aufs: Unmount inactive devices

This implements the new Put() operation such that
Get()/Put() maintains a refcount for each ID, mounting
only on first Get() and unmounting on the last Get().

This means we avoid littering the system with lots of mounts
and free resources related to them.

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)

graphdriver/aufs/aufs.go

@@ -31,6 +31,7 @@ import (
 	"os/exec"
 	"path"
 	"strings"
+	"sync"
 )
 
 func init() {
@@ -38,7 +39,9 @@ func init() {
 }
 
 type Driver struct {
-	root string
+	root       string
+	sync.Mutex // Protects concurrent modification to active
+	active     map[string]int
 }
 
 // New returns a new AUFS driver.
@@ -54,12 +57,17 @@ func Init(root string) (graphdriver.Driver, error) {
 		"layers",
 	}
 
+	a := &Driver{
+		root:   root,
+		active: make(map[string]int),
+	}
+
 	// Create the root aufs driver dir and return
 	// if it already exists
 	// If not populate the dir structure
 	if err := os.MkdirAll(root, 0755); err != nil {
 		if os.IsExist(err) {
-			return &Driver{root}, nil
+			return a, nil
 		}
 		return nil, err
 	}
@@ -69,7 +77,7 @@ func Init(root string) (graphdriver.Driver, error) {
 			return nil, err
 		}
 	}
-	return &Driver{root}, nil
+	return a, nil
 }
 
 // Return a nil error if the kernel supports aufs
@@ -167,6 +175,14 @@ func (a *Driver) createDirsFor(id string) error {
 
 // Unmount and remove the dir information
 func (a *Driver) Remove(id string) error {
+	// Protect the a.active from concurrent access
+	a.Lock()
+	defer a.Unlock()
+
+	if a.active[id] != 0 {
+		utils.Errorf("Warning: removing active id %s\n", id)
+	}
+
 	// Make sure the dir is umounted first
 	if err := a.unmount(id); err != nil {
 		return err
@@ -210,19 +226,45 @@ func (a *Driver) Get(id string) (string, error) {
 		ids = []string{}
 	}
 
+	// Protect the a.active from concurrent access
+	a.Lock()
+	defer a.Unlock()
+
+	count := a.active[id]
+
 	// If a dir does not have a parent ( no layers )do not try to mount
 	// just return the diff path to the data
 	out := path.Join(a.rootPath(), "diff", id)
 	if len(ids) > 0 {
 		out = path.Join(a.rootPath(), "mnt", id)
-		if err := a.mount(id); err != nil {
-			return "", err
+
+		if count == 0 {
+			if err := a.mount(id); err != nil {
+				return "", err
+			}
 		}
 	}
+
+	a.active[id] = count + 1
+
 	return out, nil
 }
 
 func (a *Driver) Put(id string) {
+	// Protect the a.active from concurrent access
+	a.Lock()
+	defer a.Unlock()
+
+	if count := a.active[id]; count > 1 {
+		a.active[id] = count - 1
+	} else {
+		ids, _ := getParentIds(a.rootPath(), id)
+		// We only mounted if there are any parents
+		if ids != nil && len(ids) > 0 {
+			a.unmount(id)
+		}
+		delete(a.active, id)
+	}
 }
 
 // Returns an archive of the contents for the id