volume: Don't decrement refcount below 0

With both rootless and live restore enabled, there's some race condition
which causes the container to be `Unmount`ed before the refcount is
restored.

This makes sure we don't underflow the refcount (uint64) when
decrementing it.

The root cause of this race condition still needs to be investigated and
fixed, but at least this unflakies the `TestLiveRestore`.

Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>
(cherry picked from commit 294fc9762e6650f8f8c59f1a210e6b37a02936f3)
Signed-off-by: Paweł Gronowski <pawel.gronowski@docker.com>

volume/local/local.go

@@ -9,6 +9,7 @@ import (
 	"os"
 	"path/filepath"
 	"reflect"
+	"runtime/debug"
 	"strings"
 	"sync"
 
@@ -335,8 +336,15 @@ func (v *localVolume) Unmount(id string) error {
 	// ultimately there's nothing that can be done. If we don't decrement the count
 	// this volume can never be removed until a daemon restart occurs.
 	if v.needsMount() {
-		v.active.count--
-		logger.WithField("active mounts", v.active).Debug("Decremented active mount count")
+		// TODO: Remove once the real bug is fixed: https://github.com/moby/moby/issues/46508
+		if v.active.count > 0 {
+			v.active.count--
+			logger.WithField("active mounts", v.active).Debug("Decremented active mount count")
+		} else {
+			logger.Error("An attempt to decrement a zero mount count")
+			logger.Error(string(debug.Stack()))
+			return nil
+		}
 	}
 
 	if v.active.count > 0 {

volume/mounts/mounts.go

@@ -5,6 +5,7 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"runtime/debug"
 	"syscall"
 
 	"github.com/containerd/containerd/log"
@@ -83,11 +84,22 @@ func (m *MountPoint) Cleanup() error {
 		return nil
 	}
 
+	logger := log.G(context.TODO()).WithFields(log.Fields{"active": m.active, "id": m.ID})
+
+	// TODO: Remove once the real bug is fixed: https://github.com/moby/moby/issues/46508
+	if m.active == 0 {
+		logger.Error("An attempt to decrement a zero mount count")
+		logger.Error(string(debug.Stack()))
+		return nil
+	}
+
 	if err := m.Volume.Unmount(m.ID); err != nil {
 		return errors.Wrapf(err, "error unmounting volume %s", m.Volume.Name())
 	}
 
 	m.active--
+	logger.Debug("MountPoint.Cleanup Decrement active count")
+
 	if m.active == 0 {
 		m.ID = ""
 	}