// Copyright 2020 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

//go:build zos && s390x

package unix_test

import (
	"flag"
	"fmt"
	"io"
	"net"
	"os"
	"os/exec"
	"path/filepath"
	"runtime"
	"strconv"
	"syscall"
	"testing"
	"time"
	"unsafe"

	"golang.org/x/sys/unix"
)

// Tests that below functions, structures and constants are consistent
// on all Unix-like systems.
func _() {
	// program scheduling priority functions and constants
	var (
		_ func(int, int, int) error   = unix.Setpriority
		_ func(int, int) (int, error) = unix.Getpriority
	)
	const (
		_ int = unix.PRIO_USER
		_ int = unix.PRIO_PROCESS
		_ int = unix.PRIO_PGRP
	)

	// termios constants
	const (
		_ int = unix.TCIFLUSH
		_ int = unix.TCIOFLUSH
		_ int = unix.TCOFLUSH
	)

	// fcntl file locking structure and constants
	var (
		_ = unix.Flock_t{
			Type:   int16(0),
			Whence: int16(0),
			Start:  int64(0),
			Len:    int64(0),
			Pid:    int32(0),
		}
	)
	const (
		_ = unix.F_GETLK
		_ = unix.F_SETLK
		_ = unix.F_SETLKW
	)
}

func TestErrnoSignalName(t *testing.T) {
	testErrors := []struct {
		num  syscall.Errno
		name string
	}{
		{syscall.EPERM, "EDC5139I"},
		{syscall.EINVAL, "EDC5121I"},
		{syscall.ENOENT, "EDC5129I"},
	}

	for _, te := range testErrors {
		t.Run(fmt.Sprintf("%d/%s", te.num, te.name), func(t *testing.T) {
			e := unix.ErrnoName(te.num)
			if e != te.name {
				t.Errorf("ErrnoName(%d) returned %s, want %s", te.num, e, te.name)
			}
		})
	}

	testSignals := []struct {
		num  syscall.Signal
		name string
	}{
		{syscall.SIGHUP, "SIGHUP"},
		{syscall.SIGPIPE, "SIGPIPE"},
		{syscall.SIGSEGV, "SIGSEGV"},
	}

	for _, ts := range testSignals {
		t.Run(fmt.Sprintf("%d/%s", ts.num, ts.name), func(t *testing.T) {
			s := unix.SignalName(ts.num)
			if s != ts.name {
				t.Errorf("SignalName(%d) returned %s, want %s", ts.num, s, ts.name)
			}
		})
	}
}

func TestSignalNum(t *testing.T) {
	testSignals := []struct {
		name string
		want syscall.Signal
	}{
		{"SIGHUP", syscall.SIGHUP},
		{"SIGPIPE", syscall.SIGPIPE},
		{"SIGSEGV", syscall.SIGSEGV},
		{"NONEXISTS", 0},
	}
	for _, ts := range testSignals {
		t.Run(fmt.Sprintf("%s/%d", ts.name, ts.want), func(t *testing.T) {
			got := unix.SignalNum(ts.name)
			if got != ts.want {
				t.Errorf("SignalNum(%s) returned %d, want %d", ts.name, got, ts.want)
			}
		})

	}
}

func TestFcntlInt(t *testing.T) {
	t.Parallel()
	file, err := os.Create(filepath.Join(t.TempDir(), "TestFnctlInt"))
	if err != nil {
		t.Fatal(err)
	}
	defer file.Close()
	f := file.Fd()
	flags, err := unix.FcntlInt(f, unix.F_GETFD, 0)
	if err != nil {
		t.Fatal(err)
	}
	if flags&unix.FD_CLOEXEC == 0 {
		t.Errorf("flags %#x do not include FD_CLOEXEC", flags)
	}
}

// TestFcntlFlock tests whether the file locking structure matches
// the calling convention of each kernel.
func TestFcntlFlock(t *testing.T) {
	name := filepath.Join(t.TempDir(), "TestFcntlFlock")
	fd, err := unix.Open(name, unix.O_CREAT|unix.O_RDWR|unix.O_CLOEXEC, 0)
	if err != nil {
		t.Fatalf("Open failed: %v", err)
	}
	defer unix.Close(fd)
	flock := unix.Flock_t{
		Type:  unix.F_RDLCK,
		Start: 0, Len: 0, Whence: 1,
	}
	if err := unix.FcntlFlock(uintptr(fd), unix.F_GETLK, &flock); err != nil {
		t.Fatalf("FcntlFlock failed: %v", err)
	}
}

