Return empty cursor if feed echoes input cursor (bluesky-social#2989)

* Throw if custom feed echoes input cursor out

* Return empty cursor instead of throwing

packages/bsky/src/api/app/bsky/feed/getFeed.ts

@@ -177,7 +177,7 @@ const skeletonFromFeedGen = async (
 ): Promise<AlgoResponse> => {
   const { feed, headers } = params
   const found = await ctx.hydrator.feed.getFeedGens([feed], true)
-  const feedDid = await found.get(feed)?.record.did
+  const feedDid = found.get(feed)?.record.did
   if (!feedDid) {
     throw new InvalidRequestError('could not find feed')
   }
@@ -220,7 +220,14 @@ const skeletonFromFeedGen = async (
         headers,
       },
     )
+
     skeleton = result.data
+
+    if (result.data.cursor === params.cursor) {
+      // Prevents loops if the custom feed echoes the the input cursor back.
+      skeleton.cursor = undefined
+    }
+
     if (result.headers['content-language']) {
       resHeaders = {
         'content-language': result.headers['content-language'],

packages/bsky/tests/__snapshots__/feed-generation.test.ts.snap

@@ -397,9 +397,9 @@ Array [
         "muted": false,
       },
     },
-    "description": "Provides all feed candidates, blindly ignoring pagination limit",
+    "description": "Echoes back the same cursor it received",
     "did": "user(0)",
-    "displayName": "Bad Pagination",
+    "displayName": "Bad Pagination Cursor",
     "indexedAt": "1970-01-01T00:00:00.000Z",
     "labels": Array [],
     "likeCount": 0,
@@ -439,9 +439,9 @@ Array [
         "muted": false,
       },
     },
-    "description": "Provides even-indexed feed candidates",
+    "description": "Provides all feed candidates, blindly ignoring pagination limit",
     "did": "user(0)",
-    "displayName": "Even",
+    "displayName": "Bad Pagination Limit",
     "indexedAt": "1970-01-01T00:00:00.000Z",
     "labels": Array [],
     "likeCount": 0,
@@ -481,15 +481,57 @@ Array [
         "muted": false,
       },
     },
+    "description": "Provides even-indexed feed candidates",
+    "did": "user(0)",
+    "displayName": "Even",
+    "indexedAt": "1970-01-01T00:00:00.000Z",
+    "labels": Array [],
+    "likeCount": 0,
+    "uri": "record(7)",
+    "viewer": Object {},
+  },
+  Object {
+    "cid": "cids(7)",
+    "creator": Object {
+      "avatar": "https://bsky.public.url/img/avatar/plain/user(2)/cids(1)@jpeg",
+      "createdAt": "1970-01-01T00:00:00.000Z",
+      "description": "its me!",
+      "did": "user(1)",
+      "displayName": "ali",
+      "handle": "alice.test",
+      "indexedAt": "1970-01-01T00:00:00.000Z",
+      "labels": Array [
+        Object {
+          "cid": "cids(2)",
+          "cts": "1970-01-01T00:00:00.000Z",
+          "src": "user(1)",
+          "uri": "record(3)",
+          "val": "self-label-a",
+        },
+        Object {
+          "cid": "cids(2)",
+          "cts": "1970-01-01T00:00:00.000Z",
+          "src": "user(1)",
+          "uri": "record(3)",
+          "val": "self-label-b",
+        },
+      ],
+      "viewer": Object {
+        "blockedBy": false,
+        "followedBy": "record(2)",
+        "following": "record(1)",
+        "muted": false,
+      },
+    },
     "description": "Provides all feed candidates",
     "did": "user(0)",
     "displayName": "All",
     "indexedAt": "1970-01-01T00:00:00.000Z",
     "labels": Array [],
     "likeCount": 2,
-    "uri": "record(7)",
+    "uri": "record(8)",
     "viewer": Object {
-      "like": "record(8)",
+      "like": "record(9)",
     },
   },
 ]
@@ -1950,7 +1992,7 @@ Object {
       },
       "description": "Provides all feed candidates, blindly ignoring pagination limit",
       "did": "user(0)",
-      "displayName": "Bad Pagination",
+      "displayName": "Bad Pagination Limit",
       "indexedAt": "1970-01-01T00:00:00.000Z",
       "labels": Array [],
       "likeCount": 0,

