fix(decoding): box the FSE sequence tables so decode fits a 32 KiB stack by myers · Pull Request #550 · structured-world/structured-zstd

myers · 2026-10-03T15:37:23Z

Summary

A decode now fits a 32 KiB kernel task stack in a panic = "abort" build. On main it does not: FrameDecoder is 13,952 B inline, an abort build keeps a second copy of the frame state in decode_all's frame, and decoding 300 KB on a 32 KiB thread dies with "has overflowed its stack" (it passes at 48 KiB).
The three sequence FSE tables (~12.8 KiB) move behind a Box in DecoderScratch and Dictionary. FrameDecoder is 888 B inline and the same decode fits 16 KiB.
The stack budget is pinned in CI, under panic = "abort", where the regression actually shows.

Changes

DecoderScratch::fse and Dictionary::fse are Box<FSEScratch>. The per-block path is unchanged: Workspace::split still hands out &mut FSEScratch once per block, and dictionary reinit is still one memcpy, now heap to heap.
workspace_bytes / heap_bytes count the boxed tables. Without that, a fresh DecoderScratch reported 556 B while holding ~13.4 KiB, and that figure is what a C caller budgets against. The charge sits at each owner, not inside FSEScratch::heap_bytes, so a future by-value owner does not double-count.
New example stack_budget decodes on a 32 KiB thread through decode_all, StreamingDecoder and decode_all_with_dict_bytes, over a frame with compressed, RLE and Raw blocks. The Linux test job runs it with CARGO_PROFILE_RELEASE_PANIC=abort. cargo test always unwinds, and the unwind build elides the copy, so no harness test can see this regression; the unit test pins the root cause (inline size of FrameDecoder, FrameDecoderState, DecoderScratch ≤ 2 KiB) instead.
Checked both ways: with the tables back inline the example aborts with "has overflowed its stack"; with them boxed it passes.

Performance

Not a speed change. profile_decode against main, separately built binaries, min of five interleaved runs, aarch64 (Apple silicon):

input	main	this PR
z000033.zst (417 KB frame), 300 iters	0.711 s	0.711 s
z000006.zst (small frame, new decoder per iteration), 200k iters	3.680 s	3.537 s

The small-frame difference comes from separate builds and is not claimed as a speedup, only as no regression. Not measured against libzstd, and not on x86_64.

Testing

fmt; clippy with -D warnings (all targets; library without default features); the crate tests on macOS aarch64; cargo build --no-default-features --target x86_64-unknown-none; the stack_budget example under panic = "abort".

Summary by CodeRabbit

Improvements
- Reduced stack usage during decoding, supporting decoding on threads with limited stack space. Some decoding scratch data now uses heap memory instead.
Tests
- Added checks for decoder memory usage and decoding on small-stack threads, covering regular, streaming, and dictionary-based decoding.
- Added a Linux CI check that runs the small-stack decoding example in release mode.

`FSEScratch` holds the three sequence FSE tables as inline arrays (~12.8 KiB), and `DecoderScratch` held it by value, so `FrameDecoder` is 13,952 B inline. Under `panic = "abort"`, which every `*-none` target uses, LLVM also materialises a copy of the frame state inside `FrameDecoder::decode_all`'s frame, so a decode needs roughly twice that before the first block is read. On a 32 KiB kernel task stack that overflows: a release `panic = "abort"` build decoding 300 KB of input on a 32 KiB thread dies with "has overflowed its stack" (it passes at 48 KiB). Hold the tables in a `Box` in both owners, `DecoderScratch` and `Dictionary`. `FrameDecoder` drops to 888 B inline and the same decode fits a 16 KiB thread. The dictionary construction paths (`add_dict_from_bytes`, `decode_all_with_dict_bytes`) carried the same inline copy and shrink with it. The per-block hot path is unchanged: `Workspace::split` hands out `&mut FSEScratch` once per block, and dictionary reinit still copies the tables in one memcpy, now heap to heap. Decoded bytes and error behaviour are unchanged. Host tests cannot see the overflow, because `cargo test` always unwinds and the unwind build elides the copy. The unit test therefore pins the root cause, the inline size of the types every decode entry point moves around (it fails at 13,952 B without this change); the stack itself is pinned by the `stack_budget` example in the next commit. Timing, `profile_decode` against upstream main, separately built, min of five interleaved runs: z000033 (417 KB frame) 0.711 s vs 0.711 s; z000006 (small frame, new decoder per iteration) 3.537 s vs 3.680 s. The small-frame difference is from separate builds and is not claimed as a speedup, only as no regression.

