cmd/{buildlet,runqemubuildlet}: use a functional healthAddr default

Reverse buildlets now listen publicly, which allows the QEMU host
forwarding to route to the buildlet.

Also, print a newline at the end of the healthz response for legibility.

For golang/go#47018

Change-Id: I71ae1bf4d7cbee4867c42e863cb9f8c2569e1b69
Reviewed-on: https://go-review.googlesource.com/c/build/+/336590
Trust: Alexander Rakoczy <[email protected]>
Run-TryBot: Alexander Rakoczy <[email protected]>
Reviewed-by: Heschi Kreinick <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
TryBot-Result: Go Bot <[email protected]>

Author: toothrot

cmd/buildlet/buildlet.go

@@ -57,7 +57,7 @@ var (
 	reverseType  = flag.String("reverse-type", "", "if non-empty, go into reverse mode where the buildlet dials the coordinator instead of listening for connections. The value is the dashboard/builders.go Hosts map key, naming a HostConfig. This buildlet will receive work for any BuildConfig specifying this named HostConfig.")
 	coordinator  = flag.String("coordinator", "localhost:8119", "address of coordinator, in production use farmer.golang.org. Only used in reverse mode.")
 	hostname     = flag.String("hostname", "", "hostname to advertise to coordinator for reverse mode; default is actual hostname")
-	healthAddr   = flag.String("health-addr", "localhost:8080", "For reverse buildlets, address to listen for /healthz requests separately from the reverse dialer to the coordinator.")
+	healthAddr   = flag.String("health-addr", "0.0.0.0:8080", "For reverse buildlets, address to listen for /healthz requests separately from the reverse dialer to the coordinator.")
 )
 
 // Bump this whenever something notable happens, or when another
@@ -2013,7 +2013,7 @@ func runOrLog(cmd *exec.Cmd) {
 
 // handleHealthz always returns 200 OK.
 func handleHealthz(w http.ResponseWriter, _ *http.Request) {
-	w.Write([]byte("ok"))
+	fmt.Fprintln(w, "ok")
 }
 
 // serveReverseHealth serves /healthz requests on healthAddr for

cmd/runqemubuildlet/main.go

@@ -92,7 +92,7 @@ func windows10Cmd(base string) *exec.Cmd {
 		"-device", "usb-mouse,bus=usb-bus.0",
 		"-device", "usb-kbd,bus=usb-bus.0",
 		"-device", "virtio-net-pci,netdev=net0",
-		"-netdev", "user,id=net0,hostfwd=tcp::8080-:8080",
+		"-netdev", "user,id=net0,hostfwd=tcp:127.0.0.1:8080-:8080",
 		"-bios", filepath.Join(base, "Images/QEMU_EFI.fd"),
 		"-device", "nvme,drive=drive0,serial=drive0,bootindex=0",
 		"-drive", fmt.Sprintf("if=none,media=disk,id=drive0,file=%s,cache=writethrough", filepath.Join(base, "Images/win10.qcow2")),