// TestPassFD tests passing a file descriptor over a Unix socket.
//
// This test involved both a parent and child process. The parent
// process is invoked as a normal test, with "go test", which then
// runs the child process by running the current test binary with args
// "-test.run=^TestPassFD$" and an environment variable used to signal
// that the test should become the child process instead.
func TestPassFD(t *testing.T) {
	if runtime.GOOS == "darwin" && runtime.GOARCH == "arm64" {
		t.Skip("cannot exec subprocess on iOS, skipping test")
	}

	if os.Getenv("GO_WANT_HELPER_PROCESS") == "1" {
		passFDChild()
		return
	}

	if runtime.GOOS == "aix" {
		// Unix network isn't properly working on AIX
		// 7.2 with Technical Level < 2
		out, err := exec.Command("oslevel", "-s").Output()
		if err != nil {
			t.Skipf("skipping on AIX because oslevel -s failed: %v", err)
		}

		if len(out) < len("7200-XX-ZZ-YYMM") { // AIX 7.2, Tech Level XX, Service Pack ZZ, date YYMM
			t.Skip("skipping on AIX because oslevel -s hasn't the right length")
		}
		aixVer := string(out[:4])
		tl, err := strconv.Atoi(string(out[5:7]))
		if err != nil {
			t.Skipf("skipping on AIX because oslevel -s output cannot be parsed: %v", err)
		}
		if aixVer < "7200" || (aixVer == "7200" && tl < 2) {
			t.Skip("skipped on AIX versions previous to 7.2 TL 2")
		}
	}

	fds, err := unix.Socketpair(unix.AF_LOCAL, unix.SOCK_STREAM, 0)
	if err != nil {
		t.Fatalf("Socketpair: %v", err)
	}
	writeFile := os.NewFile(uintptr(fds[0]), "child-writes")
	readFile := os.NewFile(uintptr(fds[1]), "parent-reads")
	defer writeFile.Close()
	defer readFile.Close()

	cmd := exec.Command(os.Args[0], "-test.run=^TestPassFD$", "--", t.TempDir())
	cmd.Env = []string{"GO_WANT_HELPER_PROCESS=1"}
	if lp := os.Getenv("LD_LIBRARY_PATH"); lp != "" {
		cmd.Env = append(cmd.Env, "LD_LIBRARY_PATH="+lp)
	}
	cmd.ExtraFiles = []*os.File{writeFile}

	out, err := cmd.CombinedOutput()
	if len(out) > 0 || err != nil {
		t.Fatalf("child process: %q, %v", out, err)
	}

	c, err := net.FileConn(readFile)
	if err != nil {
		t.Fatalf("FileConn: %v", err)
	}
	defer c.Close()

	uc, ok := c.(*net.UnixConn)
	if !ok {
		t.Fatalf("unexpected FileConn type; expected UnixConn, got %T", c)
	}

	buf := make([]byte, 32) // expect 1 byte
	oob := make([]byte, 32) // expect 24 bytes
	closeUnix := time.AfterFunc(5*time.Second, func() {
		t.Logf("timeout reading from unix socket")
		uc.Close()
	})
	_, oobn, _, _, err := uc.ReadMsgUnix(buf, oob)
	if err != nil {
		t.Fatalf("ReadMsgUnix: %v", err)
	}
	closeUnix.Stop()

	scms, err := unix.ParseSocketControlMessage(oob[:oobn])
	if err != nil {
		t.Fatalf("ParseSocketControlMessage: %v", err)
	}
	if len(scms) != 1 {
		t.Fatalf("expected 1 SocketControlMessage; got scms = %#v", scms)
	}
	scm := scms[0]
	gotFds, err := unix.ParseUnixRights(&scm)
	if err != nil {
		t.Fatalf("unix.ParseUnixRights: %v", err)
	}
	if len(gotFds) != 1 {
		t.Fatalf("wanted 1 fd; got %#v", gotFds)
	}

	f := os.NewFile(uintptr(gotFds[0]), "fd-from-child")
	defer f.Close()

	got, err := io.ReadAll(f)
	want := "Hello from child process!\n"
	if string(got) != want {
		t.Errorf("child process ReadAll: %q, %v; want %q", got, err, want)
	}
}

// passFDChild is the child process used by TestPassFD.
func passFDChild() {
	defer os.Exit(0)

