[proxy] print all stderr from callWeave to the proxy logs, even if successful

proxy/common.go

@@ -18,13 +18,16 @@ var (
 	weaveWaitEntrypoint = []string{"/w/w"}
 )
 
-func callWeave(args ...string) ([]byte, error) {
+func callWeave(args ...string) ([]byte, []byte, error) {
 	args = append([]string{"--local"}, args...)
 	Log.Debug("Calling weave", args)
 	cmd := exec.Command("./weave", args...)
 	cmd.Env = []string{"PROCFS=/hostproc", "PATH=/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"}
-	out, err := cmd.CombinedOutput()
-	return out, err
+	var stdout, stderr bytes.Buffer
+	cmd.Stdout = &stdout
+	cmd.Stderr = &stderr
+	err := cmd.Run()
+	return stdout.Bytes(), stderr.Bytes(), err
 }
 
 func marshalRequestBody(r *http.Request, body interface{}) error {

proxy/proxy.go

@@ -57,9 +57,9 @@ func NewProxy(c Config) (*Proxy, error) {
 	p.client = client
 
 	if !p.WithoutDNS {
-		dockerBridgeIP, err := callWeave("docker-bridge-ip")
+		dockerBridgeIP, stderr, err := callWeave("docker-bridge-ip")
 		if err != nil {
-			return nil, err
+			return nil, fmt.Errorf(string(stderr))
 		}
 		p.dockerBridgeIP = string(dockerBridgeIP)
 	}

proxy/start_container_interceptor.go

@@ -30,9 +30,11 @@ func (i *startContainerInterceptor) InterceptResponse(r *http.Response) error {
 	args := []string{"attach"}
 	args = append(args, cidrs...)
 	args = append(args, "--or-die", container.ID)
-	if output, err := callWeave(args...); err != nil {
-		Log.Warningf("Attaching container %s to weave network failed: %s", container.ID, string(output))
-		return errors.New(string(output))
+	if _, stderr, err := callWeave(args...); err != nil {
+		Log.Warningf("Attaching container %s to weave network failed: %s", container.ID, string(stderr))
+		return errors.New(string(stderr))
+	} else if len(stderr) > 0 {
+		Log.Warningf("Attaching container %s to weave network: %s", container.ID, string(stderr))
 	}
 
 	return i.proxy.client.KillContainer(docker.KillContainerOptions{ID: container.ID, Signal: docker.SIGUSR2})