windows: Support all REPARSE_POINT attrib files properly. (#4203)

This change adopts the upstream fix in this regard at
https://go-review.googlesource.com/#/c/41834/ for Minio's
purposes.

Go's current os.Stat() lacks support for lot of strange
windows files such as

 - share symlinks on SMB2
 - symlinks on docker nanoserver
 - de-duplicated files on NTFS de-duplicated volume.

This PR attempts to incorporate the change mentioned here

   https://blogs.msdn.microsoft.com/oldnewthing/20100212-00/?p=14963/

The article suggests to use Windows I/O manager to
dereference the symbolic link.

Fixes #4122

pkg/x/os/stat_windows_test.go

@@ -0,0 +1,209 @@
+// +build windows
+
+/*
+ * Minio Cloud Storage, (C) 2017 Minio, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package os
+
+import (
+	"io/ioutil"
+	os1 "os"
+	"path/filepath"
+	"syscall"
+	"testing"
+	"unsafe"
+)
+
+var (
+	modnetapi32 = syscall.NewLazyDLL("netapi32.dll")
+
+	procNetShareAdd = modnetapi32.NewProc("NetShareAdd")
+	procNetShareDel = modnetapi32.NewProc("NetShareDel")
+)
+
+func netShareAdd(serverName *uint16, level uint32, buf *byte, parmErr *uint16) (neterr error) {
+	r0, _, _ := syscall.Syscall6(procNetShareAdd.Addr(), 4, uintptr(unsafe.Pointer(serverName)), uintptr(level), uintptr(unsafe.Pointer(buf)), uintptr(unsafe.Pointer(parmErr)), 0, 0)
+	if r0 != 0 {
+		neterr = syscall.Errno(r0)
+	}
+	return
+}
+
+func netShareDel(serverName *uint16, netName *uint16, reserved uint32) (neterr error) {
+	r0, _, _ := syscall.Syscall(procNetShareDel.Addr(), 3, uintptr(unsafe.Pointer(serverName)), uintptr(unsafe.Pointer(netName)), uintptr(reserved))
+	if r0 != 0 {
+		neterr = syscall.Errno(r0)
+	}
+	return
+}
+
+type shareInfo2 struct {
+	Netname     *uint16
+	Type        uint32
+	Remark      *uint16
+	Permissions uint32
+	MaxUses     uint32
+	CurrentUses uint32
+	Path        *uint16
+	Passwd      *uint16
+}
+
+func sameFile(fi1, fi2 os1.FileInfo) bool {
+	fii1, ok1 := fi1.(*fileStat)
+	fii2, ok2 := fi2.(*fileStat)
+	if !ok1 || !ok2 {
+		return false
+	}
+	return fii1.vol == fii2.vol && fii1.idxhi == fii2.idxhi && fii1.idxlo == fii2.idxlo
+}
+
+func TestNetworkSymbolicLink(t *testing.T) {
+	dir, err := ioutil.TempDir("", "TestNetworkSymbolicLink")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os1.RemoveAll(dir)
+
+	oldwd, err := os1.Getwd()
+	if err != nil {
+		t.Fatal(err)
+	}
+	err = os1.Chdir(dir)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os1.Chdir(oldwd)
+
+	shareName := "GoSymbolicLinkTestShare" // hope no conflicts.
+	sharePath := filepath.Join(dir, shareName)
+	testDir := "TestDir"
+
+	err = os1.MkdirAll(filepath.Join(sharePath, testDir), 0777)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	wShareName, err := syscall.UTF16PtrFromString(shareName)
+	if err != nil {
+		t.Fatal(err)
+	}
+	wSharePath, err := syscall.UTF16PtrFromString(sharePath)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	p := shareInfo2{
+		Netname:     wShareName,
+		Type:        0x00, // STYPE_DISKTREE
+		Remark:      nil,
+		Permissions: 0,
+		MaxUses:     1,
+		CurrentUses: 0,
+		Path:        wSharePath,
+		Passwd:      nil,
+	}
+
+	err = netShareAdd(nil, 2, (*byte)(unsafe.Pointer(&p)), nil)
+	if err != nil {
+		if err == syscall.ERROR_ACCESS_DENIED {
+			t.Skip("you don't have enough privileges to add network share")
+		}
+		t.Fatal(err)
+	}
+	defer func() {
+		err := netShareDel(nil, wShareName, 0)
+		if err != nil {
+			t.Fatal(err)
+		}
+	}()
+
+	UNCPath := `\\localhost\` + shareName + `\`
+
+	fi1, err := Stat(sharePath)
+	if err != nil {
+		t.Fatal(err)
+	}
+	fi2, err := Stat(UNCPath)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if !sameFile(fi1, fi2) {
+		t.Fatalf("%q and %q should be the same directory, but not", sharePath, UNCPath)
+	}
+
+	target := filepath.Join(UNCPath, testDir)
+	link := "link"
+
+	err = os1.Symlink(target, link)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os1.Remove(link)
+
+	got, err := os1.Readlink(link)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if got != target {
+		t.Errorf(`os1.Readlink("%s"): got %v, want %v`, link, got, target)
+	}
+}
+
+func TestShareNotExistError(t *testing.T) {
+	if testing.Short() {
+		t.Skip("slow test that uses network; skipping")
+	}
+	_, err := Stat(`\\no_such_server\no_such_share\no_such_file`)
+	if err == nil {
+		t.Fatal("Stat succeeded, but expected to fail")
+	}
+	if !os1.IsNotExist(err) {
+		t.Fatalf("os1.Stat failed with %q, but os1.IsNotExist(err) is false", err)
+	}
+}
+
+func TestStatPagefile(t *testing.T) {
+	_, err := Stat(`c:\pagefile.sys`)
+	if err == nil {
+		return
+	}
+	if os1.IsNotExist(err) {
+		t.Skip(`skipping because c:\pagefile.sys is not found`)
+	}
+	t.Fatal(err)
+}
+
+func TestStatSymlinkLoop(t *testing.T) {
+	err := os1.Symlink("x", "y")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os1.Remove("y")
+
+	err = os1.Symlink("y", "x")
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os1.Remove("x")
+
+	_, err = Stat("x")
+	if err != nil {
+		if _, ok := err.(*os1.PathError); !ok {
+			t.Errorf("expected *PathError, got %T: %v\n", err, err)
+		}
+	}
+}