	// Look for our fd. It should be fd 3, but we work around an fd leak
	// bug here (http://golang.org/issue/2603) to let it be elsewhere.
	var uc *net.UnixConn
	for fd := uintptr(3); fd <= 10; fd++ {
		f := os.NewFile(fd, "unix-conn")
		var ok bool
		netc, _ := net.FileConn(f)
		uc, ok = netc.(*net.UnixConn)
		if ok {
			break
		}
	}
	if uc == nil {
		fmt.Println("failed to find unix fd")
		return
	}

	// Make a file f to send to our parent process on uc.
	// We make it in tempDir, which our parent will clean up.
	flag.Parse()
	tempDir := flag.Arg(0)
	f, err := os.Create(filepath.Join(tempDir, "file"))
	if err != nil {
		fmt.Println(err)
		return
	}

	f.Write([]byte("Hello from child process!\n"))
	f.Seek(0, 0)

	rights := unix.UnixRights(int(f.Fd()))
	dummyByte := []byte("x")
	n, oobn, err := uc.WriteMsgUnix(dummyByte, rights, nil)
	if err != nil {
		fmt.Printf("WriteMsgUnix: %v", err)
		return
	}
	if n != 1 || oobn != len(rights) {
		fmt.Printf("WriteMsgUnix = %d, %d; want 1, %d", n, oobn, len(rights))
		return
	}
}

// TestUnixRightsRoundtrip tests that UnixRights, ParseSocketControlMessage,
// and ParseUnixRights are able to successfully round-trip lists of file descriptors.
func TestUnixRightsRoundtrip(t *testing.T) {
	testCases := [...][][]int{
		{{42}},
		{{1, 2}},
		{{3, 4, 5}},
		{{}},
		{{1, 2}, {3, 4, 5}, {}, {7}},
	}
	for _, testCase := range testCases {
		b := []byte{}
		var n int
		for _, fds := range testCase {
			// Last assignment to n wins
			n = len(b) + unix.CmsgLen(4*len(fds))
			b = append(b, unix.UnixRights(fds...)...)
		}
		// Truncate b
		b = b[:n]

		scms, err := unix.ParseSocketControlMessage(b)
		if err != nil {
			t.Fatalf("ParseSocketControlMessage: %v", err)
		}
		if len(scms) != len(testCase) {
			t.Fatalf("expected %v SocketControlMessage; got scms = %#v", len(testCase), scms)
		}
		for i, scm := range scms {
			gotFds, err := unix.ParseUnixRights(&scm)
			if err != nil {
				t.Fatalf("ParseUnixRights: %v", err)
			}
			wantFds := testCase[i]
			if len(gotFds) != len(wantFds) {
				t.Fatalf("expected %v fds, got %#v", len(wantFds), gotFds)
			}
			for j, fd := range gotFds {
				if fd != wantFds[j] {
					t.Fatalf("expected fd %v, got %v", wantFds[j], fd)
				}
			}
		}
	}
}

func TestRlimit(t *testing.T) {
	var rlimit, zero unix.Rlimit
	err := unix.Getrlimit(unix.RLIMIT_NOFILE, &rlimit)
	if err != nil {
		t.Fatalf("Getrlimit: save failed: %v", err)
	}
	if zero == rlimit {
		t.Fatalf("Getrlimit: save failed: got zero value %#v", rlimit)
	}
	set := rlimit
	set.Cur = set.Max - 1
	if runtime.GOOS == "darwin" && set.Cur > 10240 {
		// The max file limit is 10240, even though
		// the max returned by Getrlimit is 1<<63-1.
		// This is OPEN_MAX in sys/syslimits.h.
		set.Cur = 10240
	}
	err = unix.Setrlimit(unix.RLIMIT_NOFILE, &set)
	if err != nil {
		t.Fatalf("Setrlimit: set failed: %#v %v", set, err)
	}
	var get unix.Rlimit
	err = unix.Getrlimit(unix.RLIMIT_NOFILE, &get)
	if err != nil {
		t.Fatalf("Getrlimit: get failed: %v", err)
	}
	set = rlimit
	set.Cur = set.Max - 1
	if set != get {
		// Seems like Darwin requires some privilege to
		// increase the soft limit of rlimit sandbox, though
		// Setrlimit never reports an error.
		switch runtime.GOOS {
		case "darwin":
		default:
			t.Fatalf("Rlimit: change failed: wanted %#v got %#v", set, get)
		}
	}
	err = unix.Setrlimit(unix.RLIMIT_NOFILE, &rlimit)
	if err != nil {
		t.Fatalf("Setrlimit: restore failed: %#v %v", rlimit, err)
	}
}

