Remote datasets: cap fetch body size

src/app/infrastructure/remote-data.test.ts

@@ -8,15 +8,18 @@ function fakeResponse(opts: {
   statusText?: string;
   body?: string;
   contentType?: string | null;
+  contentLength?: number;
 }): Response {
+  const headers: Record<string, string | null> = {
+    'content-type': opts.contentType ?? null,
+    'content-length': opts.contentLength !== undefined ? String(opts.contentLength) : null,
+  };
   return {
     ok: opts.ok,
     status: opts.status ?? (opts.ok ? 200 : 500),
     statusText: opts.statusText ?? '',
     text: () => Promise.resolve(opts.body ?? ''),
-    headers: {
-      get: (k: string) => (k.toLowerCase() === 'content-type' ? (opts.contentType ?? null) : null),
-    },
+    headers: { get: (k: string) => headers[k.toLowerCase()] ?? null },
   } as unknown as Response;
 }
 
@@ -84,6 +87,36 @@ describe('fetchRemoteData', () => {
     });
   });
 
+  test('rejects an oversized body declared by Content-Length, before reading it', async () => {
+    const text = vi.fn(() => Promise.resolve('x'));
+    vi.stubGlobal(
+      'fetch',
+      vi.fn(() =>
+        Promise.resolve({
+          ok: true,
+          status: 200,
+          statusText: '',
+          text,
+          headers: { get: (k: string) => (k.toLowerCase() === 'content-length' ? '999' : null) },
+        } as unknown as Response),
+      ),
+    );
+    await expect(
+      fetchRemoteData('https://example.com/huge.csv', { maxBytes: 100 }),
+    ).rejects.toMatchObject({ reason: 'too-large' });
+    expect(text).not.toHaveBeenCalled(); // never read the body
+  });
+
+  test('rejects an oversized body as a backstop when no length is declared', async () => {
+    vi.stubGlobal(
+      'fetch',
+      vi.fn(() => Promise.resolve(fakeResponse({ ok: true, body: 'abcdefghij' }))),
+    );
+    await expect(
+      fetchRemoteData('https://example.com/chunked.csv', { maxBytes: 5 }),
+    ).rejects.toMatchObject({ reason: 'too-large' });
+  });
+
   test('the thrown error is a RemoteFetchError', async () => {
     vi.stubGlobal(
       'fetch',

src/app/infrastructure/remote-data.ts

@@ -16,7 +16,7 @@
  */
 
 /** Why a remote fetch failed, in terms the UI maps to copy + a recovery path. */
-export type RemoteFetchReason = 'network' | 'http' | 'empty' | 'timeout';
+export type RemoteFetchReason = 'network' | 'http' | 'empty' | 'timeout' | 'too-large';
 
 /** A classified remote-fetch failure. `status` is set only for `http`. */
 export class RemoteFetchError extends Error {
@@ -42,9 +42,20 @@ export interface RemoteData {
 /** Default ceiling on a single fetch before it is aborted as a timeout. */
 const DEFAULT_TIMEOUT_MS = 30_000;
 
+/**
+ * Hard ceiling on a snapshotted remote body. A URL dataset is held whole in memory
+ * while profiling and then stored whole in IndexedDB, so an unbounded download
+ * (a multi-hundred-MB file) would jank or exhaust the tab. Checked against the
+ * declared `Content-Length` before reading, and against the body as a backstop for
+ * chunked responses that declare no length.
+ */
+export const MAX_REMOTE_BYTES = 50 * 1024 * 1024;
+
 export interface FetchRemoteDataOptions {
   /** Abort the fetch after this many ms (default 30 s). */
   timeoutMs?: number;
+  /** Reject a body larger than this many bytes (default {@link MAX_REMOTE_BYTES}). */
+  maxBytes?: number;
 }
 
 function describeNetworkError(err: unknown): string {
@@ -52,6 +63,11 @@ function describeNetworkError(err: unknown): string {
   return `Couldn't reach the URL (${reason}).`;
 }
 
+/** A byte count as a rounded MB string for diagnostic messages (e.g. `120 MB`). */
+function megabytes(bytes: number): string {
+  return `${Math.round(bytes / (1024 * 1024))} MB`;
+}
+
 /**
  * Fetch a remote dataset resource once and return its raw body. Throws
  * `RemoteFetchError` on any failure — the caller distinguishes recovery by
@@ -63,6 +79,7 @@ export async function fetchRemoteData(
   options: FetchRemoteDataOptions = {},
 ): Promise<RemoteData> {
   const timeoutMs = options.timeoutMs ?? DEFAULT_TIMEOUT_MS;
+  const maxBytes = options.maxBytes ?? MAX_REMOTE_BYTES;
   const controller = new AbortController();
   const timer = setTimeout(() => controller.abort(), timeoutMs);
 
@@ -88,12 +105,24 @@ export async function fetchRemoteData(
     );
   }
 
+  // Reject an oversized body before reading it, when the server declares the size —
+  // so a huge download never lands in memory in the first place.
+  const declared = Number(response.headers.get('content-length'));
+  if (Number.isFinite(declared) && declared > maxBytes) {
+    throw new RemoteFetchError('too-large', `The file is about ${megabytes(declared)}.`);
+  }
+
   const text = await response.text();
   if (text.trim() === '') {
     throw new RemoteFetchError('empty', 'The URL returned no data.');
   }
 
-  // TODO: guard very large responses (cap or warn) before snapshotting into
-  // IndexedDB — a multi-hundred-MB file would load fully into memory + storage.
+  // Backstop for a chunked response that declared no Content-Length: the body is
+  // already in memory, but this still keeps it out of IndexedDB. `text.length`
+  // (UTF-16 units) slightly under-counts multibyte bytes — fine for a ceiling.
+  if (text.length > maxBytes) {
+    throw new RemoteFetchError('too-large', `The file is over ${megabytes(maxBytes)}.`);
+  }
+
   return { text, contentType: response.headers.get('content-type') };
 }