seed:node posts batches in order beside fetching

At most two posts waiting; a failed post stops the chain, so nothing is
stored past a missing batch. The first remote node did 7k headers/s while
waiting for every post.

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

commands/SeedNode.ts

@@ -99,6 +99,10 @@ export default class SeedNode extends BaseCommand {
       const started = Date.now()
       let headers = 0
       let client: XoverClient | undefined
+      let chain: Promise<void> = Promise.resolve()
+      const posting = new Set<Promise<void>>()
+      // Throws once a post has failed, so the range stops fetching.
+      const failedSoFar = () => Promise.race([chain, Promise.resolve()])
       try {
         client = await XoverClient.connect({
           host: nntpConfig.host,
@@ -117,12 +121,23 @@ export default class SeedNode extends BaseCommand {
           chunk: this.chunk,
           submitEvery: this.submitEvery,
           isDeferred: (header: OverviewHeader) => indexerConfig.deferBodyMessageIds.some((re) => re.test(header['message-id'] ?? '')),
+          // Batches are posted in order, one after the other, while fetching goes on; with two
+          // waiting, fetching waits too. A failed post stops the chain, so no batch is stored
+          // past a missing one (the range would resume beyond it).
           sink: async (batch: SeedBatch) => {
             if (this.stopping) throw new Stopping()
-            await api.post(`/api/v1/seed/ranges/${lease!.id}/batches`, batch)
-            headers += batch.headers
+            while (posting.size >= 2) await Promise.race(posting)
+            const post = chain.then(async () => {
+              await api.post(`/api/v1/seed/ranges/${lease!.id}/batches`, batch)
+              headers += batch.headers
+            })
+            chain = post
+            posting.add(post)
+            post.catch(() => {}).finally(() => posting.delete(post))
+            await failedSoFar()
           },
         })
+        await chain
         await api.post(`/api/v1/seed/ranges/${lease.id}/done`)
         const seconds = (Date.now() - started) / 1000
         log(`done: ${headers.toLocaleString('en')} headers in ${seconds.toFixed(0)} s (${Math.round(headers / seconds).toLocaleString('en')}/s)`)
@@ -134,6 +149,8 @@ export default class SeedNode extends BaseCommand {
         }
         const reason = error instanceof Stopping ? 'node stopping' : error.message
         if (!(error instanceof Stopping)) log(`Range ${lease.id} failed: ${reason}`)
+        // Let posts in flight finish (or fail) first, so the range resumes after what was stored.
+        await chain.catch(() => {})
         await api.post(`/api/v1/seed/ranges/${lease.id}/release`, { error: reason }).catch(() => {})
         if (!this.stopping) await this.pause(30_000)
       } finally {