func TestSeekFailure(t *testing.T) {
	_, err := unix.Seek(-1, 0, 0)
	if err == nil {
		t.Fatalf("Seek(-1, 0, 0) did not fail")
	}
	str := err.Error() // used to crash on Linux
	t.Logf("Seek: %v", str)
	if str == "" {
		t.Fatalf("Seek(-1, 0, 0) return error with empty message")
	}
}

func TestSetsockoptString(t *testing.T) {
	// should not panic on empty string, see issue #31277
	err := unix.SetsockoptString(-1, 0, 0, "")
	if err == nil {
		t.Fatalf("SetsockoptString: did not fail")
	}
}

func TestDup(t *testing.T) {
	file, err := os.Create(filepath.Join(t.TempDir(), "TestDup"))
	if err != nil {
		t.Fatal(err)
	}
	defer file.Close()
	f := int(file.Fd())

	newFd, err := unix.Dup(f)
	if err != nil {
		t.Fatalf("Dup: %v", err)
	}

	// Create and reserve a file descriptor.
	// Dup2 automatically closes it before reusing it.
	nullFile, err := os.Open("/dev/null")
	if err != nil {
		t.Fatal(err)
	}
	dupFd := int(file.Fd())
	err = unix.Dup2(newFd, dupFd)
	if err != nil {
		t.Fatalf("Dup2: %v", err)
	}
	// Keep the dummy file open long enough to not be closed in
	// its finalizer.
	runtime.KeepAlive(nullFile)

	b1 := []byte("Test123")
	b2 := make([]byte, 7)
	_, err = unix.Write(dupFd, b1)
	if err != nil {
		t.Fatalf("Write to dup2 fd failed: %v", err)
	}
	_, err = unix.Seek(f, 0, 0)
	if err != nil {
		t.Fatalf("Seek failed: %v", err)
	}
	_, err = unix.Read(f, b2)
	if err != nil {
		t.Fatalf("Read back failed: %v", err)
	}
	if string(b1) != string(b2) {
		t.Errorf("Dup: stdout write not in file, expected %v, got %v", string(b1), string(b2))
	}
}

func TestGetwd(t *testing.T) {
	fd, err := os.Open(".")
	if err != nil {
		t.Fatalf("Open .: %s", err)
	}
	defer fd.Close()
	// Directory list for test. Do not worry if any are symlinks or do not
	// exist on some common unix desktop environments. That will be checked.
	dirs := []string{"/", "/usr/bin", "/etc", "/var", "/opt"}
	switch runtime.GOOS {
	case "android":
		dirs = []string{"/", "/system/bin"}
	case "darwin":
		switch runtime.GOARCH {
		case "arm64":
			dirs = []string{t.TempDir(), t.TempDir()}
		}
	}
	oldwd := os.Getenv("PWD")
	for _, d := range dirs {
		// Check whether d exists, is a dir and that d's path does not contain a symlink
		fi, err := os.Stat(d)
		if err != nil || !fi.IsDir() {
			t.Logf("Test dir %s stat error (%v) or not a directory, skipping", d, err)
			continue
		}
		check, err := filepath.EvalSymlinks(d)
		if err != nil || check != d {
			t.Logf("Test dir %s (%s) is symlink or other error (%v), skipping", d, check, err)
			continue
		}
		err = os.Chdir(d)
		if err != nil {
			t.Fatalf("Chdir: %v", err)
		}
		pwd, err := unix.Getwd()
		if err != nil {
			t.Fatalf("Getwd in %s: %s", d, err)
		}
		os.Setenv("PWD", oldwd)
		err = fd.Chdir()
		if err != nil {
			// We changed the current directory and cannot go back.
			// Don't let the tests continue; they'll scribble
			// all over some other directory.
			fmt.Fprintf(os.Stderr, "fchdir back to dot failed: %s\n", err)
			os.Exit(1)
		}
		if pwd != d {
			t.Fatalf("Getwd returned %q want %q", pwd, d)
		}
	}
}

func TestMkdev(t *testing.T) {
	major := uint32(42)
	minor := uint32(7)
	dev := unix.Mkdev(major, minor)

