vmselect: exit early from queue on context cancel (#4223)

* vmselect: exit early from queue on context cancel

When `-search.maxConcurrentRequests` is reached, vmselect puts
request in the queue. It is expected, that requests in the queue
will be processed as soon as it would be enough capacity to do so.

However, it could happen that while request was waiting its turn,
the client could have already cancel it (close the connection,
or just close the tab with UI). In this case, we should de-queue
such requests to avoid spending extra resources on them.

Signed-off-by: hagen1778 <roman@victoriametrics.com>

* app/vmselect: address review comments

Signed-off-by: hagen1778 <roman@victoriametrics.com>

---------

Signed-off-by: hagen1778 <roman@victoriametrics.com>

app/vmselect/main.go

@@ -209,6 +209,13 @@ func requestHandler(w http.ResponseWriter, r *http.Request) bool {
 			timerpool.Put(t)
 			qt.Printf("wait in queue because -search.maxConcurrentRequests=%d concurrent requests are executed", *maxConcurrentRequests)
 			defer func() { <-concurrencyLimitCh }()
+		case <-r.Context().Done():
+			timerpool.Put(t)
+			remoteAddr := httpserver.GetQuotedRemoteAddr(r)
+			requestURI := httpserver.GetRequestURI(r)
+			logger.Infof("client has cancelled the request after %.3f seconds: remoteAddr=%s, requestURI: %q",
+				d.Seconds(), remoteAddr, requestURI)
+			return true
 		case <-t.C:
 			timerpool.Put(t)
 			concurrencyLimitTimeout.Inc()