Boxing `FSEScratch` moved ~12.8 KiB of fixed-size sequence tables from inline to the heap, but the two accounting paths still described them as inline and counted only their build-scratch vectors. A fresh `DecoderScratch` reported 556 B while holding ~13.4 KiB, and `Dictionary::heap_bytes` under-reported by the same amount. That figure is what a C caller budgets against through the context-size surface, so under-reporting it by more than the rest of a fresh scratch combined is the kind of error that shows up as an OOM in the caller's arena rather than here. Charge `size_of::<FSEScratch>()` at each owner rather than inside `FSEScratch::heap_bytes`: the struct does not know it is boxed, and folding the charge in would double-count for any future by-value owner. Both doc comments now say where the bytes live. The regression test pins both owners, and fails without the change ("workspace_bytes() = 556 omits the 12800 B boxed FSE tables").

`cargo test` always builds with `panic = "unwind"`, and the frame growth the previous commits remove only appears in an abort build, so no test run by the harness can catch it coming back. Add a `stack_budget` example that decodes on a 32 KiB thread through `decode_all`, `StreamingDecoder` and `decode_all_with_dict_bytes`, over a frame with compressed, RLE and Raw blocks, and run it in the Linux test job with `CARGO_PROFILE_RELEASE_PANIC=abort`. Checked both ways: with the FSE tables back inline the example aborts with "has overflowed its stack"; with them boxed it passes.

coderabbitai · 2026-10-03T15:37:41Z

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📝 Walkthrough

Walkthrough

Decoder and dictionary FSE scratch storage now uses heap allocation. Memory accounting and tests cover the boxed storage. A new example checks three decoding paths on budget-sized threads, and Linux CI runs it with a 32 KiB stack budget.

Changes

Decoder stack budget

Layer / File(s)	Summary
Box FSE scratch storage and account for it `zstd/src/decoding/dictionary.rs`, `zstd/src/decoding/scratch.rs`, `zstd/src/decoding/frame_decoder/tests.rs`, `zstd/src/encoding/frame_compressor/tests.rs`	`Dictionary` and `DecoderScratch` now store `FSEScratch` in a `Box`. Their memory accounting includes the boxed value. Tests check inline size and memory accounting, and dictionary fixtures use the boxed field.
Exercise decoder paths within a stack budget `zstd/examples/stack_budget.rs`, `ffi-bench/Cargo.toml`, `.github/workflows/ci.yml`	The example runs `decode_all`, `StreamingDecoder`, and `decode_all_with_dict_bytes` on threads with a configurable stack budget. The example is registered as an `ffi-bench` target, and Linux CI runs it in release mode with panic abort and a 32 KiB budget.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix

Suggested reviewers: polaz

Merge Risk: ⚪ Minimal · up to eafd5

The docs-only CI failure concern is resolved by the code-change gate. No remaining concrete merge-blocking issue is established by the supplied review context.

Security Architecture Review

Security architecture risk: 🔵 Low · up to 47285

The heap move preserves the inspected parsing and ownership controls and accounts for the new allocation. No security regression was identified in those paths. Public-field compatibility and allocation behavior on heap-constrained targets remain unverified.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

inferred — The resource trade-off applies per decoder-scratch or dictionary owner. Aggregate heap pressure depends on caller-controlled construction, retention, and concurrency. Any allocation-failure impact would follow the hosting environment's failure policy; production tenant, kernel, and service exposure is not established by the supplied evidence.

Trust Boundaries and Controls

observed — Caller-supplied dictionary bytes still follow the existing serialized-versus-raw dispatch. Serialized parsing retains minimum-length, magic, nonzero-ID, entropy-table, and repeat-offset validation before returning the constructed Dictionary. Boxing changes allocation behavior without changing these inspected acceptance controls.

Resilience and Maintainability Implications

inferred — Ownership remains local and exclusive for mutable decoder state. Same-backend reset reuses the Box and clears dictionary-source and cached flags; backend replacement drops the previous owner. Failed dictionary parsing does not publish the partially built object, whose owned allocations are released through Rust drop semantics.
observed — The inspected partial-decode recovery path validates resume identity and bounds before restoration. Entropy snapshots materialize dictionary-sourced tables into owned local copies rather than aliasing the boxed scratch allocation, preserving recovery-state isolation.

