lib/file: improve error message when attempting to create dir on nonexistent drive on windows

This replaces built-in os.MkdirAll with a patched version that stops the recursion
when reaching the volume part of the path. The original version would continue recursion,
and for extended length paths end up with \\? as the top-level directory, and the error
message would then be something like:
mkdir \\?: The filename, directory name, or volume label syntax is incorrect.

lib/file/mkdir_other.go

@@ -0,0 +1,11 @@
+//go:build !windows
+// +build !windows
+
+package file
+
+import "os"
+
+// MkdirAll just calls os.MkdirAll on non-Windows.
+func MkdirAll(path string, perm os.FileMode) error {
+	return os.MkdirAll(path, perm)
+}

lib/file/mkdir_windows.go

@@ -0,0 +1,77 @@
+//go:build windows
+// +build windows
+
+package file
+
+import (
+	"os"
+	"path/filepath"
+	"syscall"
+)
+
+// MkdirAll creates a directory named path, along with any necessary parents.
+//
+// Improves os.MkdirAll by avoiding trying to create a folder \\? when the
+// volume of a given extended length path does not exist.
+//
+// Based on source code from golang's os.MkdirAll
+// (https://github.com/golang/go/blob/master/src/os/path.go)
+func MkdirAll(path string, perm os.FileMode) error {
+	// Fast path: if we can tell whether path is a directory or file, stop with success or error.
+	dir, err := os.Stat(path)
+	if err == nil {
+		if dir.IsDir() {
+			return nil
+		}
+		return &os.PathError{
+			Op:   "mkdir",
+			Path: path,
+			Err:  syscall.ENOTDIR,
+		}
+	}
+
+	// Slow path: make sure parent exists and then call Mkdir for path.
+	i := len(path)
+	for i > 0 && os.IsPathSeparator(path[i-1]) { // Skip trailing path separator.
+		i--
+	}
+	if i > 0 {
+		path = path[:i]
+
+		if path == filepath.VolumeName(path) {
+			// Make reference to a drive's root directory include the trailing slash.
+			// In extended-length form without trailing slash ("\\?\C:"), os.Stat
+			// and os.Mkdir both fails. With trailing slash ("\\?\C:\") works,
+			// and regular paths with or without it ("C:" and "C:\") both works.
+			path = path + string(os.PathSeparator)
+		} else {
+			// See if there is a parent to be created first.
+			// Not when path refer to a drive's root directory, because we don't
+			// want to return noninformative error trying to create \\?.
+			j := i
+			for j > 0 && !os.IsPathSeparator(path[j-1]) { // Scan backward over element.
+				j--
+			}
+			if j > 1 {
+				// Create parent.
+				err = MkdirAll(path[:j-1], perm)
+				if err != nil {
+					return err
+				}
+			}
+		}
+	}
+
+	// Parent now exists; invoke Mkdir and use its result.
+	err = os.Mkdir(path, perm)
+	if err != nil {
+		// Handle arguments like "foo/." by
+		// double-checking that directory doesn't exist.
+		dir, err1 := os.Lstat(path)
+		if err1 == nil && dir.IsDir() {
+			return nil
+		}
+		return err
+	}
+	return nil
+}

