Rebuild: a batch locks its range with an advisory lock, not the row

BatchStore.store held the range's row (SELECT … FOR UPDATE) for the
whole batch transaction (15-20 s on the HDD), so the prefetcher's
spooled_chunks updates and the heartbeats of that range waited on it
(0.5 s on average): the prefetcher's NNTP connections sat idle. Now an
advisory lock per range serializes its batches, and the resume point is
checked again in the final, conditional update.

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

app/services/batch_store.ts

@@ -23,7 +23,11 @@ class BatchStore {
   async store(rangeId: number, nodeId: number, batch: EdgeBatch): Promise<StoredBatch> {
     await ChunkPartitions.ensure()
     return db.transaction(async (trx) => {
-      const range = await trx.from('ranges').where('id', rangeId).forUpdate().first()
+      // One batch of a range at a time, without holding the range's row for the whole transaction: the
+      // prefetcher's and heartbeats' updates of it waited on it (0.5 s on average, NNTP connections idle;
+      // LOG.md 2026-10-09 (13)). The resume point is checked again when it moves, at the end.
+      await trx.rawQuery(`SELECT pg_advisory_xact_lock(hashtextextended('range:' || ?::text, 0))`, [rangeId])
+      const range = await trx.from('ranges').where('id', rangeId).first()
       if (!range || range.status !== 'leased' || Number(range.node_id) !== nodeId)
         throw new StaleBatch(`Range ${rangeId} is not leased to this node`)
       if (Number(range.next_id) !== batch.from)
@@ -80,14 +84,18 @@ class BatchStore {
         await trx.insertQuery().table('fragments').multiInsert(rows)
       }
 
-      await trx
+      const moved = await trx
         .from('ranges')
         .where('id', rangeId)
+        .where('status', 'leased')
+        .where('node_id', nodeId)
+        .where('next_id', batch.from)
         .update({
           next_id: batch.next,
           headers: db.raw('headers + ?', [batch.headers]),
           heartbeat_at: new Date(),
         })
+      if (Number(moved) !== 1 && !(Array.isArray(moved) && moved.length === 1)) throw new StaleBatch(`Range ${rangeId} moved on meanwhile`)
       return result
     })
   }