os, syscall: implement Stat and Lstat

File.Stat is left as a stub for now.

Tests are a bit stubbed down because os.ReadDir, os.Symlink, and t.TempDir are not yet (fully) implemented.
TODO: reimport tests from upstream as those materialize.

src/os/stat_windows.go

@@ -0,0 +1,89 @@
+// Copyright 2009 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.
+
+package os
+
+import (
+	"internal/syscall/windows"
+	"syscall"
+	"unsafe"
+)
+
+// Sync is a stub, not yet implemented
+func (f *File) Sync() error {
+	return ErrNotImplemented
+}
+
+// stat implements both Stat and Lstat of a file.
+func stat(funcname, name string, createFileAttrs uint32) (FileInfo, error) {
+	if len(name) == 0 {
+		return nil, &PathError{Op: funcname, Path: name, Err: syscall.Errno(syscall.ERROR_PATH_NOT_FOUND)}
+	}
+	if isWindowsNulName(name) {
+		return &devNullStat, nil
+	}
+	namep, err := syscall.UTF16PtrFromString(fixLongPath(name))
+	if err != nil {
+		return nil, &PathError{Op: funcname, Path: name, Err: err}
+	}
+
+	// Try GetFileAttributesEx first, because it is faster than CreateFile.
+	// See https://golang.org/issues/19922#issuecomment-300031421 for details.
+	var fa syscall.Win32FileAttributeData
+	err = syscall.GetFileAttributesEx(namep, syscall.GetFileExInfoStandard, (*byte)(unsafe.Pointer(&fa)))
+	if err == nil && fa.FileAttributes&syscall.FILE_ATTRIBUTE_REPARSE_POINT == 0 {
+		// Not a symlink.
+		fs := &fileStat{
+			FileAttributes: fa.FileAttributes,
+			CreationTime:   fa.CreationTime,
+			LastAccessTime: fa.LastAccessTime,
+			LastWriteTime:  fa.LastWriteTime,
+			FileSizeHigh:   fa.FileSizeHigh,
+			FileSizeLow:    fa.FileSizeLow,
+		}
+		if err := fs.saveInfoFromPath(name); err != nil {
+			return nil, err
+		}
+		return fs, nil
+	}
+	// GetFileAttributesEx fails with ERROR_SHARING_VIOLATION error for
+	// files, like c:\pagefile.sys. Use FindFirstFile for such files.
+	if err == windows.ERROR_SHARING_VIOLATION {
+		var fd syscall.Win32finddata
+		sh, err := syscall.FindFirstFile(namep, &fd)
+		if err != nil {
+			return nil, &PathError{Op: "FindFirstFile", Path: name, Err: err}
+		}
+		syscall.FindClose(sh)
+		fs := newFileStatFromWin32finddata(&fd)
+		if err := fs.saveInfoFromPath(name); err != nil {
+			return nil, err
+		}
+		return fs, nil
+	}
+
+	// Finally use CreateFile.
+	h, err := syscall.CreateFile(namep, 0, 0, nil,
+		syscall.OPEN_EXISTING, createFileAttrs, 0)
+	if err != nil {
+		return nil, &PathError{Op: "CreateFile", Path: name, Err: err}
+	}
+	defer syscall.CloseHandle(h)
+
+	return newFileStatFromGetFileInformationByHandle(name, h)
+}
+
+// statNolog implements Stat for Windows.
+func statNolog(name string) (FileInfo, error) {
+	return stat("Stat", name, syscall.FILE_FLAG_BACKUP_SEMANTICS)
+}
+
+// lstatNolog implements Lstat for Windows.
+func lstatNolog(name string) (FileInfo, error) {
+	attrs := uint32(syscall.FILE_FLAG_BACKUP_SEMANTICS)
+	// Use FILE_FLAG_OPEN_REPARSE_POINT, otherwise CreateFile will follow symlink.
+	// See https://docs.microsoft.com/en-us/windows/desktop/FileIO/symbolic-link-effects-on-file-systems-functions#createfile-and-createfiletransacted
+	attrs |= syscall.FILE_FLAG_OPEN_REPARSE_POINT
+	return stat("Lstat", name, attrs)
+}