feat(fs): the fs module — a per-app file tree behind a nine-op spec, sim host, pocket-fs reference core

[siwei-yuan]

Stacked on #231 (the db module) — review the last commit pair; the first two are #231's. Will rebase to a clean diff once #231 merges.

The fifth module-shaped vertical slice (after ui, strike, audio and db), built spec-first per the module discipline: a per-app file tree mounted as globalThis.fs, capability id data.fs.

The boundary

contracts/spec/fs.ts pins nine synchronous ops — read / write / remove / list / stat / mkdir / rename / usage / lastError — plus:

• universal names: a segment is any well-formed Unicode — Chinese names, dot-prefixed names, spaces — except . and .. (the escape hatches), control characters, and oversize (64 UTF-8 bytes / segment, 8 segments, 160 bytes / path). No name is reserved to the host. Names are byte-for-byte identities (no case folding, no normalization) — the folding-filesystem caveat is documented, and the deterministic hosts are byte-exact so goldens catch collisions early;
• isolation by construction, independent of names: every path is relative and resolves under the root the host binds at mount; .., absolute paths, and /-in-name are unrepresentable, so apps cannot spell each other's trees — the ATTACH-refusal principle generalized. The reference core lstat-refuses symlinks planted by host-side actors. Privilege is the binding: on Pocket Pi the device agent is the same module bound at /workspace, apps at /workspace/apps/<id>/data/;
• payload encoding: text as a JSON string (stored as UTF-8), bytes as the db module's {"$b": base64} spelling; 64 KiB per op crossing (the SDK chunks larger files transparently);
• atomicity with a clean tree: truncate writes land in the module's own temp directory — outside the bound root, same filesystem — then rename over the target (the power-loss contract LittleFS hosts inherit). The module owns tmp and clears it on construction, so the app's tree never shows host machinery and a crash orphan cannot outlive the next boot;
• paged list: sorted by Unicode code point (= UTF-8 byte order; the sim host carries the comparator since JS sorts UTF-16 code units), FS_MAX_DIR_ENTRIES (256) per call with offset + eof;
• frame contract: no clock, no events, and stat carries no mtime — a timestamp is the fs spelling of Date.now, excluded for the same golden-test reason. No watch().

One data root serves both data modules: a database is an ordinary file (<root>/<name>.sqlite) in the app's home — its own asset, visible like any of its files (backup = a file copy). Overwriting it corrupts the app's own data, the same trust class as deleting its own files; SQLite fails loudly on a corrupt image.

What ships

• @pocketjs/framework/fs — the Bun shape: file() (.text/.bytes/.json/.size/.exists) and write(), plus the node:fs sync subset Bun implements (readFileSync, writeFileSync, appendFileSync, mkdirSync, readdirSync, rmSync, renameSync, statSync, existsSync), and usage(). Everything returns synchronously per the frame contract, and await unwraps a plain value, so Bun-idiomatic code (await Bun.file(p).text()) migrates unchanged. Throws where the namespace is unmounted, like db: file code that silently drops writes is a corruption bug.
• framework/src/bytes.ts — the base64 codec extracted from db-api plus a strict UTF-8 decoder (QuickJS has no TextDecoder), shared by both data SDKs.
• hosts/sim/fs.ts — in-memory tree behind the op namespace, injected via bootWorld extraGlobals; byte-exact names, code-point-sorted listings.
• engine/crates/pocket-fs — the reference core: Storage::Memory / Storage::Dir { root, tmp } over std::fs, root confinement, per-segment lstat symlink refusal, atomic truncate writes through the host-owned temp dir, per-app quota. mount is a default feature; default-features = false drops the pocket-mod dependency for firmware with its own QuickJS wiring — verified to cargo check clean for riscv32imafc-esp-espidf.
• hosts/esp32p4/examples/data-smoke — the ppa-smoke pattern's second instance: both module cores driven on a real ESP32-P4 over LittleFS (see the commit for measured numbers; persistence proven across resets, including SQLite transaction atomicity through interrupted runs — the sample count stays a multiple of 288).
• docs/FS.md — the boundary, the data/ + tmp/ layout, and the three-move adoption path.
• data.fs registered ahead of any stock TARGET advertising it — the audio.pcm / data.sqlite precedent: the sim host and the reference core implement and test the whole contract.

Verified

• bun run test 11/11 stages green (tests/fs.test.ts 15 pass: the op contract and the SDK; suite 331+ pass, contract byte-compare included)
• cargo test -p pocket-fs 12/12, including a live QuickJS guest round-trip, universal-name round-trips, orphan-sweep, and the symlink-escape refusal
• cargo check --workspace clean, clippy clean
• cargo check --target riscv32imafc-esp-espidf (no default features) clean
• On-device: DATA-SMOKE: PASS boot=1 / PASS boot=2 on a Waveshare ESP32-P4 rev 1.3 (fs contract ~0.45 s, 288-row tx ~0.4–0.6 s)

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[siwei-yuan]

On-device verification added: hosts/esp32p4/examples/data-smoke (the ppa-smoke pattern — one building block, one hardware verifier) runs both module cores with default-features = false against a LittleFS partition on a Waveshare ESP32-P4 rev 1.3:

DATA-SMOKE: fs ok in 505.072ms; boot 1; usedBytes 12299
DATA-SMOKE: db ok in 1.044217s (288-row tx 538.36ms)
DATA-SMOKE: heap before 582008 after 580780 (delta 1228)
DATA-SMOKE: PASS boot=1

PASS boot=2 after another reset — a boot counter (fs truncate-write) and a per-boot 288-row transaction persist across resets, verified as boots × 288 on every start. The traversal/dot-name refusals, the ATTACH refusal, and the .db/ fence location all hold on-device.

