fix(upload): stop buffering a chunk body before anything checks its size

The chunk route drained the whole request into an array and concatenated it
before calling uploadChunk — which is where every check lives. So a 300MB body
sent against an unknown upload id was read in full, cost ~300MB of heap, and
was only then answered with an error. The per-file cap was real but applied
after the damage, and nothing looked at Content-Length at all.

uploadChunk now takes the request stream itself and consumes nothing until the
upload id, the chunk index and the declared Content-Length have all been
checked against the remaining allowance. A body that clears those is streamed
straight to the chunk file under a hard byte cap, so a sender that lies about
its length — or sends none, the Transfer-Encoding: chunked case — is cut off
at the allowance instead of being read to the end.

A Buffer is still accepted, so the existing callers and chunkedUploadSizeCap
tests are untouched. Worth noting the old code silently did nothing when
handed a stream: fs.promises.writeFile accepts an async iterable, so the body
was written while `chunkData.length` was undefined and the cap comparison was
NaN > max, i.e. always false.

Reachable only for an authenticated admin holding photos.upload on an event
they own, and only once the CSRF gate lets application/octet-stream through.

Relates to issue 1403

backend/__tests__/services/chunkedUploadStreaming.test.js

@@ -0,0 +1,150 @@
+/**
+ * A rejected chunk must not cost its own size in memory (#1403).
+ *
+ * The route used to drain the whole request into an array and `Buffer.concat`
+ * it before calling uploadChunk, which is where every check lives — the
+ * per-file cap, the chunk index, and even "does this upload id exist". So a
+ * 300MB body against an unknown upload id was read in full, added ~300MB to
+ * RSS, and was then answered with an error. The size cap was real but only
+ * applied after the damage.
+ *
+ * The contract these tests pin: uploadChunk consumes NOTHING until every check
+ * has passed, and once it does start reading it stops at the remaining
+ * allowance rather than trusting the sender.
+ */
+const path = require('path');
+const os = require('os');
+const fs = require('fs').promises;
+const { Readable } = require('stream');
+
+process.env.STORAGE_PATH = path.join(os.tmpdir(), `picpeak-chunk-stream-test-${process.pid}`);
+
+const chunkedUpload = require('../../src/services/chunkedUploadService');
+
+const MB = 1024 * 1024;
+
+const init = (overrides = {}) => chunkedUpload.initializeUpload({
+  filename: 'clip.mp4',
+  fileSize: 1,
+  mimeType: 'video/mp4',
+  eventId: 1,
+  totalChunks: 2,
+  maxFileSizeBytes: 1 * MB,
+  ...overrides,
+});
+
+/**
+ * A readable that reports how much of it was actually pulled. Bytes are
+ * generated lazily, so "never read" really means the body never materialized.
+ */
+function countingSource(totalBytes, sliceSize = 64 * 1024) {
+  let remaining = totalBytes;
+  const source = new Readable({
+    read() {
+      if (remaining <= 0) return this.push(null);
+      const n = Math.min(sliceSize, remaining);
+      remaining -= n;
+      source.bytesRead += n;
+      this.push(Buffer.alloc(n));
+    },
+  });
+  source.bytesRead = 0;
+  return source;
+}
+
+describe('chunked upload streams the body under a cap (#1403)', () => {
+  afterAll(async () => {
+    await fs.rm(process.env.STORAGE_PATH, { recursive: true, force: true }).catch(() => {});
+  });
+
+  describe('refused before the body is read', () => {
+    it('reads nothing for an unknown upload id', async () => {
+      const source = countingSource(8 * MB);
+      await expect(chunkedUpload.uploadChunk('does-not-exist', 0, source, { declaredBytes: 8 * MB }))
+        .rejects.toThrow('Upload not found or expired');
+      expect(source.bytesRead).toBe(0);
+    });
+
+    it('reads nothing for an out-of-range chunk index', async () => {
+      const { uploadId } = await init();
+      const source = countingSource(8 * MB);
