mountlib: use procfs to validate mount on linux - #5593

Current way of checking whether mountpoint has been already mounted (directory
list) can result in race if rclone runs under Automount (classic or systemd).

This patch adopts Linux ProcFS for the check. Note that mountpoint is considered
empty if it's tagged as "mounted" by autofs. Also ProcFS is used to check whether
rclone mount was successful (ie. tagged by a string containing "rclone").

On macOS/BSD where ProcFS is unavailable the old method is still used.

This patch also moves a few utility functions unchanged to utils.go:
CheckOverlap, CheckAllowings, SetVolumeName.

cmd/mountlib/utils.go

@@ -1,36 +1,14 @@
 package mountlib
 
 import (
-	"io"
-	"os"
+	"path/filepath"
 	"runtime"
+	"strings"
 
 	"github.com/pkg/errors"
 	"github.com/rclone/rclone/fs"
 )
 
-// CheckMountEmpty checks if folder is empty
-func CheckMountEmpty(mountpoint string) error {
-	fp, fpErr := os.Open(mountpoint)
-
-	if fpErr != nil {
-		return errors.Wrap(fpErr, "Can not open: "+mountpoint)
-	}
-	defer fs.CheckClose(fp, &fpErr)
-
-	_, fpErr = fp.Readdirnames(1)
-
-	if fpErr == io.EOF {
-		return nil
-	}
-
-	msg := "Directory is not empty: " + mountpoint + " If you want to mount it anyway use: --allow-non-empty option"
-	if fpErr == nil {
-		return errors.New(msg)
-	}
-	return errors.Wrap(fpErr, msg)
-}
-
 // ClipBlocks clips the blocks pointed to the OS max
 func ClipBlocks(b *uint64) {
 	var max uint64
@@ -53,3 +31,74 @@ func ClipBlocks(b *uint64) {
 		*b = max
 	}
 }
+
+// CheckOverlap checks that root doesn't overlap with mountpoint
+func (m *MountPoint) CheckOverlap() error {
+	name := m.Fs.Name()
+	if name != "" && name != "local" {
+		return nil
+	}
+	rootAbs := absPath(m.Fs.Root())
+	mountpointAbs := absPath(m.MountPoint)
+	if strings.HasPrefix(rootAbs, mountpointAbs) || strings.HasPrefix(mountpointAbs, rootAbs) {
+		const msg = "mount point %q and directory to be mounted %q mustn't overlap"
+		return errors.Errorf(msg, m.MountPoint, m.Fs.Root())
+	}
+	return nil
+}
+
+// absPath is a helper function for MountPoint.CheckOverlap
+func absPath(path string) string {
+	if abs, err := filepath.EvalSymlinks(path); err == nil {
+		path = abs
+	}
+	if abs, err := filepath.Abs(path); err == nil {
+		path = abs
+	}
+	path = filepath.ToSlash(path)
+	if !strings.HasSuffix(path, "/") {
+		path += "/"
+	}
+	return path
+}
+
+// CheckAllowings informs about ignored flags on Windows. If not on Windows
+// and not --allow-non-empty flag is used, verify that mountpoint is empty.
+func (m *MountPoint) CheckAllowings() error {
+	opt := &m.MountOpt
+	if runtime.GOOS == "windows" {
+		if opt.AllowNonEmpty {
+			fs.Logf(nil, "--allow-non-empty flag does nothing on Windows")
+		}
+		if opt.AllowRoot {
+			fs.Logf(nil, "--allow-root flag does nothing on Windows")
+		}
+		if opt.AllowOther {
+			fs.Logf(nil, "--allow-other flag does nothing on Windows")
+		}
+		return nil
+	}
+	if !opt.AllowNonEmpty {
+		return CheckMountEmpty(m.MountPoint)
+	}
+	return nil
+}
+
+// SetVolumeName with sensible default
+func (m *MountPoint) SetVolumeName(vol string) {
+	if vol == "" {
+		vol = m.Fs.Name() + ":" + m.Fs.Root()
+	}
+	m.MountOpt.SetVolumeName(vol)
+}
+
+// SetVolumeName removes special characters from volume name if necessary
+func (o *Options) SetVolumeName(vol string) {
+	vol = strings.ReplaceAll(vol, ":", " ")
+	vol = strings.ReplaceAll(vol, "/", " ")
+	vol = strings.TrimSpace(vol)
+	if runtime.GOOS == "windows" && len(vol) > 32 {
+		vol = vol[:32]
+	}
+	o.VolumeName = vol
+}