Hardening Proposals

proposed — If heap-constrained kernel targets are supported, document the additional fixed allocation and allocation-failure policy, and consider fallible construction or caller-side resource admission. This is a deployment-dependent hardening option, not an observed validation bypass.

Pre-merge checks | 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly and concisely describes the main change: boxing the FSE sequence tables to reduce decoder stack usage and support decoding on a 32 KiB stack.
Docstring Coverage		Docstring coverage is 80.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 15 functions across 5 files. (1 skipped: 1 …
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

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Review comments at @zstd/examples/stack_budget.rs:
- Around line 78-81: Update the fixture reads in the stack-budget example to
include each attempted file path in its failure message; use a shared read
helper for the dictionary, frame, and original files while preserving their
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structured-world-cla · 2026-10-06T11:38:10Z

Thank you for the pull request. Before it can be merged, every author of its commits signs the Contributor License Agreement once. You keep the copyright in your contribution; the agreement is a license to the maintainer.

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Review comments at @.github/workflows/ci.yml:
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polaz · 2026-10-10T07:25:54Z

Hi @myers, sorry to bother you again, and thank you for your patience. The 404 you hit on the signing page has been fixed since then, so whenever you have a minute, could you please try signing once more at https://sw.foundation/cla? One signature covers #550, #551 and #552. If anything still gets in the way, just let me know and I'll fix it right away. Thank you!

myers · 2026-10-11T18:10:23Z

@polaz done

Preserve contributor history and both sets of decoder tests. Adapt the stack-budget example to the current StreamingDecoder constructor.

## 🤖 New release * `structured-zstd`: 0.1.1 -> 0.2.0 <details><summary>Changelog</summary> <blockquote> ## [0.2.0](v0.1.1...v0.2.0) - 2026-10-12 ### Added - *(cli)* --trace writes one CSV line per frame ([#579](#579)) - *(decoding)* [**breaking**] hand the source back from a refused dictionary ([#577](#577)) - *(decoding)* [**breaking**] configurable window ceiling up to 2 GiB ([#568](#568)) ### Fixed - *(decoding)* box the FSE sequence tables so decode fits a 32 KiB stack ([#550](#550)) - *(decoding)* keep the scalar sequence decoder's debug frame off a kernel stack ([#551](#551)) ### Performance - *(cli)* move data in 128 KiB pieces, not 8 KiB ([#573](#573)) </blockquote> </details> --- This PR was generated with [release-plz](https://github.com/release-plz/release-plz/). Co-authored-by: sw-release-bot[bot] <255865126+sw-release-bot[bot]@users.noreply.github.com>

myers added 3 commits October 3, 2026 11:16

coderabbitai Bot reviewed Oct 3, 2026

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Comment thread zstd/examples/stack_budget.rs Outdated

Name the fixture path when the stack-budget example cannot read it

271ee73

polaz added 2 commits October 7, 2026 03:09

Merge branch 'main' into box-fse-tables

2a085ae

Merge branch 'main' into box-fse-tables

7ae4b6b

coderabbitai Bot reviewed Oct 8, 2026

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Comment thread .github/workflows/ci.yml Outdated

ci: skip the stack budget step on docs-only pull requests

f87351c

fix(decode): reconcile structured-world#550 with main

eafd515

Preserve contributor history and both sets of decoder tests. Adapt the stack-budget example to the current StreamingDecoder constructor.

polaz mentioned this pull request Oct 11, 2026

ci: run required CodeQL checks on fork PRs #580

Closed

polaz added 2 commits October 11, 2026 16:21

Merge branch 'main' into box-fse-tables

d4f9c44

Merge branch 'main' into box-fse-tables

492c1a4

polaz enabled auto-merge (squash) October 12, 2026 07:05

polaz merged commit 7cf17ac into structured-world:main Oct 12, 2026
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sw-release-bot Bot mentioned this pull request Oct 12, 2026

chore: release v0.2.0 #574

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fix(decoding): box the FSE sequence tables so decode fits a 32 KiB stack - #550

fix(decoding): box the FSE sequence tables so decode fits a 32 KiB stack#550
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