lib/file/mkdir_windows_test.go

@@ -0,0 +1,132 @@
+//go:build windows
+// +build windows
+
+package file
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+// Basic test from golang's os/path_test.go
+func TestMkdirAll(t *testing.T) {
+	tmpDir, tidy := testDir(t)
+	defer tidy()
+
+	path := tmpDir + "/dir/./dir2"
+	err := MkdirAll(path, 0777)
+	if err != nil {
+		t.Fatalf("MkdirAll %q: %s", path, err)
+	}
+
+	// Already exists, should succeed.
+	err = MkdirAll(path, 0777)
+	if err != nil {
+		t.Fatalf("MkdirAll %q (second time): %s", path, err)
+	}
+
+	// Make file.
+	fpath := path + "/file"
+	f, err := Create(fpath)
+	if err != nil {
+		t.Fatalf("create %q: %s", fpath, err)
+	}
+	defer f.Close()
+
+	// Can't make directory named after file.
+	err = MkdirAll(fpath, 0777)
+	if err == nil {
+		t.Fatalf("MkdirAll %q: no error", fpath)
+	}
+	perr, ok := err.(*os.PathError)
+	if !ok {
+		t.Fatalf("MkdirAll %q returned %T, not *PathError", fpath, err)
+	}
+	if filepath.Clean(perr.Path) != filepath.Clean(fpath) {
+		t.Fatalf("MkdirAll %q returned wrong error path: %q not %q", fpath, filepath.Clean(perr.Path), filepath.Clean(fpath))
+	}
+
+	// Can't make subdirectory of file.
+	ffpath := fpath + "/subdir"
+	err = MkdirAll(ffpath, 0777)
+	if err == nil {
+		t.Fatalf("MkdirAll %q: no error", ffpath)
+	}
+	perr, ok = err.(*os.PathError)
+	if !ok {
+		t.Fatalf("MkdirAll %q returned %T, not *PathError", ffpath, err)
+	}
+	if filepath.Clean(perr.Path) != filepath.Clean(fpath) {
+		t.Fatalf("MkdirAll %q returned wrong error path: %q not %q", ffpath, filepath.Clean(perr.Path), filepath.Clean(fpath))
+	}
+
+	path = tmpDir + `\dir\.\dir2\`
+	err = MkdirAll(path, 0777)
+	if err != nil {
+		t.Fatalf("MkdirAll %q: %s", path, err)
+	}
+}
+
+func unusedDrive(t *testing.T) string {
+	letter := FindUnusedDriveLetter()
+	require.NotEqual(t, letter, 0)
+	return string(letter) + ":"
+}
+
+func checkMkdirAll(t *testing.T, path string, valid bool, errormsg string) {
+	if valid {
+		assert.NoError(t, MkdirAll(path, 0777))
+	} else {
+		err := MkdirAll(path, 0777)
+		assert.Error(t, err)
+		assert.Equal(t, errormsg, err.Error())
+	}
+}
+
+func checkMkdirAllSubdirs(t *testing.T, path string, valid bool, errormsg string) {
+	checkMkdirAll(t, path, valid, errormsg)
+	checkMkdirAll(t, path+`\`, valid, errormsg)
+	checkMkdirAll(t, path+`\parent`, valid, errormsg)
+	checkMkdirAll(t, path+`\parent\`, valid, errormsg)
+	checkMkdirAll(t, path+`\parent\child`, valid, errormsg)
+	checkMkdirAll(t, path+`\parent\child\`, valid, errormsg)
+}
+
+// Testing paths on existing drive
+func TestMkdirAllOnDrive(t *testing.T) {
+	path, tidy := testDir(t)
+	defer tidy()
+
+	dir, err := os.Stat(path)
+	require.NoError(t, err)
+	require.True(t, dir.IsDir())
+
+	drive := filepath.VolumeName(path)
+
+	checkMkdirAll(t, drive, true, "")
+	checkMkdirAll(t, drive+`\`, true, "")
+	checkMkdirAll(t, `\\?\`+drive, true, "")
+	checkMkdirAll(t, `\\?\`+drive+`\`, true, "")
+	checkMkdirAllSubdirs(t, path, true, "")
+	checkMkdirAllSubdirs(t, `\\?\`+path, true, "")
+}
+
+// Testing paths on unused drive
+// This is where there is a difference from golang's os.MkdirAll. It would
+// recurse extended-length paths down to the "\\?" prefix and return the
+// noninformative error:
+// "mkdir \\?: The filename, directory name, or volume label syntax is incorrect."
+// Our version stops the recursion at drive's root directory, and reports:
+// "mkdir \\?\A:\: The system cannot find the path specified."
+func TestMkdirAllOnUnusedDrive(t *testing.T) {
+	path := unusedDrive(t)
+	errormsg := fmt.Sprintf("mkdir %s\\: The system cannot find the path specified.", path)
+	checkMkdirAllSubdirs(t, path, false, errormsg)
+	errormsg = fmt.Sprintf("mkdir \\\\?\\%s\\: The system cannot find the path specified.", path)
+	checkMkdirAllSubdirs(t, `\\?\`+path, false, errormsg)
+}