LimitListener to limit max number of connections

This also drops tcp keep-alive in ListenAndServe but it's no longer
necessary since we now close idle connections long before that.

cmd/prometheus/config.go

@@ -84,6 +84,10 @@ func init() {
 		&cfg.web.ReadTimeout, "web.read-timeout", 30*time.Second,
 		"Maximum duration before timing out read of the request, and closing idle connections.",
 	)
+	cfg.fs.IntVar(
+		&cfg.web.MaxConnections, "web.max-connections", 100,
+		"Maximum number of simultaneous connections.",
+	)
 	cfg.fs.StringVar(
 		&cfg.prometheusURL, "web.external-url", "",
 		"The URL under which Prometheus is externally reachable (for example, if Prometheus is served via a reverse proxy). Used for generating relative and absolute links back to Prometheus itself. If the URL has a path portion, it will be used to prefix all HTTP endpoints served by Prometheus. If omitted, relevant URL components will be derived automatically.",

web/web.go

@@ -37,6 +37,7 @@ import (
 	"github.com/prometheus/common/model"
 	"github.com/prometheus/common/route"
 	"golang.org/x/net/context"
+	"golang.org/x/net/netutil"
 
 	"github.com/prometheus/prometheus/config"
 	"github.com/prometheus/prometheus/notifier"
@@ -112,6 +113,7 @@ type Options struct {
 
 	ListenAddress        string
 	ReadTimeout          time.Duration
+	MaxConnections       int
 	ExternalURL          *url.URL
 	RoutePrefix          string
 	MetricsPath          string
@@ -253,7 +255,13 @@ func (h *Handler) Run() {
 		ErrorLog:    log.NewErrorLogger(),
 		ReadTimeout: h.options.ReadTimeout,
 	}
-	h.listenErrCh <- server.ListenAndServe()
+	listener, err := net.Listen("tcp", h.options.ListenAddress)
+	if err != nil {
+		h.listenErrCh <- err
+	} else {
+		limitedListener := netutil.LimitListener(listener, h.options.MaxConnections)
+		h.listenErrCh <- server.Serve(limitedListener)
+	}
 }
 
 func (h *Handler) alerts(w http.ResponseWriter, r *http.Request) {