---
id: "bug-1504"
status: "fixed"
status_updated: "2026-09-24"
status_note: ""
component: "lib"
severity: "crash"
timestamp: "2026-08-07T08:00:00Z"
last_seen: "2026-09-19T09:00:00Z"
occurrences: 1
files: ["src/schema/controller.capnp"]
tags: ["scheduler-core", "task-12.1", "capnp-schema", "codegen-limitation", "sigsegv", "uninitialized-memory", "controller.capnp", "snapshot"]
related: ["bug-1493", "bug-1505"]
legacy: true
---

# bug-1504: sched_sim_startup_test SIGSEGV in write_SnapshotData (lib/controller.capnp.c) -- deterministic…

## Description
sched_sim_startup_test SIGSEGV in write_SnapshotData (lib/controller.capnp.c) -- deterministic crash inside libCapnC_Runtime's read_ptr/capn_setp, discovered during scheduler-core task 12.1 review remediation (full clean rebuild needed for a libedit dependency swap).

## Environment
_Not recorded (migrated from buglog.json)._

## Reproduction
_Not recorded (migrated from buglog.json)._

## Root cause
SnapshotData.internalState was declared `:Data` in the capnp schema. This codebase's capnpc-c backend silently no-ops Data-field encoding inside generated encode_XXX() composite helpers (documented, deliberate limitation -- see controller.capnp's own prior comment citing ReplayHeader.configHash). But encode_SnapshotData_ptr()'s local `struct SnapshotData d;` is never zero-initialized before the (partially no-op) encode_SnapshotData() populates it, so the skipped Data field's capn_ptr is left as UNINITIALIZED STACK GARBAGE. write_SnapshotData() then unconditionally capn_setp()'s that garbage onto the wire. This is genuine UB: whether it crashes depends entirely on what garbage bytes land in that stack slot at each call site -- ReplayHeader.configHash has the identical gap but never crashed (lucky garbage); SnapshotData.internalState's stack layout in this one test's call chain was unlucky. A prior 'rm -rf xce.build' full rebuild (needed to pick up a new nix dependency) perturbed the whole binary's layout enough to flip this from dormant to a deterministic, reproducible SIGSEGV.

## Fix
Changed SnapshotData.internalState from `Data` to `List(UInt8)` in controller.capnp, with a new companion internalStateLen (@11, UInt32) wire field for the maplistcount -- following the exact working precedent of FetchFileReply.content/bytesRead, which this same capnpc-c backend DOES correctly auto-generate real (non-no-op) encode/decode for (capn_new_list8/capn_set8 loop, not a placeholder comment). Updated typemap.h's sched_snapshot_data_t.internal_state_len from size_t to uint32_t to match the wire type, and updated test_e2e_capnp.c's test_snapshot_data_round_trip (which previously worked around the Data no-op with a manual low-level SnapshotData_get/set_internalState accessor) to rely on the now-working ordinary codec bridge instead. No hand-written low-level accessor code needed any more for this field.

## Verification
_Not recorded (migrated from buglog.json)._

## References
- Related: bug-1493, bug-1505