fix(review): worker-thread HEIC conversion with bomb guard, concurrency cap, and magic-byte routing (PR #151)

- Event-loop blockage: HEIC decode/encode (CPU-synchronous libheif WASM +
  jpeg-js) now runs in a per-conversion worker_threads Worker
  (src/web/heic-jpeg-worker.ts, spawned by heic-jpeg-converter.ts) with
  resourceLimits and a 30s hard timeout that terminates the worker —
  verified end-to-end under tsx and against compiled dist/ output with a
  real iPhone HEIC (event-loop max stall 52ms during conversion).
- No server-side concurrency cap: conversions now acquire a slot from the
  existing global runWithConversionLimit() pool (document-conversion-limiter),
  bounding peak decode memory/CPU across simultaneous uploads.
- Decompression bomb: header-declared dimensions are read via heic-decode's
  allocation-free `.all` path and rejected above 64MP BEFORE decode() can
  allocate width*height*4 bytes (a <300-byte crafted file can declare
  30000x30000 = 3.6GB). Regression-tested with a crafted ISOBMFF fixture
  against the real heic-decode WASM (test/heic-jpeg-core.test.ts).
- Mislabeled HEIC (documented Android/MIUI case): conversion now routes on
  ftyp magic-byte sniff of the raw buffer regardless of declared
  ext/Content-Type, so a HEIF uploaded as image/jpeg converts instead of
  415ing; the magic-mismatch 415 only fires for genuinely unrecognized bytes.
- Brand allowlist narrowed to what heic-decode's isHeic() accepts
  (heim/heis/hevm/hevs dropped — they could only ever fail conversion).
- Converted-output size: the JPEG result is checked against
  MAX_PASTE_IMAGE_BYTES (jpeg-js can inflate a within-limit HEIC past the cap).
- Deps: heic-convert replaced with its underlying heic-decode + jpeg-js
  (the wrapper could not expose the pre-decode dimension check); lockfile
  synced, drops pngjs.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/web/heic-jpeg-worker.ts

@@ -0,0 +1,93 @@
+/**
+ * @fileoverview HEIC/HEIF → JPEG conversion core + worker-thread entry.
+ *
+ * Spawned per conversion by `heic-jpeg-converter.ts` so the CPU-synchronous
+ * libheif WASM decode + jpeg-js encode never run on the server's main thread
+ * (on the event loop they would freeze every session's SSE/PTY/WS handling
+ * for seconds per photo). Input arrives via `workerData`; the result (or
+ * error message) is posted back as a single message and the thread exits.
+ *
+ * The conversion logic lives in this same file (exported, guarded bootstrap)
+ * rather than a sibling module: the worker runs from `.ts` source under tsx
+ * in dev, where relative `.js` imports don't resolve inside worker threads —
+ * only `node:` builtins are imported at top level. Unit tests import
+ * `convertHeicBufferToJpeg` directly; the bootstrap only runs when spawned
+ * with our `workerData` shape.
+ *
+ * Decompression-bomb guard: heic-decode's `.all` path exposes the
+ * header-declared {width, height} per image WITHOUT decoding pixels, while
+ * its plain decode path allocates `width * height * 4` bytes straight from
+ * those header values — a <1KB crafted file declaring 30000×30000 would
+ * demand a 3.6GB allocation. We reject anything above MAX_HEIC_DECODE_PIXELS
+ * before calling `decode()`.
+ */
+
+import { parentPort, workerData } from 'node:worker_threads';
+
+/** Max header-declared pixel count we will decode (64MP ≈ 256MB RGBA). */
+export const MAX_HEIC_DECODE_PIXELS = 64_000_000;
+
+/** JPEG quality used for converted HEIC uploads (matches heic-convert's default). */
+export const HEIC_JPEG_QUALITY = 0.92;
+
+export interface HeicWorkerInput {
+  heicInput: Uint8Array;
+  quality: number;
+}
+
+export type HeicWorkerResult = { ok: true; data: Uint8Array } | { ok: false; error: string };
+
+/**
+ * Convert HEIC/HEIF bytes to JPEG bytes. Throws on non-HEIC input, empty
+ * containers, over-limit dimensions, and non-JPEG encoder output.
+ */
+export async function convertHeicBufferToJpeg(input: Uint8Array, quality: number = HEIC_JPEG_QUALITY): Promise<Buffer> {
+  const { default: decode } = await import('heic-decode');
+  const buffer = Buffer.isBuffer(input) ? input : Buffer.from(input.buffer, input.byteOffset, input.byteLength);
+  const images = await decode.all({ buffer });
+  try {
+    if (images.length === 0) throw new Error('no image found in HEIC container');
+    const { width, height } = images[0];
+    if (
+      !Number.isSafeInteger(width) ||
+      !Number.isSafeInteger(height) ||
+      width <= 0 ||
+      height <= 0 ||
+      width * height > MAX_HEIC_DECODE_PIXELS
+    ) {
+      throw new Error(
+        `HEIC dimensions ${width}x${height} exceed the ${Math.floor(MAX_HEIC_DECODE_PIXELS / 1_000_000)}MP decode limit`
+      );
+    }
+    const decoded = await images[0].decode();
+    const { encode } = await import('jpeg-js');
+    // Same output path as heic-convert's JPEG format (jpeg-js at quality*100).
+    const jpeg = encode(
+      { data: decoded.data, width: decoded.width, height: decoded.height },
+      Math.floor(quality * 100)
+    ).data;
+    const jpegBytes = Buffer.isBuffer(jpeg) ? jpeg : Buffer.from(jpeg);
+    if (jpegBytes.length < 3 || jpegBytes[0] !== 0xff || jpegBytes[1] !== 0xd8 || jpegBytes[2] !== 0xff) {
+      throw new Error('HEIC conversion did not produce JPEG bytes');
+    }
+    return jpegBytes;
+  } finally {
+    images.dispose();
+  }
+}
+
+// ── Worker bootstrap ──────────────────────────────────────────────────────
+// Runs only when spawned by heic-jpeg-converter.ts: requires a parent port
+// AND our exact workerData shape, so importing this module from the main
+// thread (or a test runner's own worker pool) stays inert.
+const request = workerData as HeicWorkerInput | null | undefined;
+if (parentPort && request && request.heicInput instanceof Uint8Array && typeof request.quality === 'number') {
+  const port = parentPort;
+  try {
+    const jpegBytes = await convertHeicBufferToJpeg(request.heicInput, request.quality);
+    port.postMessage({ ok: true, data: jpegBytes } satisfies HeicWorkerResult);
+  } catch (err: unknown) {
+    const error = err instanceof Error ? err.message : String(err);
+    port.postMessage({ ok: false, error } satisfies HeicWorkerResult);
+  }
+}