mount: implement UnmountAll()

`UnmountAll()` unmounts all mounts underneath the specified path, but
(contrary to `RecursiveUnmount()`) does not unmount `path` itself.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

mount/mount_unix.go

@@ -4,7 +4,9 @@ package mount
 
 import (
 	"fmt"
+	"path"
 	"sort"
+	"strings"
 
 	"github.com/moby/sys/mountinfo"
 	"golang.org/x/sys/unix"
@@ -37,35 +39,56 @@ func Unmount(target string) error {
 	}
 }
 
-// RecursiveUnmount unmounts the target and all mounts underneath, starting
-// with the deepest mount first. The argument does not have to be a mount
-// point itself.
-func RecursiveUnmount(target string) error {
-	// Fast path, works if target is a mount point that can be unmounted.
+// UnmountAll unmounts all mounts and submounts underneath parent,
+func UnmountAll(parent string) error {
+	// Get all mounts in "parent"
+	mounts, err := mountinfo.GetMounts(mountinfo.PrefixFilter(parent))
+	if err != nil {
+		return err
+	}
+
+	// Fast path: try to unmount top-level mounts first. This works if target is
+	// a mount point that can be unmounted.
 	// On Linux, mntDetach flag ensures a recursive unmount.  For other
 	// platforms, if there are submounts, we'll get EBUSY (and fall back
-	// to the slow path). NOTE we do not ignore EINVAL here as target might
-	// not be a mount point itself (but there can be mounts underneath).
-	if err := unix.Unmount(target, mntDetach); err == nil {
-		return nil
+	// to the slow path). We're not using RecursiveUnmount() here, to avoid
+	// repeatedly calling mountinfo.GetMounts()
+
+	var skipParents []string
+	for _, m := range mounts {
+		// Skip parent itself, and skip non-top-level mounts
+		if m.Mountpoint == parent || path.Dir(m.Mountpoint) != parent {
+			continue
+		}
+		if err := unix.Unmount(m.Mountpoint, mntDetach); err == nil {
+			skipParents = append(skipParents, m.Mountpoint)
+		}
 	}
 
-	// Slow path: get all submounts, sort, unmount one by one.
-	mounts, err := mountinfo.GetMounts(mountinfo.PrefixFilter(target))
-	if err != nil {
-		return err
+	// Remove all sub-mounts of paths that were successfully unmounted from the list
+	subMounts := mounts[:0]
+	for _, m := range mounts {
+		for _, p := range skipParents {
+			if m.Mountpoint == parent || m.Mountpoint == p {
+				// Skip parent itself, and mounts that already were unmounted
+				continue
+			}
+			if !strings.HasPrefix(m.Mountpoint, p) {
+				subMounts = append(subMounts, m)
+			}
+		}
 	}
 
 	// Make the deepest mount be first
-	sort.Slice(mounts, func(i, j int) bool {
-		return len(mounts[i].Mountpoint) > len(mounts[j].Mountpoint)
+	sort.Slice(subMounts, func(i, j int) bool {
+		return len(subMounts[i].Mountpoint) > len(subMounts[j].Mountpoint)
 	})
 
 	var (
 		suberr    error
 		lastMount = len(mounts) - 1
 	)
-	for i, m := range mounts {
+	for i, m := range subMounts {
 		err = Unmount(m.Mountpoint)
 		if err != nil {
 			if i == lastMount {
@@ -85,3 +108,20 @@ func RecursiveUnmount(target string) error {
 	}
 	return nil
 }
+
+// RecursiveUnmount unmounts the target and all mounts underneath, starting
+// with the deepest mount first. The argument does not have to be a mount
+// point itself.
+func RecursiveUnmount(target string) error {
+	// Fast path, works if target is a mount point that can be unmounted.
+	// On Linux, mntDetach flag ensures a recursive unmount.  For other
+	// platforms, if there are submounts, we'll get EBUSY (and fall back
+	// to the slow path). NOTE we do not ignore EINVAL here as target might
+	// not be a mount point itself (but there can be mounts underneath).
+	if err := unix.Unmount(target, mntDetach); err == nil {
+		return nil
+	}
+
+	// Slow path: unmount all mounts inside target one by one.
+	return UnmountAll(target)
+}