Par2 merge: a set held by two merged releases; one failed set doesn't stop a round

A par2 index and its repost both hold a set: moving both rows to the target
broke par2_sets' primary key, and the round failed on it every minute from
20:28 UTC (no merges at all). Duplicates among the others go first; a set that
fails is logged and the round goes on.

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

tests/functional/par2_merger.spec.ts

@@ -63,6 +63,29 @@ test.group('Par2Merger', (group) => {
     assert.deepEqual(await new Par2Merger().round().then((r) => r.merges), 0)
   })
 
+  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[] = []
+    for (const [key, files] of [['bb000001', 1], ['bb000002', 1], ['bb000003', 3]] as const) {
+      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, posted_at: at, readable: false })
+        .returning('id')
+      ids.push(Number(row.id))
+    }
+    const setId = hash('7')
+    // The par2 index and its repost each hold the set; the third release holds the data file.
+    await db.table('par2_sets').multiInsert([
+      { set_id: setId, release_id: ids[0], files: 1 },
+      { set_id: setId, release_id: ids[1], files: 1 },
+    ])
+    await db.table('par2_files').insert({ set_id: setId, file_id: hash('6'), hash16k: hash('5'), length: 1000, name: 'x.rar' })
+    await db.table('file_hashes').insert({ candidate_id: 3001, release_id: ids[2], hash16k: hash('5'), size: 1000 })
+    assert.equal(await new Par2Merger().mergeSet(setId), 2)
+    const held = await db.from('par2_sets').where('set_id', setId).select('release_id')
+    assert.deepEqual(held.map((r) => Number(r.release_id)), [ids[2]])
+  })
+
   test('a set file that matches several releases nearby is ambiguous: not merged', async ({ assert }) => {
     const at = new Date('2022-07-08T03:46:00Z')
     const ids: number[] = []