Rebuild: coordinator:run --shard=i/n splits prefetching over processes

The coordinator's event loop was 16.5% busy at ~10 MB/s of compressed
XOVER (socket reads, buffers, GC, spool writes; nothing stands out), so
one process tops out around 60 MB/s: less than 50 connections bring
(~1M articles/s, ~70-86 MB/s). A shard prefetches the ranges with
id % n = i - 1 with its own NNTP_CONNECTIONS; only shard 1 plans, merges
and settles. Two shards of 5 connections fetched what one of 10 did.

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

commands/coordinator_run.ts

@@ -1,4 +1,4 @@
-import { BaseCommand } from '@adonisjs/core/ace'
+import { BaseCommand, flags } from '@adonisjs/core/ace'
 import type { CommandOptions } from '@adonisjs/core/types/ace'
 import { setTimeout as sleep } from 'node:timers/promises'
 import { Prefetcher } from '#services/prefetcher'
@@ -24,8 +24,14 @@ export default class CoordinatorRun extends BaseCommand {
   static description = 'Prefetches ranges into the spool and plans new ones'
   static options: CommandOptions = { startApp: true, staysAlive: true }
 
+  @flags.string({ description: 'This process\'s share of the prefetching, "i/n" (ranges with id % n = i - 1); only 1/n plans, merges and settles' })
+  declare shard?: string
+
   async run() {
-    const prefetcher = new Prefetcher()
+    const [index, count] = (this.shard ?? '1/1').split('/').map(Number)
+    if (!(count >= 1 && index >= 1 && index <= count)) throw new Error(`--shard: "i/n" with 1 <= i <= n, not "${this.shard}"`)
+    const prefetcher = new Prefetcher({ index: index - 1, count })
+    const main = index === 1
     let stopping = false
     let stopped: () => void
     const done = new Promise<void>((resolve) => (stopped = resolve))
@@ -36,7 +42,7 @@ export default class CoordinatorRun extends BaseCommand {
       prefetcher.stop()
       await done
     })
-    this.logger.info('Coordinator started.')
+    this.logger.info(count > 1 ? `Coordinator started (shard ${index}/${count}${main ? '' : ', prefetching only'}).` : 'Coordinator started.')
     // Two loops: the prefetcher's ticks don't wait for merging or planning (a merge round
     // of up to MERGE_BUDGET_MS left the connections idle once their targets were fetched).
     const prefetching = (async () => {
@@ -53,14 +59,14 @@ export default class CoordinatorRun extends BaseCommand {
     let mergedAt = 0
     while (!stopping) {
       try {
-        if (Date.now() - plannedAt >= PLAN_EVERY_MS) {
+        if (main && Date.now() - plannedAt >= PLAN_EVERY_MS) {
           plannedAt = Date.now()
           const { groups, ranges } = await Planner.planAll()
           if (ranges > 0) this.logger.info(`Planned ${ranges} ranges over ${groups} groups.`)
           const settled = await Settler.run()
           if (settled > 0) this.logger.info(`Settled ${settled} releases.`)
         }
-        if (Date.now() - mergedAt >= MERGE_EVERY_MS) {
+        if (main && Date.now() - mergedAt >= MERGE_EVERY_MS) {
           mergedAt = Date.now()
           // Rounds until a whole pass over the file keys decides nothing more, or MERGE_BUDGET_MS.
           // (Going on while only waiting files were left kept Postgres busy all the time.)