Rebuild: a chunk's deadline doubles each time it's hit

At 90 s a chunk that always takes longer would be given up forever
(range 155437's worker gave up waiting on its stream). Each deadline a
chunk hits doubles its next one (90, 180, 360 ... up to 16x), reset
once it comes in.

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

app/services/prefetcher.ts

@@ -59,6 +59,8 @@ export class Prefetcher {
   private fetchingNow = 0
   private writingNow = 0
   private sampler?: NodeJS.Timeout
+  /** Deadlines a chunk hit, by range and chunk start: each doubles its next deadline, so a slow chunk still comes. */
+  private lateChunks = new Map<string, number>()
   /** Requests in flight: what, since when, on which connection (the report names the oldest). */
   private inFlight = new Set<{ rangeId: number; from: number; to: number; since: number; client: XoverClient }>()
 
@@ -190,11 +192,14 @@ export class Prefetcher {
         const entry = { rangeId, from: span.from, to: span.to, since: Date.now(), client: c }
         this.inFlight.add(entry)
         let late = false
+        const key = `${rangeId}:${span.from}`
+        const deadlineMs = coordinatorConfig.nntpChunkDeadlineMs * 2 ** Math.min(this.lateChunks.get(key) ?? 0, 4)
         const deadline = setTimeout(() => {
           late = true
           this.stats.deadlines++
+          this.lateChunks.set(key, (this.lateChunks.get(key) ?? 0) + 1)
           c.close()
-        }, coordinatorConfig.nntpChunkDeadlineMs)
+        }, deadlineMs)
         let data: Buffer
         let groupMs = 0
         try {
@@ -204,11 +209,12 @@ export class Prefetcher {
           data = await c.xoverRaw(span.from, span.to)
         } catch (error) {
           if (!late) throw error
-          throw new Error(`Chunk ${span.from}–${span.to} of range ${rangeId} not in after ${coordinatorConfig.nntpChunkDeadlineMs / 1000} s; fetching it again`)
+          throw new Error(`Chunk ${span.from}–${span.to} of range ${rangeId} not in after ${deadlineMs / 1000} s; fetching it again`)
         } finally {
           clearTimeout(deadline)
           this.inFlight.delete(entry)
         }
+        this.lateChunks.delete(key)
         if (!cancelled) {
           const t = c.lastTiming
           this.stats.requests++