packages/bsky/tests/feed-generation.test.ts

@@ -30,7 +30,8 @@ describe('feed generation', () => {
   let feedUriAllRef: RecordRef
   let feedUriEven: string
   let feedUriOdd: string // Unsupported by feed gen
-  let feedUriBadPagination: string
+  let feedUriBadPaginationLimit: string
+  let feedUriBadPaginationCursor: string
   let feedUriPrime: string // Taken-down
   let feedUriPrimeRef: RecordRef
   let feedUriNeedsAuth: string
@@ -47,10 +48,15 @@ describe('feed generation', () => {
     await network.processAll()
     alice = sc.dids.alice
     const allUri = AtUri.make(alice, 'app.bsky.feed.generator', 'all')
-    const feedUriBadPagination = AtUri.make(
+    const feedUriBadPaginationLimit = AtUri.make(
       alice,
       'app.bsky.feed.generator',
-      'bad-pagination',
+      'bad-pagination-limit',
+    )
+    const feedUriBadPaginationCursor = AtUri.make(
+      alice,
+      'app.bsky.feed.generator',
+      'bad-pagination-cursor',
     )
     const evenUri = AtUri.make(alice, 'app.bsky.feed.generator', 'even')
     const primeUri = AtUri.make(alice, 'app.bsky.feed.generator', 'prime')
@@ -62,15 +68,20 @@ describe('feed generation', () => {
     gen = await network.createFeedGen({
       [allUri.toString()]: feedGenHandler('all'),
       [evenUri.toString()]: feedGenHandler('even'),
-      [feedUriBadPagination.toString()]: feedGenHandler('bad-pagination'),
+      [feedUriBadPaginationLimit.toString()]: feedGenHandler(
+        'bad-pagination-limit',
+      ),
+      [feedUriBadPaginationCursor.toString()]: feedGenHandler(
+        'bad-pagination-cursor',
+      ),
       [primeUri.toString()]: feedGenHandler('prime'),
       [needsAuthUri.toString()]: feedGenHandler('needs-auth'),
     })
 
     const feedSuggestions = [
       { uri: allUri.toString(), order: 1 },
       { uri: evenUri.toString(), order: 2 },
-      { uri: feedUriBadPagination.toString(), order: 3 },
+      { uri: feedUriBadPaginationLimit.toString(), order: 3 },
       { uri: primeUri.toString(), order: 4 },
     ]
     await network.bsky.db.db
@@ -116,17 +127,31 @@ describe('feed generation', () => {
       },
       sc.getHeaders(alice),
     )
-    const badPagination = await pdsAgent.api.app.bsky.feed.generator.create(
-      { repo: alice, rkey: 'bad-pagination' },
-      {
-        did: gen.did,
-        displayName: 'Bad Pagination',
-        description:
-          'Provides all feed candidates, blindly ignoring pagination limit',
-        createdAt: new Date().toISOString(),
-      },
-      sc.getHeaders(alice),
-    )
+
+    const badPaginationLimit =
+      await pdsAgent.api.app.bsky.feed.generator.create(
+        { repo: alice, rkey: 'bad-pagination-limit' },
+        {
+          did: gen.did,
+          displayName: 'Bad Pagination Limit',
+          description:
+            'Provides all feed candidates, blindly ignoring pagination limit',
+          createdAt: new Date().toISOString(),
+        },
+        sc.getHeaders(alice),
+      )
+    const badPaginationCursor =
+      await pdsAgent.api.app.bsky.feed.generator.create(
+        { repo: alice, rkey: 'bad-pagination-cursor' },
+        {
+          did: gen.did,
+          displayName: 'Bad Pagination Cursor',
+          description: 'Echoes back the same cursor it received',
+          createdAt: new Date().toISOString(),
+        },
+        sc.getHeaders(alice),
+      )
+
     // Taken-down
     const prime = await pdsAgent.api.app.bsky.feed.generator.create(
       { repo: alice, rkey: 'prime' },
@@ -157,7 +182,8 @@ describe('feed generation', () => {
     feedUriAllRef = new RecordRef(all.uri, all.cid)
     feedUriEven = even.uri
     feedUriOdd = odd.uri
-    feedUriBadPagination = badPagination.uri
+    feedUriBadPaginationLimit = badPaginationLimit.uri
+    feedUriBadPaginationCursor = badPaginationCursor.uri
     feedUriPrime = prime.uri
     feedUriPrimeRef = new RecordRef(prime.uri, prime.cid)
     feedUriNeedsAuth = needsAuth.uri
@@ -203,12 +229,13 @@ describe('feed generation', () => {
 
     const paginatedAll = results(await paginateAll(paginator))
 
