Tracing the Data Boundary: A Cross-Language Debugging Odyssey Through Filecoin's PoRep PSProve Pipeline

Introduction

In the complex ecosystem of decentralized storage proving systems, the most insidious bugs often live at the boundaries between programming languages, between serialization formats, and between the assumptions of different developers working on different components of a distributed pipeline. This article synthesizes a deep-dive debugging investigation spanning 26 messages in an opencode coding session, where an AI assistant methodically traced a production bug through Go struct definitions, Rust serde deserialization, protobuf field mappings, gRPC dispatch logic, and Docker build configurations — all in pursuit of a single, elusive error: "porep failed to validate".

The bug was exquisitely specific. PoRep (Proof of Replication) tasks processed through the PSProve (ProofShare) proving pipeline failed when routed through the CuZK GPU-accelerated proving engine, while three closely related code paths worked correctly: SnapDeals PSProve tasks through CuZK, normal (non-PSProve) PoRep C2 through CuZK, and PSProve PoRep without CuZK through the standard FFI. This pattern — one specific combination failing while all others succeed — immediately suggested a plumbing issue at the intersection of these components rather than a fundamental problem with any single one.

The Architecture: Two Paths to a Proof

To understand the investigation, one must first understand the architecture. The Filecoin proving pipeline has two phases: Commit Phase 1 (C1), which generates a "vanilla proof" containing Merkle proofs at challenge positions, and Commit Phase 2 (C2), which wraps the vanilla proof into a SNARK proof suitable for on-chain verification. The CuZK engine accelerates the C2 computation using GPUs.

The system has two distinct paths for generating PoRep C2 proofs through CuZK. The normal path (cuzk_funcs.go) is straightforward: a single machine calls ffi.SealCommitPhase1() to produce raw Rust JSON bytes, wraps those bytes using the shared wrapC1Output function, and sends the wrapped output to the CuZK daemon via gRPC. The raw bytes never pass through Go's JSON marshaling machinery — they remain in the exact format produced by Rust's serde_json.

The PSProve path (task_prove.go) is more circuitous, reflecting its market-based architecture. A client machine generates the C1 output locally, but instead of passing the raw bytes directly to CuZK, it deserializes them into a Go Commit1OutRaw struct via json.Unmarshal, then re-marshals them into a ProofData blob for upload to the ProofShare market. A provider machine downloads this blob, deserializes it back into Go structs, and then re-marshals the PoRep field to produce the vproof bytes that get wrapped for CuZK. Between the client's C1 generation and the provider's CuZK call, the data undergoes two full round-trips through Go's JSON marshaler and unmarshaler.

This structural difference — raw bytes versus round-tripped structs — became the central focus of the investigation.

Phase 1: The JSON Round-Trip Hypothesis

The assistant's initial hypothesis, formed early in the investigation, was that the Go JSON round-trip introduced subtle byte-level differences that the Rust CuZK engine could not tolerate. This was a compelling theory because the types involved have different serialization semantics in Go and Rust.

The Go types PoseidonDomain and Sha256Domain (both [32]byte arrays) have custom MarshalJSON methods that serialize as base64-encoded strings, but critically lack corresponding UnmarshalJSON methods ([msg 1615]). The type alias type HasherDomain = any in merkle.go further complicates matters: when Rust JSON arrays like [1, 2, 3, ..., 32] (how serde_json serializes [u8; 32]) are deserialized by Go into any fields, they become []interface{} slices of float64 values, bypassing the custom marshalers entirely. The Commitment type (used for CommR, CommD, ReplicaID) is a concrete [32]byte — Go's default unmarshaler expects base64 for this type, not JSON arrays.

The assistant launched a subagent task to investigate how Rust actually serializes these types ([msg 1621]), confirming that Commitment ([u8; 32]) is serialized as a JSON array of 32 integers. This seemed to confirm the hypothesis: the round-trip should be lossy for concrete [32]byte fields.

But then the assistant discovered an existing test in porep_vproof_test.go that explicitly validates the JSON round-trip ([msg 1623]). The test unmarshals Rust-style JSON into Go structs, marshals back, and compares at the map[string]interface{} level. It passes. More importantly, the non-CuZK PSProve path — which uses the same Go-re-marshaled JSON — works correctly. If the JSON round-trip were corrupt, this path would also fail.

This was a critical moment of hypothesis elimination. The JSON round-trip was functionally correct. The investigation had to pivot.

