[P1][adapters] Loop OPFS reads; fail recovery on short read instead of truncating #23

Summary

recover() treats any short read (fewer bytes than file.size()) as a torn tail and truncates. The OPFS adapter's read() performs one handle.read() call and does not loop to fill the buffer — short reads are legal per the OPFS API. A transient short read at reopen can destroy fsync'd commits. The DST short-read test currently asserts this data loss as acceptable — that expectation must flip.

Audit ID: B10 (High)
Wave: 2 (adapter + recovery; pairs with #10 OPFS E2E)

Root cause

OPFS adapter:

read(offset, length) {
  const buffer = new Uint8Array(length);
  const read = handle.read(buffer, { at: offset });
  return buffer.subarray(0, read);
}

Kernel recovery:

const log = file.read(0, file.size());
// ...
if (end > log.length) break; // treats short read as torn

Append already loops (opfs.ts:64-67); read does not.

Proposed fix

1. OPFS adapter — loop reads (mirror append)

read(offset, length) {
  const buffer = new Uint8Array(length);
  let filled = 0;
  while (filled < length) {
    const n = handle.read(buffer.subarray(filled), { at: offset + filled });
    if (n === 0) break;
    filled += n;
  }
  return buffer.subarray(0, filled);
}

2. Kernel recover() — fail loud on short read

If log.length !== file.size() after read → throw (libredb: incomplete WAL read) instead of treating as torn tail.

Distinguish:

• read returned < size → IO/read error (throw)
• read returned == size but last record incomplete → torn tail (truncate)

3. Fix DST short-read test

Update src/sim/dst.ts / SimFS armShortRead expectations: short read during full-file read should error, not truncate commits.

Related issues

• Verify OPFS browser persistence end-to-end on Chrome, Firefox, and Safari #10 — browser OPFS E2E verification (flush/power semantics)
• B8 — torn vs corrupt classification
• B9 — fsync after truncate

Acceptance criteria

• OPFS read loops until length or EOF.
• Recovery throws on size mismatch after read.
• DST short-read profile updated.
• bun run gate green.
• Changeset required.

Agent constraints

• node-fs reads whole file today (readFileSync) — size mismatch unlikely but kernel check still valuable.

Verification

bun run gate