Targets 3-4 of docs/plans/fuzzing.md, per the sibling layout:
- fuzz/shared/src/read_opseq.rs — SequentialReader op sequences
(Next/NextBorrowed/Field/Reset/End, Arbitrary-derived) over hostile
buffers under the fixed packed schema menu. Invariants: cursor
discipline (failed read leaves position untouched, state replay
deterministic), None sticky at plan end, plan-order full walks with
a spin bound, read_field leaves a usable reader, ADR-007 reader
independence (shared Arc, isolated cursors), and the plan §6-1
record-count ≥4-verified-bytes bound encoded as an explicit End-op
assertion.
- fuzz/shared/src/layout_build.rs — LayoutBuilder::build with
adversarial var_sizes over a five-schema menu (string/bytes, nested
struct, byte-disc union, record+array, fixed control). Invariants:
Offset-class failures only, position disjointness + total-size
bounds, variable fields record their 4-byte prefix, failed writes
leave the buffer byte-identical, write→read pair round trip.
- derive_var_sizes discovers the synthetic keys ('p.__discriminator')
the builder actually wants by parsing the quoted key from the
Offset reason.
- 73 committed seeds (58 read_opseq + 15 layout_build) hand-encoded
against the pinned arbitrary 1.4.2 derive layout (4-byte LE
multiply-shift variant selectors, keep-going vec elements,
take-rest last field) and pinned by decode_lands_on_the_intended_variants
replay tests; gen_fuzz_seeds.py mirrors the encoders.
- Engine fix (finding W2-1): plan_read_array returned Ok for a
fixed-stride array whose count*stride window extended past the
buffer — the struct/union arms bounds-check, the array arm did not;
a truncated array deferred the failure to the next field (wrong
path) or masked it entirely as an Ok walk. Now an Access error
naming the array, regression test in sequential_reader.rs.
- Packed-mode 'encoding: offset-indirect' pinned as the documented
inline-length-prefix no-op (finding W2-2, bast-format.md Default
strategy selection); open design question recorded as plan §6-7.
Verification: fuzz corpus replay 19/19; main crate 570 tests incl.
the new regression; clippy -D warnings clean (crate + shared); wasm
build clean; cargo fuzz build clean (nightly confined to fuzz/).
Smoke campaigns (10 min detached each): read_opseq 52.1k execs exit 0
empty artifacts, layout_build 42.4k execs exit 0 empty artifacts; no
crash/oom/timeout on any fork job.
glm-5.3-flash