A Surgical Git Probe: Tracing PoRep Proof Failures Through Recent Commits

In the middle of a high-stakes debugging session, an AI assistant issues a single, carefully crafted bash command. The target is a remote host running a GPU-accelerated proving engine for Filecoin's proof-of-replication (PoRep) protocol. Every single proof is failing — a 100% failure rate that has persisted across multiple service restarts and configuration changes. The assistant has already ruled out the most obvious suspect, the Pre-Compiled Constraint Evaluator (PCE), by disabling it entirely and observing that proofs still fail. Now it needs to understand what did change, and the answer lies in the git history of a downstream dependency.

The message at index 353 is deceptively simple on its surface:

[assistant] [bash] cd /tmp/czk/extern/bellperson && git log --oneline -10 2>/dev/null && echo "---" && git diff HEAD~3 -- src/util_cs/witness_cs.rs src/groth16/prover/mod.rs src/groth16/prover/supraseal.rs 2>/dev/null | head -80
1660e842 fix finalize reg
fdba807a fix batch ref free
a4eec2b1 rfin: allow missing files
ab7ee418 add protobufc to docs
43473a5a cuzk: Fix post/snap pce
341946b0 cuzk: fix snap
17729280 cuzk: wire post
f57276e0 cuzk: fix over-scheduling with shared MaxPending limiter
35805f02 cuzk: GPU memory pool caching, proof self-check, and diagnostic instrumentation
7b987124 wrap C1 output in JSON envelope for cuzk PoRep proving
---

A git log followed by a targeted diff. But beneath this compact command lies a sophisticated chain of reasoning, a narrowing of hypotheses, and a critical juncture in a debugging narrative that spans multiple machines, GPU architectures, and constraint system implementations.

The Debugging Arc That Led Here

To understand why this message was written, one must trace the assistant's journey through the preceding messages. The session had been focused on implementing PCE extraction for all proof types — WinningPoSt, WindowPoSt, and SnapDeals — and resolving a crash in the WindowPoSt path caused by a mismatch in the is_extensible() flag between RecordingCS and WitnessCS. That fix had been successfully deployed, but it introduced a new and more perplexing problem: PoRep partitioned proofs, which had been working on the local development machine, were now failing catastrophically on the remote test host.

The assistant's initial hypothesis was that the PCE changes were responsible. The WindowPoSt fix had modified WitnessCS::new() and RecordingCS::new() to start with zero inputs instead of one, and had added explicit alloc_input("one") calls before synthesis. If the cached PCE on disk had been extracted with the old code (one pre-allocated input), but the witness was now being generated with the new code (zero inputs plus an explicit allocation), the input counts would be off by one, causing corruption. This was a plausible and well-reasoned theory.

To test it, the assistant added CUZK_DISABLE_PCE=1 to the remote service's environment and restarted the daemon. The result was decisive: even with PCE completely disabled, proofs continued to fail at the same 100% rate. The PCE was not the culprit. This was a critical finding — it meant the bug was deeper, in the partitioned pipeline itself or in the GPU proving path, and it had been introduced by some other change in the recent deployment.

The User's Critical Clue

At this point, the user provided an essential piece of context: the partitioned pipeline had been working on the local machine, and it kept working with the old PCE cached. The user had never tested the new PCE on the local machine. This suggested that the WindowPoSt fix changes might have had an impact, but the mechanism was unclear — especially since the PCE path was now ruled out.

The assistant synthesized these observations in message 352, concluding that the issue must be in changes that affect the non-PCE path. The WindowPoSt fix modified WitnessCS::new() and RecordingCS::new(), but with PCE disabled, the standard bellperson path uses ProvingAssignment directly — it never calls WitnessCS::new(). So how could those changes affect the non-PCE path? Unless there were other changes in the bellperson repository that touched the prover code or the GPU proving path.

The Message's Purpose: A Targeted Git Investigation

This is the precise reasoning that motivated message 353. The assistant needed to determine whether any recent commits to the bellperson repository — the underlying Groth16 proving library — had modified files that could affect partitioned proving. The command is a model of surgical precision:

  1. git log --oneline -10: Shows the ten most recent commits, giving a timeline of what has changed recently. The output reveals a series of commits with descriptive messages: "fix finalize reg", "fix batch ref free", "rfin: allow missing files", "add protobufc to docs", and critically, "cuzk: Fix post/snap pce" — the WindowPoSt fix itself.
  2. git diff HEAD~3 -- src/util_cs/witness_cs.rs src/groth16/prover/mod.rs src/groth16/prover/supraseal.rs: This is the heart of the investigation. The assistant selects three files that are most likely to affect partitioned proving behavior: - witness_cs.rs — the witness constraint system, which was modified by the WindowPoSt fix - prover/mod.rs — the prover module, which orchestrates proof generation - prover/supraseal.rs — the GPU-accelerated proving path, which is the most likely source of GPU-related bugs The choice of HEAD~3 (three commits before HEAD) is deliberate. Looking at the git log, HEAD~3 corresponds to commit ab7ee418 ("add protobufc to docs"). The WindowPoSt fix commit 43473a5a ("cuzk: Fix post/snap pce") is the fifth commit from the top, so it falls outside the HEAD~3 window. This means the diff will not include the WindowPoSt fix changes — it will only show changes from the three most recent commits that are not the WindowPoSt fix. Why HEAD~3 specifically? The assistant is trying to isolate whether any other recent changes — beyond the WindowPoSt fix — could have broken the non-PCE partitioned path. By excluding the WindowPoSt fix from the diff window, the assistant can see if there are any unrelated modifications to the prover code that might be responsible.

What the Output Reveals

The output is telling. The git log shows ten commits, and after the "---" separator, there is nothing — the git diff produced no output at all. The | head -80 pipe would have captured up to 80 lines of diff, but there was nothing to capture. This means that in the last three commits (HEAD through HEAD~2), none of the three targeted files were modified.