+      await expect(chunkedUpload.uploadChunk(uploadId, 99, source, { declaredBytes: 8 * MB }))
+        .rejects.toMatchObject({ code: 'INVALID_CHUNK', statusCode: 400 });
+      expect(source.bytesRead).toBe(0);
+    });
+
+    it('reads nothing when Content-Length already exceeds the cap', async () => {
+      const { uploadId } = await init();
+      const source = countingSource(8 * MB);
+      await expect(chunkedUpload.uploadChunk(uploadId, 0, source, { declaredBytes: 8 * MB }))
+        .rejects.toMatchObject({ code: 'FILE_TOO_LARGE', statusCode: 413 });
+      expect(source.bytesRead).toBe(0);
+      expect(chunkedUpload.getUploadStatus(uploadId)).toBeNull();
+    });
+
+    it('counts what earlier chunks already banked when checking Content-Length', async () => {
+      const { uploadId } = await init();
+      await chunkedUpload.uploadChunk(uploadId, 0, Buffer.alloc(0.75 * MB));
+      const source = countingSource(0.5 * MB);
+      // 0.75MB banked + 0.5MB declared > the 1MB cap.
+      await expect(chunkedUpload.uploadChunk(uploadId, 1, source, { declaredBytes: 0.5 * MB }))
+        .rejects.toMatchObject({ code: 'FILE_TOO_LARGE', statusCode: 413 });
+      expect(source.bytesRead).toBe(0);
+    });
+  });
+
+  describe('a sender that lies, or says nothing', () => {
+    it('stops at the allowance instead of reading the whole body', async () => {
+      const { uploadId } = await init();
+      // No declaredBytes at all — the Transfer-Encoding: chunked case.
+      const source = countingSource(8 * MB);
+      await expect(chunkedUpload.uploadChunk(uploadId, 0, source))
+        .rejects.toMatchObject({ code: 'FILE_TOO_LARGE', statusCode: 413 });
+      // The overshoot is whatever the readable had already buffered ahead when
+      // the cap tripped — a small constant tied to highWaterMark, NOT a
+      // function of the body size. That is the whole claim: 8MB offered, ~1MB
+      // read. The slack is deliberately loose so this doesn't turn into a
+      // Node-version canary.
+      expect(source.bytesRead).toBeLessThan(2 * MB);
+    });
+
+    it('leaves no partial chunk file behind when it cuts a body off', async () => {
+      const { uploadId } = await init();
+      const meta = chunkedUpload.getUploadStatus(uploadId);
+      await expect(chunkedUpload.uploadChunk(uploadId, 0, countingSource(8 * MB)))
+        .rejects.toMatchObject({ statusCode: 413 });
+      // abortUpload removes the whole directory; assert nothing survived it.
+      await expect(fs.readdir(path.join(process.env.STORAGE_PATH, 'chunks', uploadId)))
+        .rejects.toMatchObject({ code: 'ENOENT' });
+      expect(meta).not.toBeNull();
+    });
+  });
+
+  describe('the happy path still works', () => {
+    it('writes a streamed chunk and reports progress', async () => {
+      const { uploadId } = await init();
+      const result = await chunkedUpload.uploadChunk(uploadId, 0, countingSource(0.25 * MB), {
+        declaredBytes: 0.25 * MB,
+      });
+      expect(result).toMatchObject({ chunkIndex: 0, received: 1, expected: 2, complete: false });
+
+      const chunkPath = path.join(process.env.STORAGE_PATH, 'chunks', uploadId, 'chunk_000000');
+      expect((await fs.stat(chunkPath)).size).toBe(0.25 * MB);
+    });
+
+    it('still accepts a Buffer, the shape the service was written for', async () => {
+      const { uploadId } = await init();
+      const result = await chunkedUpload.uploadChunk(uploadId, 0, Buffer.alloc(0.25 * MB));
+      expect(result).toMatchObject({ chunkIndex: 0, received: 1 });
+    });
+
+    it('lets a re-sent chunk replace itself without double-counting', async () => {
+      const { uploadId } = await init();
+      await chunkedUpload.uploadChunk(uploadId, 0, countingSource(0.6 * MB), { declaredBytes: 0.6 * MB });
+      // Same index again: the first copy's 0.6MB must not count toward the cap.
+      await expect(
+        chunkedUpload.uploadChunk(uploadId, 0, countingSource(0.6 * MB), { declaredBytes: 0.6 * MB }),
+      ).resolves.toBeTruthy();
+    });
+  });
+});