maps: stop reading a community picture at its size cap

An answer without a content-length was buffered whole before its size was
checked. The read now stops at the cap.

server/src/nest/maps/trek-community.service.ts

@@ -1,3 +1,4 @@
+import { readCapped } from '../../utils/cappedFetch';
 import { trekCommunityByKeys, trekPlacesBaseUrl, trekPlacesById } from './trek-places.client';
 import { identifiedFetch } from './trek-places.fetch';
 import { Injectable } from '@nestjs/common';
@@ -81,8 +82,9 @@ export class TrekCommunityService {
         await res.body?.cancel();
         return null;
       }
-      const bytes = Buffer.from(await res.arrayBuffer());
-      return bytes.length && bytes.length <= IMAGE_MAX_BYTES ? bytes : null;
+      // A chunked answer declares no length, so the read itself stops at the cap.
+      const { bytes, truncated } = await readCapped(res, IMAGE_MAX_BYTES);
+      return bytes.length && !truncated ? bytes : null;
     } catch {
       return null;
     } finally {

server/tests/unit/nest/trek-community.service.test.ts

@@ -91,4 +91,18 @@ describe('TrekCommunityService.imageBytes', () => {
     fetchWithKey.mockRejectedValueOnce(new Error('timeout'));
     expect(await svc.imageBytes(A)).toBeNull();
   });
+
+  it('TCOM-007: an answer that declares no length stops being read at the cap', async () => {
+    // An endless body in 1 MB chunks, without a content-length, as a chunked answer comes.
+    let pulled = 0;
+    const body = new ReadableStream<Uint8Array>({
+      pull(controller) {
+        pulled += 1;
+        controller.enqueue(new Uint8Array(1_000_000));
+      },
+    });
+    fetchWithKey.mockResolvedValueOnce(new Response(body, { status: 200 }));
+    expect(await new TrekCommunityService().imageBytes(A)).toBeNull();
+    expect(pulled).toBeLessThanOrEqual(6);
+  });
 });