This is a significant negative result. It tells the assistant that:

Assumptions Embedded in the Investigation

The assistant makes several assumptions in this message, some explicit and some implicit:

That the bug is in code that was recently changed. This is a reasonable debugging heuristic — when something that was working stops working, look at what changed. The assistant has already confirmed that the remote host is running the latest build, so any recent commit could be the source of the regression.

That the three selected files are the most likely locations for the bug. This is a well-informed judgment. witness_cs.rs was directly modified by the WindowPoSt fix. prover/mod.rs orchestrates the proving flow and could contain logic that differs between partitioned and monolithic modes. supraseal.rs handles GPU proving and is the most likely location for GPU-specific bugs like race conditions or memory corruption.

That HEAD~3 is the right window to examine. This assumption is more subtle. The assistant chooses three commits, which excludes the WindowPoSt fix commit (which is the fifth from the top). This is a deliberate choice to isolate other changes. But it also means the assistant is implicitly treating the WindowPoSt fix as a known quantity — the assistant already understands what that commit changed and is now looking for something else.

That the bug is in bellperson, not in the cuzk wrapper code. The assistant checks the bellperson repository specifically, not the cuzk repository. This is based on the reasoning that the non-PCE path flows through bellperson's ProvingAssignment, and any bug there would affect partitioned proving. However, the cuzk repository also contains pipeline orchestration code (in pipeline.rs) that could have bugs specific to the partitioned path.

Input Knowledge Required

To fully understand this message, a reader needs:

  1. Knowledge of the debugging context: That PoRep partitioned proofs are failing 100% on a remote multi-GPU host but working on a local single-GPU machine. That PCE has been ruled out as the cause. That the WindowPoSt fix modified WitnessCS and RecordingCS initialization.
  2. Understanding of the codebase architecture: That bellperson is the underlying Groth16 proving library. That WitnessCS is used by the PCE fast path while ProvingAssignment is used by the standard path. That supraseal.rs contains the GPU proving code. That the partitioned pipeline splits a PoRep proof into multiple circuits that are proven independently.
  3. Familiarity with git: Understanding git log --oneline, git diff HEAD~3, and the concept of commit ranges. Knowing that HEAD~3 means "three commits before HEAD" and that the diff shows changes between that commit and HEAD.
  4. Awareness of the GPU architecture difference: The remote host has two GPUs (RTX 4000 Ada) while the local machine has one (RTX 5070 Ti). This difference is critical because GPU-related bugs often manifest only on multi-GPU systems.

Output Knowledge Created

The message produces two pieces of knowledge:

  1. The recent commit history of bellperson: Ten commits are displayed, showing a sequence of fixes and features including the WindowPoSt PCE fix, GPU memory pool caching, and proof self-check instrumentation.
  2. The negative diff result: The three targeted files were not modified in the last three commits. This is a crucial piece of negative evidence that forces the assistant to reconsider its hypothesis. This negative result is arguably more valuable than a positive one would have been. If the diff had shown changes to supraseal.rs, the assistant would have a clear suspect. But the empty diff means the assistant must look elsewhere — perhaps at the cuzk repository's pipeline orchestration code, perhaps at the interaction between the Rust engine and the C++ GPU code, or perhaps at an environmental difference between the local and remote machines.

The Thinking Process Visible in the Reasoning

The assistant's reasoning, visible across messages 332-353, reveals a methodical debugging process:

  1. Formulate hypothesis: The PCE changes broke PoRep proofs (input count mismatch).
  2. Test hypothesis: Disable PCE via environment variable, restart service, observe results.
  3. Evaluate results: Proofs still fail with PCE disabled. Hypothesis rejected.
  4. Incorporate new information: User confirms partitioned pipeline worked locally with old PCE, suggesting the WindowPoSt fix changes might have had an impact.
  5. Refine hypothesis: Perhaps the WindowPoSt fix changed something in the non-PCE path too.
  6. Investigate: Check git history to see what else changed recently.
  7. Analyze results: No recent changes to the three key files. The WindowPoSt fix is the only relevant change, but its effects on the non-PCE path are unclear. This is classic scientific debugging: form a hypothesis, design an experiment, run it, interpret the results, and iterate. The assistant is methodically narrowing the search space, ruling out possibilities one by one.

The Broader Significance

Message 353 represents a turning point in the debugging session. The assistant has exhausted the most obvious hypotheses and is now in uncharted territory. The git investigation reveals that the code changes are not where the assistant expected them to be. This will force a shift in focus — from the Rust-level proving code to the C++ GPU runtime, from the PCE path to the interaction between Rust workers and CUDA device selection.

Indeed, the assistant's subsequent investigation (visible in later messages) will uncover the real root cause: a GPU race condition caused by incorrect CUDA_VISIBLE_DEVICES handling on the multi-GPU remote host. The C++ code reads CUDA_VISIBLE_DEVICES once at static initialization time, so Rust's std::env::set_var() calls have no effect on the CUDA runtime. All workers end up targeting the same physical GPU 0, and the per-GPU mutexes in the Rust engine are powerless to prevent concurrent CUDA kernel execution.

But at this moment, in message 353, the assistant doesn't know that yet. It is still methodically working through the possibilities, using git as a diagnostic tool to understand what changed and where to look next. The empty diff is a dead end, but it's a productive dead end — it eliminates a class of hypotheses and points the investigation in a new direction.

This message exemplifies a crucial debugging skill: knowing when to stop looking at one layer of the system and start looking at another. The assistant could have continued investigating the PCE code, or the bellperson prover code, but the evidence pointed elsewhere. The git probe was the final check before a fundamental shift in the investigation's focus.