[proxy] We can only rely on receiving a signal when we are a container

Due to https://github.com/docker/docker/issues/9098, there is no way to
signal an exec. Instead we'll have to just poll the interface and hope.

Adds `weavewait -s` which is the way to tell weavewait to skip waiting
for a signal, which we use on execs

prog/weavewait/main.go

@@ -1,32 +1,42 @@
 package main
 
 import (
-	"errors"
 	"fmt"
 	"os"
 	"os/exec"
 	"os/signal"
 	"syscall"
 	"time"
+
+	"github.com/weaveworks/weave/net"
 )
 
 func main() {
-	usr2 := make(chan os.Signal)
-	signal.Notify(usr2, syscall.SIGUSR2)
-	select {
-	case <-usr2:
-	case <-time.After(20 * time.Second):
-		checkErr(errors.New("Container timed out waiting for signal from proxy"))
-	}
-
 	if len(os.Args) <= 1 {
 		os.Exit(0)
 	}
 
-	binary, err := exec.LookPath(os.Args[1])
+	args := os.Args[1:]
+	signalWait := 20
+	if args[0] == "-s" {
+		signalWait = 0
+		args = args[1:]
+	}
+	interfaceWait := 20 - signalWait
+
+	usr2 := make(chan os.Signal)
+	signal.Notify(usr2, syscall.SIGUSR2)
+	select {
+	case <-usr2:
+	case <-time.After(time.Duration(signalWait) * time.Second):
+	}
+	_, err := net.EnsureInterface("ethwe", interfaceWait)
 	checkErr(err)
 
-	checkErr(syscall.Exec(binary, os.Args[1:], os.Environ()))
+	binary, err := exec.LookPath(args[0])
+	checkErr(err)
+
+	checkErr(syscall.Exec(binary, args, os.Environ()))
 }
 
 func checkErr(err error) {

proxy/create_exec_interceptor.go

@@ -34,7 +34,8 @@ func (i *createExecInterceptor) InterceptRequest(r *http.Request) error {
 
 	if cidrs, ok := weaveCIDRsFromConfig(container.Config); ok || i.withIPAM {
 		Info.Printf("Exec in container %s with WEAVE_CIDR \"%s\"", container.ID, strings.Join(cidrs, " "))
-		options.Cmd = append(weaveWaitEntrypoint, options.Cmd...)
+		cmd := append(weaveWaitEntrypoint, "-s")
+		options.Cmd = append(cmd, options.Cmd...)
 	}
 
 	if err := marshalRequestBody(r, options); err != nil {