The Silence of the Unmarshalers: A Debugging Breakpoint in the PoRep PSProve Investigation

In the sprawling architecture of a Filecoin Curio deployment, where Go orchestrates Rust proving engines across distributed GPU workers, the boundary between languages is a minefield of subtle serialization mismatches. Message 1616 captures the precise moment when a debugging investigation into a production bug crystallized around a single, devastating finding: there were no custom UnmarshalJSON methods anywhere in the codebase. The grep was simple, the result was stark, and the implications rippled through every assumption the investigator had been building.

The Production Bug

The context for this investigation was a critical production failure. The user reported that PSProve tasks—a mechanism for proving storage challenges on behalf of a ProofShare market—were failing for PoRep (Proof-of-Replication) challenges with the error "porep failed to validate." The same PSProve pipeline worked flawlessly for Snap challenges. The user noted that this was the first time PoRep had been tested through the PSProve path, and the failure was consistent across all PoRep challenge tasks. The normal CuZK PoRep C2 path (non-PSProve) worked correctly, as did the non-CuZK PSProve path. The failure was specifically isolated to the intersection of PSProve + CuZK + PoRep.

This was not a simple configuration error or a resource exhaustion issue. The SNARK proof was being generated successfully—the CuZK engine returned without error—but the proof failed verification. The ffi.VerifySeal() call at line 346 of task_prove.go returned ok=false, err=nil. The proof was structurally valid enough to be produced, but cryptographically invalid. Something was corrupting the data somewhere in the pipeline.

Tracing the Two Paths

The assistant had spent several rounds methodically comparing the two code paths. The normal PoRep C2 path in cuzk_funcs.go calls ffi.SealCommitPhase1() which returns raw bytes—the exact JSON output from the Rust proving engine. These raw bytes are then wrapped using the shared wrapC1Output function and passed to the CuZK prover. The PSProve PoRep path in task_prove.go follows a different route: the Commit1OutRaw struct arrives as JSON from the market, is deserialized into Go structs via json.Unmarshal, and then re-serialized via json.Marshal before being passed to the same CuZK prover.

This structural difference—raw bytes vs. a Go round-trip—became the central hypothesis. The original Rust JSON output from ffi.SealCommitPhase1() had a specific serialization format determined by serde. If the Go json.Unmarshal → Go structs → json.Marshal round-trip produced different JSON, the CuZK Rust engine would deserialize a subtly different SealCommitPhase1Output, and the resulting SNARK proof would fail verification.

The Type System Investigation

The assistant drilled into the type definitions in merkle.go and porep_vproof_types.go. The Go types PoseidonDomain and Sha256Domain are defined as [32]byte with custom MarshalJSON methods. These custom marshalers serialize the 32-byte arrays as Go's default [32]byte JSON representation—which is a base64-encoded string. But the Rust side, using serde, serializes [u8; 32] as an array of 32 integers: [1, 2, 3, ..., 32].

This is where the complexity deepens. The Go type definitions use a curious pattern: type HasherDomain = any. This type alias means that generic struct fields like MerkleProof[Sha256Domain] are actually instantiated as MerkleProof[any]—the concrete type information is erased. When Go's json.Unmarshal processes a JSON array of numbers into an any field, it produces a []interface{} containing float64 values, not a [32]byte or PoseidonDomain. This means the custom MarshalJSON methods on PoseidonDomain and Sha256Domain would never be invoked during the round-trip, because the values are not of those types—they are raw interface{} slices.

But this raised another question: if the custom marshalers are never invoked, how does the JSON round-trip work at all? The answer is that Go's default JSON marshaling for []interface{} produces an array of numbers, which happens to match the Rust serde format. The round-trip might actually be lossless for the domain types, despite the apparent asymmetry.

The Decisive Grep

Message 1616 is the moment the assistant tested a critical piece of this puzzle. Having observed that PoseidonDomain and Sha256Domain have custom MarshalJSON methods, the natural question was whether they also have corresponding UnmarshalJSON methods. If custom marshalers exist without custom unmarshalers, the JSON round-trip would be asymmetric: the marshaler produces base64, but the unmarshaler expects something else (arrays of numbers, or the default Go [32]byte behavior which also expects base64).

The assistant issued a targeted grep:

[grep] UnmarshalJSON
No files found

The result was absolute. There were no custom UnmarshalJSON methods anywhere in the codebase. This confirmed the asymmetry: the custom MarshalJSON methods on PoseidonDomain and Sha256Domain have no corresponding unmarshalers.

The Deeper Significance

This finding is a classic "smoking gun" in cross-language serialization debugging. The presence of custom marshalers without unmarshalers is a code smell that often indicates a half-implemented serialization strategy. Someone added custom marshaling to produce JSON in a specific format (perhaps to match Rust's serde output), but never completed the round-trip by adding the corresponding unmarshaling logic.

However, the investigation was not yet complete. The HasherDomain = any type alias complicates the picture significantly. Because the generic type parameter erases the concrete type, the custom MarshalJSON methods may never be invoked during the round-trip anyway—the values arrive as interface{} and leave as interface{}. The grep result, while confirming the absence of unmarshalers, does not by itself prove the round-trip is lossy. It merely eliminates one possible mechanism.

The true significance of message 1616 is that it represents a decisive investigative step. The assistant had formed a hypothesis, identified the evidence needed to test it, and executed the test. The negative result ("No files found") was itself a positive finding: it narrowed the search space and confirmed that whatever was causing the verification failure, it was not a simple missing unmarshaler on these domain types.

The Path Forward

In the messages following 1616, the assistant would continue to trace the serialization path, examining how ffi.SealCommitPhase1() actually produces its JSON output, whether the FFI layer itself performs any Go-side marshaling, and whether the SectorNum field (typed as int64 in Go vs u64 in Rust) introduces a type mismatch. The grep for UnmarshalJSON was one piece of a larger puzzle, but it was the piece that forced the investigation to look deeper—beyond surface-level serialization mismatches and into the actual byte-level behavior of the Go-Rust FFI boundary.

For anyone debugging cross-language serialization issues, message 1616 stands as a reminder of the power of a simple, well-targeted grep. Sometimes the most important finding is the absence of something you expected to find.