Fix stderr obtention

* The Stderr pipe should be read before waiting
* The Stderr pipe should not be used with Run/Output. See https://golang.org/pkg/os/exec/#Cmd.StderrPipe:

  Wait will close the pipe after seeing the command exit, so most callers need
  not close the pipe themselves; however, an implication is that it is incorrect
  to call Wait before all reads from the pipe have completed. For the same
  reason, it is incorrect to use Run when using StderrPipe.

common/weave/client.go

@@ -9,6 +9,7 @@ import (
 	"net/http"
 	"net/url"
 	"os"
+	realexec "os/exec"
 	"regexp"
 	"strconv"
 
@@ -196,9 +197,9 @@ func (c *client) PS() (map[string]PSEntry, error) {
 		}
 	}
 	scannerErr := scanner.Err()
+	slurp, _ := ioutil.ReadAll(stdErr)
 	cmdErr := cmd.Wait()
 	if cmdErr != nil {
-		slurp, _ := ioutil.ReadAll(stdErr)
 		return nil, fmt.Errorf("%s: %q", cmdErr, slurp)
 	}
 	if scannerErr != nil {
@@ -209,14 +210,13 @@ func (c *client) PS() (map[string]PSEntry, error) {
 
 func (c *client) Expose() error {
 	cmd := weaveCommand("--local", "ps", "weave:expose")
-	stdErr, err := cmd.StderrPipe()
-	if err != nil {
-		return err
-	}
 	output, err := cmd.Output()
 	if err != nil {
-		slurp, _ := ioutil.ReadAll(stdErr)
-		return fmt.Errorf("Error running weave ps: %s: %q", err, slurp)
+		stdErr := []byte{}
+		if exitErr, ok := err.(*realexec.ExitError); ok {
+			stdErr = exitErr.Stderr
+		}
+		return fmt.Errorf("Error running weave ps: %s: %q", err, stdErr)
 	}
 	ips := ipMatch.FindAllSubmatch(output, -1)
 	if ips != nil {
@@ -224,13 +224,12 @@ func (c *client) Expose() error {
 		return nil
 	}
 	cmd = weaveCommand("--local", "expose")
-	stdErr, err = cmd.StderrPipe()
-	if err != nil {
-		return err
-	}
-	if err := cmd.Run(); err != nil {
-		slurp, _ := ioutil.ReadAll(stdErr)
-		return fmt.Errorf("Error running weave expose: %s: %q", err, slurp)
+	if _, err := cmd.Output(); err != nil {
+		stdErr := []byte{}
+		if exitErr, ok := err.(*realexec.ExitError); ok {
+			stdErr = exitErr.Stderr
+		}
+		return fmt.Errorf("Error running weave expose: %s: %q", err, stdErr)
 	}
 	return nil
 }