Raise recent-feed total scan cap to 500

GET /posts/recent now caps its total scan at 500 eligible top-level posts
instead of 10, so corpora smaller than the cap report an exact total while
larger corpora stay bounded. Added unit coverage for the exact-total-under-cap
and default-cap cases, plus the many-top-level-posts acceptance fixture.

By coder.

psf-memo-db/acceptance/lib/handlers.js

@@ -209,6 +209,11 @@ async function loadFixture (world, name) {
     return
   }
 
+  if (name === 'many-top-level-posts') {
+    await loadManyTopLevelPosts(world)
+    return
+  }
+
   if (name !== 'three-top-level-posts-and-one-reply') {
     throw new Error(`Unknown fixture: ${name}`)
   }
@@ -376,6 +381,31 @@ async function loadManyPostsWithReplies (world) {
   }
 }
 
+async function loadManyTopLevelPosts (world) {
+  // 510 eligible top-level posts: larger than the 500 total-scan cap, so the
+  // capped total is exactly 500, while an offset near the end exhausts the
+  // index and reads all 510 entries.
+  for (let i = 0; i < 510; i++) {
+    const id = String(i).padStart(3, '0')
+    const txid = `post-${id}`
+    const blockHeight = 600000 + i
+    await world.adapters.level.postsDb.put(txid, {
+      addr: 'bitcoincash:qaddr',
+      text: `post ${id}`,
+      seen: i,
+      blockHeight
+    })
+    await world.adapters.level.postHeightsDb.put(
+      String(blockHeight).padStart(12, '0') + ':' + txid,
+      { txid, blockHeight }
+    )
+    await world.adapters.level.addrPostHeightsDb.put(
+      `bitcoincash:qaddr:${String(blockHeight).padStart(12, '0')}:${txid}`,
+      { txid, addr: 'bitcoincash:qaddr', blockHeight }
+    )
+  }
+}
+
 async function backfillIndexes (world) {
   const posts = []
   for await (const [txid, post] of world.adapters.level.postsDb.iterator()) {

psf-memo-db/src/adapters/post-query.js

@@ -11,7 +11,7 @@ import { getPostOrNull as getPostOrNullShared } from './lib/get-post-or-null.js'
 import { loadMutedAddrs, isMutedPost } from './lib/muted-posts.js'
 
 const HEIGHT_PAD = 12
-const TOTAL_SCAN_CAP = 10
+const TOTAL_SCAN_CAP = 500
 
 class PostQuery {
   constructor (localConfig = {}) {

psf-memo-db/test/unit/adapters/post-query.unit.js

@@ -162,7 +162,7 @@ describe('#PostQuery', () => {
   })
 
   describe('#scanRecentPostTxidsAndCount', () => {
-    it('should return txids and a capped total count', async () => {
+    it('should report the exact total when the index is smaller than the default cap', async () => {
       async function * mockHeights () {
         for (let i = 20; i >= 0; i--) {
           const id = String(i).padStart(3, '0')
@@ -174,7 +174,24 @@ describe('#PostQuery', () => {
       const result = await uut.scanRecentPostTxidsAndCount({ limit: 3, offset: 0 })
 
       assert.deepEqual(result.txids, ['post-020', 'post-019', 'post-018'])
-      assert.equal(result.total, 10)
+      assert.equal(result.total, 21)
+    })
+
+    it('should default the total scan cap to 500', async () => {
+      let reads = 0
+      async function * mockHeights () {
+        for (let i = 599; i >= 0; i--) {
+          reads++
+          const id = String(i).padStart(3, '0')
+          yield [`000000${600000 + i}:post-${id}`, { txid: `post-${id}` }]
+        }
+      }
+      postHeightsDb.iterator.withArgs({ reverse: true }).returns(mockHeights())
+
+      const result = await uut.scanRecentPostTxidsAndCount({ limit: 3, offset: 0 })
+
+      assert.equal(result.total, 500)
+      assert.equal(reads, 503) // offset + limit + default cap
     })
 
     it('should cap the raw postHeights scan to limit + offset + cap', async () => {