mount: support FUSE helper

mount/unmount_unix.go

@@ -2,13 +2,49 @@
 
 package mount
 
-import "golang.org/x/sys/unix"
+import (
+	"os/exec"
+
+	"golang.org/x/sys/unix"
+)
 
 func unmountBare(target string, flags int) error {
 	return unix.Unmount(target, flags)
 }
 
+// fuseSuperMagic is defined in statfs(2)
+const fuseSuperMagic = 0x65735546
+
+func isFUSE(dir string) bool {
+	var st unix.Statfs_t
+	if err := unix.Statfs(dir, &st); err != nil {
+		return false
+	}
+	return st.Type == fuseSuperMagic
+}
+
+// unmountFUSE attempts to unmount using fusermount/fusermount3 helper binary.
+//
+// For FUSE mounts, using these helper binaries is preferred, see:
+// https://github.com/containerd/containerd/pull/3765#discussion_r342083514
+func unmountFUSE(target string) error {
+	var err error
+	for _, helperBinary := range []string{"fusermount3", "fusermount"} {
+		cmd := exec.Command(helperBinary, "-u", target)
+		err = cmd.Run()
+		if err == nil {
+			return nil
+		}
+	}
+	return err
+}
+
 func unmount(target string, flags int) error {
+	if isFUSE(target) {
+		if err := unmountFUSE(target); err == nil {
+			return nil
+		}
+	}
 	err := unmountBare(target, flags)
 	if err == nil || err == unix.EINVAL {
 		// Ignore "not mounted" error here. Note the same error