syscall: use openat instead of dup to make a really new file descriptor

Update #31269

Change-Id: I0e7184420055b8dfd23688dab9f9d8cba1fa2485
Reviewed-on: https://go-review.googlesource.com/c/go/+/170892
Run-TryBot: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/syscall/getdirentries_test.go

@@ -0,0 +1,82 @@
+// Copyright 2019 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.
+
+// +build darwin freebsd netbsd openbsd
+
+package syscall_test
+
+import (
+	"fmt"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"sort"
+	"strings"
+	"syscall"
+	"testing"
+	"unsafe"
+)
+
+func TestGetdirentries(t *testing.T) {
+	for _, count := range []int{10, 1000} {
+		t.Run(fmt.Sprintf("n=%d", count), func(t *testing.T) {
+			testGetdirentries(t, count)
+		})
+	}
+}
+func testGetdirentries(t *testing.T, count int) {
+	d, err := ioutil.TempDir("", "getdirentries-test")
+	if err != nil {
+		t.Fatalf("Tempdir: %v", err)
+	}
+	defer os.RemoveAll(d)
+	var names []string
+	for i := 0; i < count; i++ {
+		names = append(names, fmt.Sprintf("file%03d", i))
+	}
+
+	// Make files in the temp directory
+	for _, name := range names {
+		err := ioutil.WriteFile(filepath.Join(d, name), []byte("data"), 0)
+		if err != nil {
+			t.Fatalf("WriteFile: %v", err)
+		}
+	}
+
+	// Read files using Getdirentries
+	var names2 []string
+	fd, err := syscall.Open(d, syscall.O_RDONLY, 0)
+	if err != nil {
+		t.Fatalf("Open: %v", err)
+	}
+	defer syscall.Close(fd)
+	var base uintptr
+	var buf [2048]byte
+	for {
+		n, err := syscall.Getdirentries(fd, buf[:], &base)
+		if err != nil {
+			t.Fatalf("Getdirentries: %v", err)
+		}
+		if n == 0 {
+			break
+		}
+		data := buf[:n]
+		for len(data) > 0 {
+			dirent := (*syscall.Dirent)(unsafe.Pointer(&data[0]))
+			data = data[dirent.Reclen:]
+			name := make([]byte, dirent.Namlen)
+			for i := 0; i < int(dirent.Namlen); i++ {
+				name[i] = byte(dirent.Name[i])
+			}
+			names2 = append(names2, string(name))
+		}
+	}
+
+	names = append(names, ".", "..") // Getdirentries returns these also
+	sort.Strings(names)
+	sort.Strings(names2)
+	if strings.Join(names, ":") != strings.Join(names2, ":") {
+		t.Errorf("names don't match\n names: %q\nnames2: %q", names, names2)
+	}
+}

src/syscall/syscall_darwin.go

@@ -368,7 +368,13 @@ func writelen(fd int, buf *byte, nbuf int) (n int, err error) {
 func Getdirentries(fd int, buf []byte, basep *uintptr) (n int, err error) {
 	// Simulate Getdirentries using fdopendir/readdir_r/closedir.
 	const ptrSize = unsafe.Sizeof(uintptr(0))
-	fd2, err := Dup(fd)
+	// We need to duplicate the incoming file descriptor
+	// because the caller expects to retain control of it, but
+	// fdopendir expects to take control of its argument.
+	// Just Dup'ing the file descriptor is not enough, as the
+	// result shares underlying state. Use openat to make a really
+	// new file descriptor referring to the same directory.
+	fd2, err := openat(fd, ".", O_RDONLY, 0)
 	if err != nil {
 		return 0, err
 	}