TestCheckPerms: utilize pipe to read stderr

Refer to the stderr manpage:
$ man 3 stderr
*Note that mixing use of FILEs and raw file descriptors can produce unexpected results and should generally be avoided.*

And actually, we noticed that the warning() will output the message to
stdout instead of stderr sometimes.

lizj@FNSTPC:~/workspace/k8s/helm$ while true; do timeout 1m go test -count=1  -run TestCheckPerms ./cmd/helm -v 2>/dev/null; done
=== RUN   TestCheckPerms
--- PASS: TestCheckPerms (0.00s)
PASS
ok  	helm.sh/helm/v3/cmd/helm	0.028s
=== RUN   TestCheckPerms
--- PASS: TestCheckPerms (0.00s)
PASS
ok  	helm.sh/helm/v3/cmd/helm	0.027s
=== RUN   TestCheckPerms
--- PASS: TestCheckPerms (0.00s)
PASS
ok  	helm.sh/helm/v3/cmd/helm	0.028s
=== RUN   TestCheckPerms
--- PASS: TestCheckPerms (0.00s)
PASS
ok  	helm.sh/helm/v3/cmd/helm	0.029s
=== RUN   TestCheckPerms
--- PASS: TestCheckPerms (0.00s)
PASS
ok  	helm.sh/helm/v3/cmd/helm	0.029s
=== RUN   TestCheckPerms
--- PASS: TestCheckPerms (0.00s)
PASS
ok  	helm.sh/helm/v3/cmd/helm	0.028s
=== RUN   TestCheckPerms
--- PASS: TestCheckPerms (0.00s)
PASS
ok  	helm.sh/helm/v3/cmd/helm	0.030s
=== RUN   TestCheckPerms
WARNING: Kubernetes configuration file is group-readable. This is insecure. Location: /tmp/helmtest093620773/testconfig
=== RUN   TestCheckPerms
WARNING: Kubernetes configuration file is group-readable. This is insecure. Location: /tmp/helmtest083469215/testconfig
=== RUN   TestCheckPerms
WARNING: Kubernetes configuration file is group-readable. This is insecure. Location: /tmp/helmtest101343249/testconfig
=== RUN   TestCheckPerms
--- PASS: TestCheckPerms (0.00s)
PASS
ok  	helm.sh/helm/v3/cmd/helm	0.032s
=== RUN   TestCheckPerms
--- PASS: TestCheckPerms (0.00s)
PASS
ok  	helm.sh/helm/v3/cmd/helm	0.040s
=== RUN   TestCheckPerms
--- PASS: TestCheckPerms (0.00s)
PASS
ok  	helm.sh/helm/v3/cmd/helm	0.031s
=== RUN   TestCheckPerms
WARNING: Kubernetes configuration file is group-readable. This is insecure. Location: /tmp/helmtest706352639/testconfig

Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
Signed-off-by: Lu Fengqi <lufq.fnst@cn.fujitsu.com>

cmd/helm/root_unix_test.go

@@ -19,7 +19,8 @@ limitations under the License.
 package main
 
 import (
-	"bufio"
+	"bytes"
+	"io"
 	"io/ioutil"
 	"os"
 	"path/filepath"
@@ -27,15 +28,27 @@ import (
 	"testing"
 )
 
-func TestCheckPerms(t *testing.T) {
-	// NOTE(bacongobbler): have to open a new file handler here as the default os.Sterr cannot be read from
-	stderr, err := os.Open("/dev/stderr")
+func checkPermsStderr() (string, error) {
+	r, w, err := os.Pipe()
 	if err != nil {
-		t.Fatalf("could not open /dev/stderr for reading: %s", err)
+		return "", err
 	}
-	defer stderr.Close()
-	reader := bufio.NewReader(stderr)
 
+	stderr := os.Stderr
+	os.Stderr = w
+	defer func() {
+		os.Stderr = stderr
+	}()
+
+	checkPerms()
+	w.Close()
+
+	var text bytes.Buffer
+	io.Copy(&text, r)
+	return text.String(), nil
+}
+
+func TestCheckPerms(t *testing.T) {
 	tdir, err := ioutil.TempDir("", "helmtest")
 	if err != nil {
 		t.Fatal(err)
@@ -51,8 +64,7 @@ func TestCheckPerms(t *testing.T) {
 	settings.KubeConfig = tfile
 	defer func() { settings.KubeConfig = tconfig }()
 
-	checkPerms()
-	text, err := reader.ReadString('\n')
+	text, err := checkPermsStderr()
 	if err != nil {
 		t.Fatalf("could not read from stderr: %s", err)
 	}
@@ -64,8 +76,7 @@ func TestCheckPerms(t *testing.T) {
 	if err := fh.Chmod(0404); err != nil {
 		t.Errorf("Could not change mode on file: %s", err)
 	}
-	checkPerms()
-	text, err = reader.ReadString('\n')
+	text, err = checkPermsStderr()
 	if err != nil {
 		t.Fatalf("could not read from stderr: %s", err)
 	}