net/http: make strace test more robust

Apparently some distros don't let you ptrace attach
to your own existing processes.

Run strace on the child directly, instead, which
reportedly is more often allowed, and makes the
code simpler too.

R=golang-dev, n13m3y3r
CC=golang-dev
https://golang.org/cl/5675050

src/pkg/net/http/fs_test.go

@@ -18,7 +18,6 @@ import (
 	"path/filepath"
 	"regexp"
 	"runtime"
-	"strconv"
 	"strings"
 	"testing"
 	"time"
@@ -387,24 +386,15 @@ func TestLinuxSendfile(t *testing.T) {
 	}
 	defer ln.Close()
 
-	child := exec.Command(os.Args[0], "-test.run=TestLinuxSendfileChild")
+	var buf bytes.Buffer
+	child := exec.Command("strace", "-f", os.Args[0], "-test.run=TestLinuxSendfileChild")
 	child.ExtraFiles = append(child.ExtraFiles, lnf)
 	child.Env = append([]string{"GO_WANT_HELPER_PROCESS=1"}, os.Environ()...)
-
+	child.Stdout = &buf
+	child.Stderr = &buf
 	err = child.Start()
 	if err != nil {
-		t.Fatal(err)
-	}
-
-	pid := child.Process.Pid
-
-	var buf bytes.Buffer
-	strace := exec.Command("strace", "-f", "-p", strconv.Itoa(pid))
-	strace.Stdout = &buf
-	strace.Stderr = &buf
-	err = strace.Start()
-	if err != nil {
-		t.Logf("skipping; failed to start strace: %v", err)
+		t.Logf("skipping; failed to start straced child: %v", err)
 		return
 	}
 
@@ -417,7 +407,6 @@ func TestLinuxSendfile(t *testing.T) {
 	// Force child to exit cleanly.
 	Get(fmt.Sprintf("http://%s/quit", ln.Addr()))
 	child.Wait()
-	strace.Wait()
 
 	rx := regexp.MustCompile(`sendfile(64)?\(\d+,\s*\d+,\s*NULL,\s*\d+\)\s*=\s*\d+\s*\n`)
 	rxResume := regexp.MustCompile(`<\.\.\. sendfile(64)? resumed> \)\s*=\s*\d+\s*\n`)