Rebuild: one advisory lock per group when writing releases, not per key

A picture group's batch has 30-55k release keys; an advisory lock each
ran Postgres out of shared memory (max_locks_per_transaction), and those
batches failed with HTTP 500 (the ranges went back, nothing stored).
A chunk only matches releases of its range's group, so one lock per
group serializes exactly the writers that could both start an upload.

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

app/services/batch_store.ts

@@ -97,8 +97,8 @@ class BatchStore {
    * probe candidates (a batch's, or files the fragment merger completed).
    * A chunk joins the release of its key whose articles the range's group
    * owns and whose posting window is within UPLOAD_GAP_MS of its own, else
-   * starts one: re-uploads are releases of their own (decision d). Keys are
-   * locked in order, so concurrent writers don't deadlock or both start one.
+   * starts one: re-uploads are releases of their own (decision d). The
+   * group is locked, so concurrent writers don't both start one.
    */
   async writeReleases(
     trx: TransactionClientContract,
@@ -111,11 +111,12 @@ class BatchStore {
     const ids = new Map<string, number>()
     const slot = (key: string, upload: number) => `${key}\n${upload}`
     const { group_id: groupId } = await trx.from('ranges').where('id', rangeId).select('group_id').firstOrFail()
+    // One writer per group at a time: a chunk only matches releases of its group, and a lock per key
+    // ran out of shared memory (a picture group's batch has 50k keys; LOG.md 2026-10-09 (12)).
+    await trx.rawQuery(`SELECT pg_advisory_xact_lock(hashtextextended('releases:' || ?::text, 0))`, [groupId])
     const sorted = [...releases].sort((a, b) => a.key.localeCompare(b.key) || a.upload - b.upload)
     for (let i = 0; i < sorted.length; i += 1000) {
       const part = sorted.slice(i, i + 1000)
-      const keys = [...new Set(part.map((r) => r.key))]
-      await trx.rawQuery(`SELECT pg_advisory_xact_lock(hashtextextended(k, 0)) FROM unnest(?::text[]) AS k ORDER BY k`, [keys])
       const windows = part.map((r, idx) => ({ idx, key: r.key, first: r.postedAt, last: r.postedLast ?? r.postedAt }))
       const { rows: matches } = await trx.rawQuery(
         `SELECT DISTINCT ON (v.idx) v.idx, r.id

app/services/fragment_merger.ts

@@ -89,7 +89,7 @@ class FragmentMerger {
     }
     await ChunkPartitions.ensure()
     await db.transaction(async (trx) => {
-      // In key order, as batches lock keys (no deadlocks).
+      // In key order (batches lock their group; the merger only ever waits on one batch at a time).
       releases.sort((a, b) => a.chunk.key.localeCompare(b.chunk.key) || a.chunk.upload - b.chunk.upload)
       for (const { rangeId, chunk, candidates } of releases) await BatchStore.writeReleases(trx, rangeId, 0, [chunk], candidates)
       const done = [...completed.flatMap((c) => c.ids), ...incomplete.flatMap((c) => c.ids)]