already done steps 1–5 and is executing step 6. The file read is not a random exploration — it is a targeted verification of a specific hypothesis. This systematic approach is the hallmark of effective debugging in complex systems. Rather than guessing at the root cause or applying random fixes, the assistant builds a chain of evidence, each link supported by code reading and logical inference. Message 1620 is one link in that chain, but it is the link that connects the hypothesis to the ground truth of the FFI boundary.