	if unix.Major(dev) != major {
		t.Errorf("Major(%#x) == %d, want %d", dev, unix.Major(dev), major)
	}
	if unix.Minor(dev) != minor {
		t.Errorf("Minor(%#x) == %d, want %d", dev, unix.Minor(dev), minor)
	}
}

func TestMountUnmount(t *testing.T) {
	// use an available fs
	var buffer struct {
		header unix.W_Mnth
		fsinfo [64]unix.W_Mntent
	}
	fsCount, err := unix.W_Getmntent_A((*byte)(unsafe.Pointer(&buffer)), int(unsafe.Sizeof(buffer)))
	if err != nil {
		t.Fatalf("W_Getmntent_A returns with error: %s", err.Error())
	} else if fsCount == 0 {
		t.Fatalf("W_Getmntent_A returns no entries")
	}
	var fs string
	var fstype string
	var mountpoint string
	var available bool = false
	for i := 0; i < fsCount; i++ {
		err = unix.Unmount(unix.ByteSliceToString(buffer.fsinfo[i].Mountpoint[:]), unix.MTM_RDWR)
		if err != nil {
			// Unmount and Mount require elevated privilege
			// If test is run without such permission, skip test
			if err == unix.EPERM {
				t.Logf("Permission denied for Unmount. Skipping test (Errno2:  %X)", unix.Errno2())
				return
			} else if err == unix.EBUSY {
				continue
			} else {
				t.Fatalf("Unmount returns with error: %s", err.Error())
			}
		} else {
			available = true
			fs = unix.ByteSliceToString(buffer.fsinfo[i].Fsname[:])
			fstype = unix.ByteSliceToString(buffer.fsinfo[i].Fstname[:])
			mountpoint = unix.ByteSliceToString(buffer.fsinfo[i].Mountpoint[:])
			t.Logf("using file system = %s; fstype = %s and mountpoint = %s\n", fs, fstype, mountpoint)
			break
		}
	}
	if !available {
		t.Fatalf("No filesystem available")
	}
	// test unmount
	buffer.header = unix.W_Mnth{}
	fsCount, err = unix.W_Getmntent_A((*byte)(unsafe.Pointer(&buffer)), int(unsafe.Sizeof(buffer)))
	if err != nil {
		t.Fatalf("W_Getmntent_A returns with error: %s", err.Error())
	}
	for i := 0; i < fsCount; i++ {
		if unix.ByteSliceToString(buffer.fsinfo[i].Fsname[:]) == fs {
			t.Fatalf("File system found after unmount")
		}
	}
	// test mount
	err = unix.Mount(fs, mountpoint, fstype, unix.MTM_RDWR, "")
	if err != nil {
		t.Fatalf("Mount returns with error: %s", err.Error())
	}
	buffer.header = unix.W_Mnth{}
	fsCount, err = unix.W_Getmntent_A((*byte)(unsafe.Pointer(&buffer)), int(unsafe.Sizeof(buffer)))
	if err != nil {
		t.Fatalf("W_Getmntent_A returns with error: %s", err.Error())
	}
	fsMounted := false
	for i := 0; i < fsCount; i++ {
		if unix.ByteSliceToString(buffer.fsinfo[i].Fsname[:]) == fs &&
			unix.ByteSliceToString(buffer.fsinfo[i].Mountpoint[:]) == mountpoint {
			fsMounted = true
		}
	}
	if !fsMounted {
		t.Fatalf("%s not mounted after Mount()", fs)
	}
}

func TestChroot(t *testing.T) {
	// create temp dir and tempfile 1
	tempDir := t.TempDir()
	f, err := os.Create(filepath.Join(tempDir, "chroot_test_file"))
	if err != nil {
		t.Fatal(err)
	}
	// chroot temp dir
	err = unix.Chroot(tempDir)
	// Chroot requires elevated privilege
	// If test is run without such permission, skip test
	if err == unix.EPERM {
		t.Logf("Denied permission for Chroot. Skipping test (Errno2:  %X)", unix.Errno2())
		return
	} else if err != nil {
		t.Fatalf("Chroot: %s", err.Error())
	}
	// check if tempDir contains test file
	files, err := os.ReadDir("/")
	if err != nil {
		t.Fatalf("ReadDir: %s", err.Error())
	}
	found := false
	for _, file := range files {
		if file.Name() == filepath.Base(f.Name()) {
			found = true
			break
		}
	}
	if !found {
		t.Fatalf("Temp file not found in temp dir")
	}
}