-    expect(paginatedAll.length).toEqual(5)
+    expect(paginatedAll.length).toEqual(6)
     expect(paginatedAll[0].uri).toEqual(feedUriOdd)
     expect(paginatedAll[1].uri).toEqual(feedUriNeedsAuth)
-    expect(paginatedAll[2].uri).toEqual(feedUriBadPagination)
-    expect(paginatedAll[3].uri).toEqual(feedUriEven)
-    expect(paginatedAll[4].uri).toEqual(feedUriAll)
+    expect(paginatedAll[2].uri).toEqual(feedUriBadPaginationCursor)
+    expect(paginatedAll[3].uri).toEqual(feedUriBadPaginationLimit)
+    expect(paginatedAll[4].uri).toEqual(feedUriEven)
+    expect(paginatedAll[5].uri).toEqual(feedUriAll)
     expect(paginatedAll.map((fg) => fg.uri)).not.toContain(feedUriPrime) // taken-down
     expect(forSnapshot(paginatedAll)).toMatchSnapshot()
   })
@@ -490,7 +517,7 @@ describe('feed generation', () => {
       expect(res.data.feeds.map((f) => f.uri)).toEqual([
         feedUriAll,
         feedUriEven,
-        feedUriBadPagination,
+        feedUriBadPaginationLimit,
       ])
     })
 
@@ -605,7 +632,7 @@ describe('feed generation', () => {
 
     it('paginates, handling feed not respecting limit.', async () => {
       const res = await agent.api.app.bsky.feed.getFeed(
-        { feed: feedUriBadPagination, limit: 3 },
+        { feed: feedUriBadPaginationLimit, limit: 3 },
         {
           headers: await network.serviceHeaders(
             alice,
@@ -640,6 +667,22 @@ describe('feed generation', () => {
       })
     })
 
+    it('returns empty cursor with feeds that echo back the same cursor from the param.', async () => {
+      const res = await agent.api.app.bsky.feed.getFeed(
+        { feed: feedUriBadPaginationCursor, cursor: '1', limit: 2 },
+        {
+          headers: await network.serviceHeaders(
+            alice,
+            ids.AppBskyFeedGetFeed,
+            gen.did,
+          ),
+        },
+      )
+
+      expect(res.data.cursor).toBeUndefined()
+      expect(res.data.feed).toHaveLength(2)
+    })
+
     it('resolves contents of taken-down feed.', async () => {
       const tryGetFeed = agent.api.app.bsky.feed.getFeed(
         { feed: feedUriPrime },
@@ -712,7 +755,13 @@ describe('feed generation', () => {
 
   const feedGenHandler =
     (
-      feedName: 'even' | 'all' | 'prime' | 'bad-pagination' | 'needs-auth',
+      feedName:
+        | 'even'
+        | 'all'
+        | 'prime'
+        | 'bad-pagination-limit'
+        | 'bad-pagination-cursor'
+        | 'needs-auth',
     ): SkeletonHandler =>
     async ({ req, params }) => {
       if (feedName === 'needs-auth' && !req.headers.authorization) {
@@ -753,17 +802,22 @@ describe('feed generation', () => {
         return true
       })
       const feedResults =
-        feedName === 'bad-pagination'
+        feedName === 'bad-pagination-limit'
           ? fullFeed.slice(offset) // does not respect limit
           : fullFeed.slice(offset, offset + limit)
       const lastResult = feedResults.at(-1)
+      const cursorResult =
+        feedName === 'bad-pagination-cursor'
+          ? cursor
+          : lastResult
+            ? (fullFeed.indexOf(lastResult) + 1).toString()
+            : undefined
+
       return {
         encoding: 'application/json',
         body: {
           feed: feedResults,
-          cursor: lastResult
-            ? (fullFeed.indexOf(lastResult) + 1).toString()
-            : undefined,
+          cursor: cursorResult,
           $auth: jwtBody(req.headers.authorization), // for testing purposes
         },
       }