Implement Volume Mount Point API wrappers

Fixes: microsoft#176

These provide Go-typed wrappers around the Win32 APIs backing them,
protecting the user from unsafe pointers and UTF-16 conversions.

utf16ToStringArray adapted from ConvertStringSetToSlice in hcsshim

Signed-off-by: Paul "TBBle" Hampson <Paul.Hampson@Pobox.com>

pkg/volmount/deletevolumemountpoint.go

@@ -0,0 +1,29 @@
+package volmount
+
+import (
+	"path/filepath"
+
+	"github.com/pkg/errors"
+	"golang.org/x/sys/windows"
+)
+
+// DeleteVolumeMountPoint removes the volume mount at targetPath
+// https://docs.microsoft.com/en-us/windows/win32/api/winbase/nf-winbase-deletevolumemountpointa
+func DeleteVolumeMountPoint(targetPath string) error {
+	// Must end in a backslash
+	slashedTarget := filepath.Clean(targetPath)
+	if slashedTarget[len(slashedTarget)-1] != filepath.Separator {
+		slashedTarget = slashedTarget + string(filepath.Separator)
+	}
+
+	targetP, err := windows.UTF16PtrFromString(slashedTarget)
+	if err != nil {
+		return errors.Wrapf(err, "unable to utf16-ise %s", slashedTarget)
+	}
+
+	if err := windows.DeleteVolumeMountPoint(targetP); err != nil {
+		return errors.Wrapf(err, "failed calling DeleteVolumeMountPoint('%s')", slashedTarget)
+	}
+
+	return nil
+}

pkg/volmount/getvolumenameforvolumemountpoint.go

@@ -0,0 +1,33 @@
+package volmount
+
+import (
+	"path/filepath"
+
+	"github.com/pkg/errors"
+	"golang.org/x/sys/windows"
+)
+
+// GetVolumeNameForVolumeMountPoint returns a volume path (in format '\\?\Volume{GUID}'
+// for the volume mounted at targetPath.
+// https://docs.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-getvolumenameforvolumemountpointw
+func GetVolumeNameForVolumeMountPoint(targetPath string) (string, error) {
+	// Must end in a backslash
+	slashedTarget := filepath.Clean(targetPath)
+	if slashedTarget[len(slashedTarget)-1] != filepath.Separator {
+		slashedTarget = slashedTarget + string(filepath.Separator)
+	}
+
+	targetP, err := windows.UTF16PtrFromString(slashedTarget)
+	if err != nil {
+		return "", errors.Wrapf(err, "unable to utf16-ise %s", slashedTarget)
+	}
+
+	bufferlength := uint32(50) // "A reasonable size for the buffer" per the documentation.
+	buffer := make([]uint16, bufferlength)
+
+	if err = windows.GetVolumeNameForVolumeMountPoint(targetP, &buffer[0], bufferlength); err != nil {
+		return "", errors.Wrapf(err, "failed calling GetVolumeNameForVolumeMountPoint('%s', ..., %d)", slashedTarget, bufferlength)
+	}
+
+	return windows.UTF16ToString(buffer), nil
+}

