cmd/tipgodoc: Kill godoc process if http.ListenAndServe fails

Currently, if for some reason http.ListenAndServe fails, any running
running godoc processes don't get killed. I don't think this would ever
actually happen because, with godoc being set up in a separate go
routine, http.ListenAndServe would always(?) fail before the godoc
server started.

This change ensures that, if a Proxy has a cmd, it is closed when
http.ListenAndServe fails.

Change-Id: I0d3bfae0c16bc583248c2052a4d7a84c95127e76
Reviewed-on: https://go-review.googlesource.com/8570
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

cmd/tipgodoc/tip.go

@@ -36,7 +36,11 @@ func main() {
 	p := new(Proxy)
 	go p.run()
 	http.Handle("/", p)
-	log.Fatal(http.ListenAndServe(":8080", nil))
+
+	if err := http.ListenAndServe(":8080", nil); err != nil {
+		p.stop()
+		log.Fatal(err)
+	}
 }
 
 // Proxy implements the tip.golang.org server: a reverse-proxy
@@ -85,6 +89,14 @@ func (p *Proxy) run() {
 	}
 }
 
+func (p *Proxy) stop() {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	if p.cmd != nil {
+		p.cmd.Process.Kill()
+	}
+}
+
 // poll runs from the run loop goroutine.
 func (p *Proxy) poll() {
 	heads := gerritMetaMap()