Phase 2: The RegisteredProof That Wasn't Used

With the serialization hypothesis eliminated, the assistant turned to the RegisteredProof enum — the numeric identifier that tells the prover which circuit and parameters to use. The CuZK protobuf API defines only a single POREP_SEAL_COMMIT proof kind, with no sub-distinction for interactive PoRep versus NiPoRep (Non-Interactive PoRep). The specific proof type is carried solely through the numeric RegisteredProof field.

The assistant traced this field through both code paths. In the normal path, it comes from sn.ProofType (an abi.RegisteredSealProof enum). In the PSProve path, it comes from request.RegisteredProof.ToABI(), which converts a string-based proof identifier to the ABI numeric type. If these produced different values — for instance, if the PSProve path used a default or incorrectly converted type — CuZK would select the wrong proving circuit, producing a proof that fails verification.

But then the assistant made a crucial discovery by reading the Rust CuZK prover code in prover.rs ([msg 1637]). The Rust function prove_porep_c2 takes vanilla_proof_json, sector_number, and miner_id as inputs. It deserializes the C1 wrapper, base64-decodes the Phase1Out field, deserializes SealCommitPhase1Output from JSON, and then calls seal::seal_commit_phase2(c1_output, prover_id, sector_id). The registered_proof field from the gRPC request is not used for PoRep — the Rust prover uses the registered_proof value embedded inside the C1 JSON output itself.

This discovery was a double-edged sword. On one hand, it ruled out an entire class of potential bugs: any mismatch in the enum mapping at the gRPC boundary would be irrelevant because the Rust side ignores that field. On the other hand, it meant the investigation had to go deeper: if the C1 output's internal registered_proof field was somehow corrupted or different after the Go round-trip, that would explain the failure. But the assistant had already proven the round-trip was correct.

Phase 3: The Duplicated Wrapper and the Type Mismatch

The investigation then focused on the C1OutputWrapper — the JSON envelope that packages the C1 output with sector metadata before sending to CuZK. In the normal path, this wrapper is constructed by the shared wrapC1Output function in cuzk_funcs.go. In the PSProve path, a local c1OutputWrapper struct is defined and populated within task_prove.go itself.

The assistant performed a meticulous side-by-side comparison of the two code paths ([msg 1636]), laying out the exact Go structs and gRPC calls for both. The comparison revealed that both paths construct nearly identical SubmitProofRequest protobuf messages, differing only in the RequestId prefix (cosmetic) and the source of the VanillaProof bytes.

But the critical difference was in the wrapper struct definitions. The assistant read the Rust C1OutputWrapper struct in prover.rs ([msg 1638]):

pub struct C1OutputWrapper {
    #[serde(rename = "SectorNum")]
    pub sector_num: u64,
    #[serde(rename = "Phase1Out")]
    pub phase1_out: String,
    #[serde(rename = "SectorSize")]
    pub sector_size: u64,
}

The Go-side local struct in task_prove.go declared SectorNum as int64 — a signed 64-bit integer. The Rust struct expects u64 — an unsigned 64-bit integer. This was the smoking gun: a cross-language type mismatch that, while benign for small positive sector numbers, represented a fundamental breakdown in the contract between the Go and Rust sides of the pipeline.

The assistant correctly reasoned that for positive sector numbers (which all real sector numbers are), the int64 value would serialize to the same JSON number as a u64, and Rust's serde_json would accept it without complaint ([msg 1639]). But the very existence of this mismatch signaled a deeper problem: the PSProve path's wrapper was a hand-rolled duplicate, not derived from the canonical Rust definition. Any future divergence between these definitions could introduce subtle bugs.

Phase 4: The Dead Ends That Focused the Investigation

Before arriving at the wrapper mismatch, the assistant systematically eliminated several other plausible hypotheses. It checked whether the Docker image was missing a mainnet build tag that might gate NiPoRep support (<msg id=1630-1632>), running greps that confirmed no such tag existed in the codebase. It investigated whether parameter file mismatches on the vast.ai workers could cause VerifySeal to produce false negatives ([msg 1633]), reasoning through the Groth16 parameter architecture to conclude that all PoRep variants use the same circuit for the same sector size. It even considered whether the redundant VerifySeal call in computePoRep might be producing a false positive — a hypothesis the user correctly rejected, noting that the snark market's own verification would catch the same issue (<msg id=1634-1635>).

Each eliminated hypothesis narrowed the search space. The user's guidance was instrumental: "compare the code to C1 paths; SPT - rust has different mappings most likely, follow filecoin-ffi/etc there are multiple enum mappings" ([msg 1634]). This direction refocused the investigation from speculative hypothesis generation to systematic structural comparison.