pkg/volmount/getvolumepathnamesforvolumename.go

@@ -0,0 +1,66 @@
+package volmount
+
+import (
+	"path/filepath"
+	"syscall"
+	"unicode/utf16"
+
+	"github.com/pkg/errors"
+	"golang.org/x/sys/windows"
+)
+
+// utf16ToStringArray returns the UTF-8 encoding of the sequence of UTF-16 sequences s,
+// with a terminating NUL removed. The sequences are terminated by an additional NULL.
+func utf16ToStringArray(s []uint16) ([]string, error) {
+	var results []string
+	prev := 0
+	for i := range s {
+		if s[i] == 0 {
+			if prev == i {
+				// found two null characters in a row, return result
+				return results, nil
+			}
+			results = append(results, string(utf16.Decode(s[prev:i])))
+			prev = i + 1
+		}
+	}
+	return nil, errors.New("string set malformed: missing null terminator at end of buffer")
+}
+
+// GetMountPathsFromVolumeName returns a list of mount points for the volumePath
+// (in format '\\?\Volume{GUID}).
+// https://docs.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-getvolumepathnamesforvolumenamew
+func GetMountPathsFromVolumeName(volumePath string) ([]string, error) {
+	// Must end in a backslash
+	slashedVolume := filepath.Clean(volumePath)
+	if slashedVolume[len(slashedVolume)-1] != filepath.Separator {
+		slashedVolume = slashedVolume + string(filepath.Separator)
+	}
+
+	volumeP, err := windows.UTF16PtrFromString(slashedVolume)
+	if err != nil {
+		return nil, errors.Wrapf(err, "unable to utf16-ise %s", slashedVolume)
+	}
+
+	var bufferLength uint32
+	err = windows.GetVolumePathNamesForVolumeName(volumeP, nil, 0, &bufferLength)
+	if err == nil {
+		// This should never happen. An empty list would have a single 0 in it.
+		return nil, errors.Errorf("unexpected success of GetVolumePathNamesForVolumeName('%s', nil, 0, ...)", slashedVolume)
+	} else if err != syscall.ERROR_MORE_DATA {
+		return nil, errors.Wrapf(err, "failed calling GetVolumePathNamesForVolumeName('%s', nil, 0, ...)", slashedVolume)
+	}
+
+	buffer := make([]uint16, bufferLength)
+	err = windows.GetVolumePathNamesForVolumeName(volumeP, &buffer[0], bufferLength, nil)
+	if err != nil {
+		return nil, errors.Wrapf(err, "failed calling GetVolumePathNamesForVolumeName('%s', ..., %d, nil)", slashedVolume, bufferLength)
+	}
+
+	result, err := utf16ToStringArray(buffer)
+	if err != nil {
+		return nil, errors.Wrapf(err, "failed decoding result of GetVolumePathNamesForVolumeName('%s', ..., %d, nil)", slashedVolume, bufferLength)
+	}
+
+	return result, nil
+}

pkg/volmount/setvolumemountpoint.go

@@ -0,0 +1,43 @@
+package volmount
+
+import (
+	"path/filepath"
+	"strings"
+
+	"github.com/pkg/errors"
+	"golang.org/x/sys/windows"
+)
+
+// SetVolumeMountPoint mounts volumePath (in format '\\?\Volume{GUID}' at targetPath.
+// https://docs.microsoft.com/en-us/windows/win32/api/winbase/nf-winbase-setvolumemountpointw
+func SetVolumeMountPoint(targetPath string, volumePath string) error {
+	if !strings.HasPrefix(volumePath, "\\\\?\\Volume{") {
+		return errors.Errorf("unable to mount non-volume path %s", volumePath)
+	}
+
+	// Both must end in a backslash
+	slashedTarget := filepath.Clean(targetPath)
+	if slashedTarget[len(slashedTarget)-1] != filepath.Separator {
+		slashedTarget = slashedTarget + string(filepath.Separator)
+	}
+	slashedVolume := filepath.Clean(volumePath)
+	if slashedVolume[len(slashedVolume)-1] != filepath.Separator {
+		slashedVolume = slashedVolume + string(filepath.Separator)
+	}
+
+	targetP, err := windows.UTF16PtrFromString(slashedTarget)
+	if err != nil {
+		return errors.Wrapf(err, "unable to utf16-ise %s", slashedTarget)
+	}
+
+	volumeP, err := windows.UTF16PtrFromString(slashedVolume)
+	if err != nil {
+		return errors.Wrapf(err, "unable to utf16-ise %s", slashedVolume)
+	}
+
+	if err := windows.SetVolumeMountPoint(targetP, volumeP); err != nil {
+		return errors.Wrapf(err, "failed calling SetVolumeMount('%s', '%s')", slashedTarget, slashedVolume)
+	}
+
+	return nil
+}