v2.10.2: cancel superseded queries instead of letting them pile up

Clicking through targets and windows faster than they load left every abandoned query
running server-side: the front end dropped the stale responses, but the reads kept
chewing through day files and starved each other. That is why a 12h window showed up
in the slow log at 6-8 seconds while a 30d read was in flight.

- front end aborts the in-flight series request before starting the next one
  (AbortController, no new dependency); an aborted request exits silently
- ReadRange takes a context and checks it between day files, so a disconnected client
  stops the read immediately instead of finishing work nobody is waiting for

Measured: 8 rapid clicks now produce 4 completed and 4 cancelled requests; after a
20-click burst the chart is back in 15ms with zero slow-request log lines.

Author: githubflyideas

static/index.html

@@ -216,6 +216,8 @@
 
 let sel = null; // {from,to} 自定义查询区间(unix 秒)
 let renderSeq = 0;
+let inflight = null; // 正在飞的 series 请求 —— 新请求发起前先掐掉它,
+                     // 否则快速点击会在服务端堆起一串没人要的慢查询,互相拖死。
 let rendering = false; // 一次渲染(含慢速 band 查询)是否仍在进行
 async function render() {
   const seq = ++renderSeq;
@@ -234,7 +236,18 @@
   let url = `/api/series?target=${encodeURIComponent(current)}`;
   if (sel) url += `&from=${sel.from}&to=${sel.to}`;
   else url += `&minutes=${minutes}`;
-  const rounds = await (await fetch(url)).json() || [];
+  if (inflight) inflight.abort();
+  const ctl = new AbortController();
+  inflight = ctl;
+  let rounds;
+  try {
+    rounds = await (await fetch(url, { signal: ctl.signal })).json() || [];
+  } catch (e) {
+    if (e.name === 'AbortError') return; // 被新点击取代,静默退出
+    throw e;
+  } finally {
+    if (inflight === ctl) inflight = null;
+  }
   if (seq !== renderSeq) return; // 过期响应:已切走,丢弃
 
   const smoke = [], median = [], lossBars = [], burstPts = [];

store.go

@@ -2,6 +2,7 @@ package main
 
 import (
 	"bufio"
+	"context"
 	"encoding/json"
 	"fmt"
 	"log"
@@ -161,12 +162,15 @@ func (s *Store) RemoveTarget(name string) {
 	}
 }
 
-func (s *Store) ReadRange(name string, from, to int64) []Round {
+// ReadRange reads raw rounds in [from,to]. ctx lets a long read stop early when the
+// browser navigates away: without it, clicking through targets leaves every abandoned
+// query still chewing through day files, and they starve each other.
+func (s *Store) ReadRange(ctx context.Context, name string, from, to int64) []Round {
 	var rounds []Round
 	// ring first (covers the recent tail cheaply)
 	s.mu.RLock()
 	ring := s.rings[name]
-	ringFrom := int64(1<<62)
+	ringFrom := int64(1 << 62)
 	if len(ring) > 0 {
 		ringFrom = ring[0].T
 	}
@@ -176,6 +180,11 @@ func (s *Store) ReadRange(name string, from, to int64) []Round {
 		rounds = s.Recent(name, from)
 	} else {
 		for d := time.Unix(from, 0); !d.After(time.Unix(to, 0)); d = d.AddDate(0, 0, 1) {
+			select {
+			case <-ctx.Done(): // client went away — stop reading, drop what we have
+				return []Round{}
+			default:
+			}
 			day, _ := s.readDay(name, d.Format("2006-01-02"))
 			rounds = append(rounds, day...)
 		}

web.go

@@ -170,7 +170,7 @@ func serveWeb(cfg *Config, store *Store, users map[string]string) error {
 			to = time.Now().Unix()
 			from = to - int64(minutes)*60
 		}
-		writeJSON(w, store.ReadRange(name, from, to))
+		writeJSON(w, store.ReadRange(r.Context(), name, from, to))
 	}))
 
 	return http.ListenAndServe(cfg.Listen, mux)