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[siwei-yuan]

Design revision after review (PR body updated to match):

1. Names are now universal. The earlier filename-safe-token grammar over-restricted: apps could not name their own files in Chinese or with a leading dot, and the restriction was never load-bearing for isolation (that's the binding + unrepresentable ../absolute paths + symlink refusal, unchanged). A segment is now any well-formed Unicode except ./.., control chars, and oversize. Nothing is reserved to the host.
2. Atomic-write temps moved out of the app's tree. Storage::Dir { root, tmp }: temps land in a host-owned directory outside the bound root (same filesystem — rename stays atomic), which the module clears on construction. The app's tree provably holds only the app's own names, and a crash orphan cannot outlive the next boot — a guarantee the old in-tree dot-file scheme could never give once dot names became legal.
3. The .db/ fence is gone (with feat(db): the db module — SQLite behind a five-op spec, sim host, pocket-db reference core #231's update): a database is an ordinary visible file in the shared data root. The fence protected an app from itself — an app that can already remove("", recursive) its whole home gains nothing from one file being unspellable, and visibility makes backup a file copy.

Re-verified end to end after the revision: full suite green, clippy clean, espidf cross-compile clean, and on-device PASS boot=1 / PASS boot=2 — including universal names (CJK + dot-prefixed) round-tripping on real LittleFS, and SQLite transaction atomicity witnessed across interrupted runs (sample count stays a multiple of 288 through resets).

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[doodlewind]

Verdict: approved. Same quality bar as #231 — the nine-op boundary is clean, the atomic-write contract is carefully built (host-owned temp outside the root, swept on construction), and the Dir backend's symlink refusal is real. I ran the same op matrix through the sim and the Memory core, found the issues below, and pushed the fixes as a17c02c. This branch is rebased onto #231's fixed head, so it will show a clean diff once #231 lands.

What the review found (and the commit fixes)

1. tests/fs.test.ts was never wired into the gate. The PR body claims 331+ passing, but bun run test's unit stage is a hand-maintained list in tools/test.ts and this file's entry was missing — the gate actually ran 316. Wired in; the stage now runs 331. (main's test-suite.test.ts guard only checks iphone2g-* files — generalizing it is the follow-up below.)
2. fsValidSegment accepted unpaired surrogates, contradicting the spec's "any well-formed Unicode". A lone surrogate has no UTF-8 spelling: a JS host stores it byte-exactly while the QuickJS→native bridge mangles it into a different name — silent identity divergence between sim and a native core, exactly the class of bug the byte-exact hosts exist to catch. The shared predicate now refuses it (pinned in tests), and the spec documents that a text payload with a lone surrogate is likewise host-dependent, so arbitrary bytes belong in the {"$b"} spelling.
3. sim list() mishandled a negative offset — passed straight to Array.slice, which wraps to slice-from-the-end, where the reference core clamps to 0. Clamped to match. A cross-host parity probe now byte-agrees on the whole op matrix (modulo JSON key order), including that astral-plane names sort by code point on both hosts.
4. docs/FS.md now states the chunking atomicity caveat. The op is the atomic unit, so an SDK write above FS_MAX_IO_BYTES crosses as truncate + appends and power loss between chunks can leave the leading chunks only — whole-file atomicity above 64 KiB is write-a-sibling + rename, the same move the module itself makes. The single-op atomicity contract was well-documented; this closes the multi-op gap.
5. concepts.md's module diagram enumerated ui/audio/strike; db and fs join it. Both PRs call themselves the 4th and 5th modules, but neither updated the page that enumerates them.
6. Minor: dropped a dead pad counter in bytes.ts carried over from the db-api extraction.

Verified consistent, left as-is: the Dir backend's lstat-per-segment symlink refusal (the security-critical path) round-trips correctly and hides planted links from read/stat/list; universal names (Chinese, dot-prefixed, spaces) round-trip byte-exact; the quota accounting agrees between hosts.

Verified

• cargo test -p pocket-fs 12/12 (incl. the Dir-storage atomicity + symlink-escape test), clippy clean, cargo check --workspace clean
• tests/fs.test.ts + tests/db.test.ts 33/33; unit stage 330/331 — the 1 fail is the pre-existing symbian-runtime Gatekeeper flake described in #231, unrelated to this diff
• bunx tsc --noEmit clean
• The on-device data-smoke (both module cores on a real ESP32-P4 over LittleFS, persistence across resets incl. SQLite tx atomicity) is taken as hardware-verified per the ppa-smoke precedent.

Left alone / follow-ups (not this PR)

• Generalize test-suite.test.ts to require every tests/*.test.ts to have a stage home (with an explicit exclusion list), so the next module can't repeat finding #1.
• The symbian-runtime 5s-timeout flake on managed macOS hosts (shared with #231).

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