os: make readdir more robust on Windows

On Windows, File.readdir currently fails if the volume information
can't be retrieved via GetVolumeInformationByHandle and if the
directory path is relative and can't be converted to an absolute
path.

This change makes readdir more robust by not failing in these cases,
as these steps are just necessary to support a potential call to
os.SameFile, but not for the actual readdir operation. os.SameFile
will still fail in these cases, but that's a separate issue tracked
in #62042.

Change-Id: I8d98d8379bdac4b2832fa433432a5f027756abaa
Reviewed-on: https://go-review.googlesource.com/c/go/+/574155
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Alex Brainman <alex.brainman@gmail.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>

src/internal/syscall/windows/symlink_windows.go

@@ -9,6 +9,7 @@ import "syscall"
 const (
 	ERROR_INVALID_PARAMETER syscall.Errno = 87
 
+	FILE_SUPPORTS_OBJECT_IDS      = 0x00010000
 	FILE_SUPPORTS_OPEN_BY_FILE_ID = 0x01000000
 
 	// symlink support for CreateSymbolicLink() starting with Windows 10 (1703, v10.0.14972)

src/internal/syscall/windows/syscall_windows.go

@@ -473,3 +473,18 @@ const (
 //sys    OpenService(mgr syscall.Handle, serviceName *uint16, access uint32) (handle syscall.Handle, err error) = advapi32.OpenServiceW
 //sys	QueryServiceStatus(hService syscall.Handle, lpServiceStatus *SERVICE_STATUS) (err error)  = advapi32.QueryServiceStatus
 //sys    OpenSCManager(machineName *uint16, databaseName *uint16, access uint32) (handle syscall.Handle, err error)  [failretval==0] = advapi32.OpenSCManagerW
+
+func FinalPath(h syscall.Handle, flags uint32) (string, error) {
+	buf := make([]uint16, 100)
+	for {
+		n, err := GetFinalPathNameByHandle(h, &buf[0], uint32(len(buf)), flags)
+		if err != nil {
+			return "", err
+		}
+		if n < uint32(len(buf)) {
+			break
+		}
+		buf = make([]uint16, n)
+	}
+	return syscall.UTF16ToString(buf), nil
+}

src/os/dir_windows.go

@@ -21,6 +21,7 @@ type dirInfo struct {
 	// buf to dirBufPool.
 	buf   *[]byte // buffer for directory I/O
 	bufp  int     // location of next record in buf
+	h     syscall.Handle
 	vol   uint32
 	class uint32 // type of entries in buf
 	path  string // absolute directory path, empty if the file system supports FILE_ID_BOTH_DIR_INFO
@@ -45,6 +46,7 @@ var dirBufPool = sync.Pool{
 }
 
 func (d *dirInfo) close() {
+	d.h = 0
 	if d.buf != nil {
 		dirBufPool.Put(d.buf)
 		d.buf = nil
@@ -56,41 +58,44 @@ func (d *dirInfo) close() {
 // Useful for testing purposes.
 var allowReadDirFileID = true
 
+func (d *dirInfo) init(h syscall.Handle) {
+	d.h = h
+	d.class = windows.FileFullDirectoryRestartInfo
+	// The previous settings are enough to read the directory entries.
+	// The following code is only needed to support os.SameFile.
+
+	// It is safe to query d.vol once and reuse the value.
+	// Hard links are not allowed to reference files in other volumes.
+	// Junctions and symbolic links can reference files and directories in other volumes,
+	// but the reparse point should still live in the parent volume.
+	var flags uint32
+	err := windows.GetVolumeInformationByHandle(h, nil, 0, &d.vol, nil, &flags, nil, 0)
+	if err != nil {
+		d.vol = 0 // Set to zero in case Windows writes garbage to it.
+		// If we can't get the volume information, we can't use os.SameFile,
+		// but we can still read the directory entries.
+		return
+	}
+	if flags&windows.FILE_SUPPORTS_OBJECT_IDS == 0 {
+		// The file system does not support object IDs, no need to continue.
+		return
+	}
+	if allowReadDirFileID && flags&windows.FILE_SUPPORTS_OPEN_BY_FILE_ID != 0 {
+		// Use FileIdBothDirectoryRestartInfo if available as it returns the file ID
+		// without the need to open the file.
+		d.class = windows.FileIdBothDirectoryRestartInfo
+	} else {
+		// If FileIdBothDirectoryRestartInfo is not available but objects IDs are supported,
+		// get the directory path so that os.SameFile can use it to open the file
+		// and retrieve the file ID.
+		d.path, _ = windows.FinalPath(h, windows.FILE_NAME_OPENED)
+	}
+}
+
 func (file *File) readdir(n int, mode readdirMode) (names []string, dirents []DirEntry, infos []FileInfo, err error) {
-	// If this file has no dirinfo, create one.
 	if file.dirinfo == nil {
-		// vol is used by os.SameFile.
-		// It is safe to query it once and reuse the value.
-		// Hard links are not allowed to reference files in other volumes.
-		// Junctions and symbolic links can reference files and directories in other volumes,
-		// but the reparse point should still live in the parent volume.
-		var vol, flags uint32
-		err = windows.GetVolumeInformationByHandle(file.pfd.Sysfd, nil, 0, &vol, nil, &flags, nil, 0)
-		runtime.KeepAlive(file)
-		if err != nil {
-			err = &PathError{Op: "readdir", Path: file.name, Err: err}
-			return
-		}
 		file.dirinfo = new(dirInfo)
-		file.dirinfo.vol = vol
-		if allowReadDirFileID && flags&windows.FILE_SUPPORTS_OPEN_BY_FILE_ID != 0 {
-			file.dirinfo.class = windows.FileIdBothDirectoryRestartInfo
-		} else {
-			file.dirinfo.class = windows.FileFullDirectoryRestartInfo
-			// Set the directory path for use by os.SameFile, as it is possible that
-			// the file system supports retrieving the file ID using GetFileInformationByHandle.
-			file.dirinfo.path = file.name
-			if !isAbs(file.dirinfo.path) {
-				// If the path is relative, we need to convert it to an absolute path
-				// in case the current directory changes between this call and a
-				// call to os.SameFile.
-				file.dirinfo.path, err = syscall.FullPath(file.dirinfo.path)
-				if err != nil {
-					err = &PathError{Op: "readdir", Path: file.name, Err: err}
-					return
-				}
-			}
-		}
+		file.dirinfo.init(file.pfd.Sysfd)
 	}
 	d := file.dirinfo
 	if d.buf == nil {
@@ -172,11 +177,13 @@ func (file *File) readdir(n int, mode readdirMode) (names []string, dirents []Di
 					f = newFileStatFromFileIDBothDirInfo((*windows.FILE_ID_BOTH_DIR_INFO)(entry))
 				} else {
 					f = newFileStatFromFileFullDirInfo((*windows.FILE_FULL_DIR_INFO)(entry))
-					// Defer appending the entry name to the parent directory path until
-					// it is really needed, to avoid allocating a string that may not be used.
-					// It is currently only used in os.SameFile.
-					f.appendNameToPath = true
-					f.path = d.path
+					if d.path != "" {
+						// Defer appending the entry name to the parent directory path until
+						// it is really needed, to avoid allocating a string that may not be used.
+						// It is currently only used in os.SameFile.
+						f.appendNameToPath = true
+						f.path = d.path
+					}
 				}
 				f.name = name
 				f.vol = d.vol