Merger: what's new an hour at a time, only the sets two releases share; common files match nothing

A round behind by two days of the probe backlog read 3.5M sets and never ended;
a par2 file shared by up to 190k files loaded every release it matched and the
merger ran out of heap every few minutes (LOG.md 2026-10-20 (85)).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/functional/par2_merger.spec.ts

@@ -78,6 +78,44 @@ test.group('Par2Merger', (group) => {
     assert.equal((await new Par2Merger().round()).sets, 1)
   })
 
+  test('a round behind by hours goes through them a window at a time, past the gaps', async ({ assert }) => {
+    const ago = (hours: number) => new Date(Date.now() - hours * 3600_000)
+    await db.table('par2_sets').multiInsert([
+      { set_id: 'a1'.repeat(16), release_id: 1, files: 1, created_at: ago(30) },
+      { set_id: 'a2'.repeat(16), release_id: 2, files: 1, created_at: ago(2.5) },
+      { set_id: 'a3'.repeat(16), release_id: 3, files: 1, created_at: ago(0.5) },
+    ])
+    await db.table('worker_cursors').insert({ name: 'merger', at: ago(48) })
+    assert.equal((await new Par2Merger().round()).sets, 3)
+    const saved = await db.from('worker_cursors').where('name', 'merger').firstOrFail()
+    assert.isAbove(new Date(saved.at).getTime(), ago(0.5).getTime())
+  })
+
+  test('a par2 file held by more than 20 files is a common one: it merges nothing', async ({ assert }) => {
+    const at = new Date('2022-07-08T03:46:00Z')
+    const ids: number[] = []
+    for (const key of ['dd000001', 'dd000002']) {
+      const [row] = await db
+        .table('releases')
+        .insert({ release_key: key.padEnd(40, '0'), identity_key: key.padEnd(40, '0'), name: key, source: 'stem', groups: '[]', group_id: 1, files: 1, posted_at: at, readable: false })
+        .returning('id')
+      ids.push(Number(row.id))
+    }
+    // The par2 is held by the first; the second holds the file, and so do 20 releases elsewhere and years apart.
+    const setId = hash('1')
+    await db.table('par2_sets').insert({ set_id: setId, release_id: ids[0], files: 1 })
+    await db.table('par2_files').insert({ set_id: setId, file_id: hash('0'), hash16k: hash('9'), length: 30214, name: 'sample.nfo' })
+    await db.table('file_hashes').insert({ candidate_id: 4000, release_id: ids[1], hash16k: hash('9'), size: 30214 })
+    for (let i = 1; i <= 20; i++) await db.table('file_hashes').insert({ candidate_id: 4000 + i, release_id: 900000 + i, hash16k: hash('9'), size: 30214 })
+    const round = await new Par2Merger().round()
+    assert.deepEqual([round.sets, round.merges], [1, 0])
+    assert.equal(await new Par2Merger().mergeSet(setId), 0)
+    assert.equal((await db.from('releases').whereIn('id', ids).select('id')).length, 2)
+    // One holder fewer: the file is one upload's again.
+    await db.from('file_hashes').where('candidate_id', 4020).delete()
+    assert.equal(await new Par2Merger().mergeSet(setId), 1)
+  })
+
   test('a set held by two of the releases merged is held once by the target', async ({ assert }) => {
     const at = new Date('2022-07-08T03:46:00Z')
     const ids: number[] = []