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/check_linux.go

@@ -0,0 +1,77 @@
+//go:build linux
+// +build linux
+
+package mountlib
+
+import (
+	"path/filepath"
+	"strings"
+	"time"
+
+	"github.com/artyom/mtab"
+	"github.com/pkg/errors"
+)
+
+const (
+	mtabPath     = "/proc/mounts"
+	pollInterval = 100 * time.Millisecond
+)
+
+// CheckMountEmpty checks if folder is not already a mountpoint.
+// On Linux we use the OS-specific /proc/mount API so the check won't access the path.
+// Directories marked as "mounted" by autofs are considered not mounted.
+func CheckMountEmpty(mountpoint string) error {
+	const msg = "Directory already mounted, use --allow-non-empty to mount anyway: %s"
+
+	mountpointAbs, err := filepath.Abs(mountpoint)
+	if err != nil {
+		return errors.Wrapf(err, "cannot get absolute path: %s", mountpoint)
+	}
+
+	entries, err := mtab.Entries(mtabPath)
+	if err != nil {
+		return errors.Wrapf(err, "cannot read %s", mtabPath)
+	}
+	for _, entry := range entries {
+		if entry.Dir == mountpointAbs && entry.Type != "autofs" {
+			return errors.Errorf(msg, mountpointAbs)
+		}
+	}
+	return nil
+}
+
+// CheckMountReady checks whether mountpoint is mounted by rclone.
+// Only mounts with type "rclone" or "fuse.rclone" count.
+func CheckMountReady(mountpoint string) error {
+	mountpointAbs, err := filepath.Abs(mountpoint)
+	if err != nil {
+		return errors.Wrapf(err, "cannot get absolute path: %s", mountpoint)
+	}
+	entries, err := mtab.Entries(mtabPath)
+	if err != nil {
+		return errors.Wrapf(err, "cannot read %s", mtabPath)
+	}
+	for _, entry := range entries {
+		if entry.Dir == mountpointAbs && strings.Contains(entry.Type, "rclone") {
+			return nil
+		}
+	}
+	return errors.New("mount not ready")
+}
+
+// WaitMountReady waits until mountpoint is mounted by rclone.
+func WaitMountReady(mountpoint string, timeout time.Duration) (err error) {
+	endTime := time.Now().Add(timeout)
+	for {
+		err = CheckMountReady(mountpoint)
+		delay := time.Until(endTime)
+		if err == nil || delay <= 0 {
+			break
+		}
+		if delay > pollInterval {
+			delay = pollInterval
+		}
+		time.Sleep(delay)
+	}
+	return
+}