http: shutdown the actual http server

PR thanos-io#1680 introduced graceful handling for the HTTP server in Thanos, but
the graceful `Shutdown` call was being performed on an `http.Server`
instance that was *not* running at all. The actual server that was
listening for requests was started through `http.Serve`, so there was no
reference to the server struct that we could use to shut it down. This
was causing all of Thanos to freeze after receiving an exit signal,
because the run-group for the HTTP server would never finalize.

This seems like an oversight because the `(*Server).srv` field was being
properly initialized with an HTTP server. Fix this by calling
`ListenAndServe` on our initialized server.

Signed-off-by: Vicent Marti <vmg@strn.cat>

pkg/server/http.go

@@ -2,7 +2,6 @@ package server
 
 import (
 	"context"
-	"net"
 	"net/http"
 	"net/http/pprof"
 	"time"
@@ -22,9 +21,8 @@ type Server struct {
 	comp   component.Component
 	prober *prober.Prober
 
-	mux      *http.ServeMux
-	srv      *http.Server
-	listener net.Listener
+	mux *http.ServeMux
+	srv *http.Server
 
 	opts options
 }
@@ -55,15 +53,9 @@ func NewHTTP(logger log.Logger, reg *prometheus.Registry, comp component.Compone
 }
 
 func (s *Server) ListenAndServe() error {
-	l, err := net.Listen("tcp", s.opts.listen)
-	if err != nil {
-		return errors.Wrap(err, "listen metrics address")
-	}
-	s.listener = l
 	s.prober.SetHealthy()
 	level.Info(s.logger).Log("msg", "listening for requests and metrics", "component", s.comp.String(), "address", s.opts.listen)
-
-	return errors.Wrapf(http.Serve(l, s.mux), "serve %s and metrics", s.comp.String())
+	return errors.Wrapf(s.srv.ListenAndServe(), "serve %s and metrics", s.comp.String())
 }
 
 func (s *Server) Shutdown(err error) {