func TestFlock(t *testing.T) {
	if os.Getenv("GO_WANT_HELPER_PROCESS") == "1" {
		defer os.Exit(0)
		if len(os.Args) != 3 {
			fmt.Printf("bad argument")
			return
		}
		fn := os.Args[2]
		f, err := os.OpenFile(fn, os.O_RDWR, 0755)
		if err != nil {
			fmt.Printf("%s", err.Error())
			return
		}
		err = unix.Flock(int(f.Fd()), unix.LOCK_EX|unix.LOCK_NB)
		// if the lock we are trying should be locked, ignore EAGAIN error
		// otherwise, report all errors
		if err != nil && err != unix.EAGAIN {
			fmt.Printf("%s", err.Error())
		}
	} else {
		// create temp dir and tempfile 1
		f, err := os.Create(filepath.Join(t.TempDir(), "flock_test_file"))
		if err != nil {
			t.Fatal(err)
		}
		fd := int(f.Fd())

		/* Test Case 1
		 * Try acquiring an occupied lock from another process
		 */
		err = unix.Flock(fd, unix.LOCK_EX)
		if err != nil {
			t.Fatalf("Flock: %s", err.Error())
		}
		cmd := exec.Command(os.Args[0], "-test.run=^TestFlock$", f.Name())
		cmd.Env = append(os.Environ(), "GO_WANT_HELPER_PROCESS=1")
		out, err := cmd.CombinedOutput()
		if len(out) > 0 || err != nil {
			t.Fatalf("child process: %q, %v", out, err)
		}
		err = unix.Flock(fd, unix.LOCK_UN)
		if err != nil {
			t.Fatalf("Flock: %s", err.Error())
		}

		/* Test Case 2
		 * Try locking with Flock and FcntlFlock for same file
		 */
		err = unix.Flock(fd, unix.LOCK_EX)
		if err != nil {
			t.Fatalf("Flock: %s", err.Error())
		}
		flock := unix.Flock_t{
			Type:   int16(unix.F_WRLCK),
			Whence: int16(0),
			Start:  int64(0),
			Len:    int64(0),
			Pid:    int32(unix.Getppid()),
		}
		err = unix.FcntlFlock(f.Fd(), unix.F_SETLK, &flock)
		if err != nil {
			t.Fatalf("FcntlFlock: %s", err.Error())
		}
	}
}

func TestSelect(t *testing.T) {
	for {
		n, err := unix.Select(0, nil, nil, nil, &unix.Timeval{Sec: 0, Usec: 0})
		if err == unix.EINTR {
			t.Logf("Select interrupted")
			continue
		} else if err != nil {
			t.Fatalf("Select: %v", err)
		}
		if n != 0 {
			t.Fatalf("Select: got %v ready file descriptors, expected 0", n)
		}
		break
	}

	dur := 250 * time.Millisecond
	var took time.Duration
	for {
		// On some platforms (e.g. Linux), the passed-in timeval is
		// updated by select(2). Make sure to reset to the full duration
		// in case of an EINTR.
		tv := unix.NsecToTimeval(int64(dur))
		start := time.Now()
		n, err := unix.Select(0, nil, nil, nil, &tv)
		took = time.Since(start)
		if err == unix.EINTR {
			t.Logf("Select interrupted after %v", took)
			continue
		} else if err != nil {
			t.Fatalf("Select: %v", err)
		}
		if n != 0 {
			t.Fatalf("Select: got %v ready file descriptors, expected 0", n)
		}
		break
	}

	// On some BSDs the actual timeout might also be slightly less than the requested.
	// Add an acceptable margin to avoid flaky tests.
	if took < dur*2/3 {
		t.Errorf("Select: got %v timeout, expected at least %v", took, dur)
	}

	rr, ww, err := os.Pipe()
	if err != nil {
		t.Fatal(err)
	}
	defer rr.Close()
	defer ww.Close()

	if _, err := ww.Write([]byte("HELLO GOPHER")); err != nil {
		t.Fatal(err)
	}

	rFdSet := &unix.FdSet{}
	fd := int(rr.Fd())
	rFdSet.Set(fd)

	for {
		n, err := unix.Select(fd+1, rFdSet, nil, nil, nil)
		if err == unix.EINTR {
			t.Log("Select interrupted")
			continue
		} else if err != nil {
			t.Fatalf("Select: %v", err)
		}
		if n != 1 {
			t.Fatalf("Select: got %v ready file descriptors, expected 1", n)
		}
		break
	}
}