Phase 5: Tracing the Data into the Rust Engine

With the Go-side plumbing thoroughly examined, the assistant crossed the language boundary into the Rust CuZK engine. It traced how the vanilla_proof field (singular, used for PoRep, as distinct from the plural vanilla_proofs used for Snap and PoSt) is dispatched within engine.rs (<msg id=1640-1642>). The engine calls pipeline::parse_c1_output(&amp;vanilla_proof) which parses the C1 wrapper, base64-decodes the Phase1Out, deserializes SealCommitPhase1Output, and calls seal::seal_commit_phase2.

The assistant's reasoning at this point was precise: "If the parse succeeds and SNARK generation succeeds, the proof should be valid" ([msg 1643]). Since CuZK successfully produces a SNARK proof (the error is "porep failed to validate," not "failed to parse" or "failed to prove"), the engine's internal logic is correct. The problem must be in the semantic content of the data — some field within the C1 output or wrapper that causes the engine to prove a different statement than what VerifySeal expects.

This led the assistant to examine the gRPC dispatch layer, searching for how vanilla_proof is populated from the SubmitProofRequest protobuf message. The grep for struct ProveRequest|vanilla_proof.*=|req_to_internal returned no results, suggesting the field mapping was not straightforward.

The Root Cause: A Synthesis

The investigation, while not definitively concluded within this chunk, had identified the critical structural differences between the two code paths. The PSProve path:

  1. Defines a local c1OutputWrapper struct instead of using the shared wrapC1Output function, creating a maintenance hazard and a potential source of cross-language type mismatches.
  2. Re-marshals the C1 output from a Go struct rather than passing the raw Rust JSON bytes from ffi.SealCommitPhase1(), subjecting the data to two additional JSON marshal/unmarshal cycles.
  3. Uses int64 for SectorNum where the Rust side expects u64, a type mismatch that is benign for positive values but signals a breakdown in cross-language coordination. The JSON round-trip itself was confirmed correct by both a dedicated unit test and the working non-CuZK PSProve path. The RegisteredProof enum mapping was ruled out because the Rust CuZK prover ignores the gRPC field for PoRep and uses the value embedded in the C1 output. The build configuration and parameter files were eliminated as causes.

The Methodology: A Model for Cross-Language Debugging

What makes this investigation remarkable is not just the technical depth but the systematic methodology. The assistant employed several debugging strategies that are essential for cross-language, distributed systems:

Differential analysis: By comparing three working code paths against one failing path, the assistant isolated the exact variable that differs. This is the scientific method applied to debugging: control experiments isolate the cause.

Hypothesis elimination: Rather than pursuing a single theory, the assistant generated multiple plausible explanations and systematically tested each one. The JSON round-trip hypothesis, the RegisteredProof mapping, the build tag theory, and the parameter file hypothesis were all examined and either confirmed or eliminated.

Cross-language tracing: The assistant traced the data flow from Go struct definitions through JSON serialization, through gRPC protobuf encoding, into Rust serde_json deserialization, and finally into the GPU proving pipeline. Each boundary was examined as a potential site for data corruption.

Using working paths as controls: The non-CuZK PSProve path served as a crucial control experiment. If the JSON round-trip were corrupt, this path would also fail. Its success proved the data format was correct, forcing the investigation to look elsewhere.

User collaboration: The user's domain knowledge and strategic guidance were instrumental. When the assistant proposed removing the VerifySeal check as a "fix," the user correctly rejected this approach and redirected the investigation toward comparing enum mappings and code paths.

Conclusion

The debugging odyssey captured in this chunk is a testament to the complexity of cross-language proving systems and the discipline required to diagnose their failures. The bug — PoRep PSProve tasks failing through CuZK — was not in any single component but in the plumbing that connects them. The JSON round-trip through Go structs, the duplicated wrapper definition, the int64/u64 type mismatch, and the Rust engine's internal deserialization all contributed to a failure mode that only manifested at the intersection of multiple subsystems.

The investigation demonstrates that in distributed, polyglot systems, the boundaries between languages and protocols are where bugs hide. Each serialization boundary — Go JSON marshal/unmarshal, base64 encoding, protobuf field mapping, Rust serde_json deserialization — is an opportunity for subtle data corruption. The most effective debugging strategy is to trace the data across every boundary, using working paths as controls and eliminating hypotheses systematically.

The root cause, while not definitively identified in this chunk, had been narrowed to a precise set of structural differences. The fix would involve either eliminating the duplicated wrapper by using the shared wrapC1Output function, passing raw bytes through the PSProve path instead of round-tripping through Go structs, or adding proper UnmarshalJSON methods to ensure symmetric serialization. The investigation had transformed an opaque production error into a concrete, fixable set of code changes — the ultimate goal of any debugging effort.