graphdriver: change (*Driver).Put signature

There are a couple of drivers that swallow errors that may occur in
their Put() implementation.

This changes the signature of (*Driver).Put for all the drivers implemented.

Signed-off-by: Vincent Batts <vbatts@hashbangbash.com>

daemon/graphdriver/aufs/aufs.go

@@ -278,7 +278,7 @@ func (a *Driver) Get(id, mountLabel string) (string, error) {
 	return out, nil
 }
 
-func (a *Driver) Put(id string) {
+func (a *Driver) Put(id string) error {
 	// Protect the a.active from concurrent access
 	a.Lock()
 	defer a.Unlock()
@@ -293,6 +293,7 @@ func (a *Driver) Put(id string) {
 		}
 		delete(a.active, id)
 	}
+	return nil
 }
 
 // Diff produces an archive of the changes between the specified

daemon/graphdriver/btrfs/btrfs.go

@@ -220,9 +220,10 @@ func (d *Driver) Get(id, mountLabel string) (string, error) {
 	return dir, nil
 }
 
-func (d *Driver) Put(id string) {
+func (d *Driver) Put(id string) error {
 	// Get() creates no runtime resources (like e.g. mounts)
 	// so this doesn't need to do anything.
+	return nil
 }
 
 func (d *Driver) Exists(id string) bool {

daemon/graphdriver/devmapper/driver.go

@@ -141,10 +141,12 @@ func (d *Driver) Get(id, mountLabel string) (string, error) {
 	return rootFs, nil
 }
 
-func (d *Driver) Put(id string) {
-	if err := d.DeviceSet.UnmountDevice(id); err != nil {
+func (d *Driver) Put(id string) error {
+	err := d.DeviceSet.UnmountDevice(id)
+	if err != nil {
 		log.Errorf("Warning: error unmounting device %s: %s", id, err)
 	}
+	return err
 }
 
 func (d *Driver) Exists(id string) bool {

daemon/graphdriver/driver.go

@@ -40,7 +40,7 @@ type ProtoDriver interface {
 	Get(id, mountLabel string) (dir string, err error)
 	// Put releases the system resources for the specified id,
 	// e.g, unmounting layered filesystem.
-	Put(id string)
+	Put(id string) error
 	// Exists returns whether a filesystem layer with the specified
 	// ID exists on this driver.
 	Exists(id string) bool

daemon/graphdriver/overlay/overlay.go

@@ -299,7 +299,7 @@ func (d *Driver) Get(id string, mountLabel string) (string, error) {
 	return mount.path, nil
 }
 
-func (d *Driver) Put(id string) {
+func (d *Driver) Put(id string) error {
 	// Protect the d.active from concurrent access
 	d.Lock()
 	defer d.Unlock()
@@ -307,21 +307,23 @@ func (d *Driver) Put(id string) {
 	mount := d.active[id]
 	if mount == nil {
 		log.Debugf("Put on a non-mounted device %s", id)
-		return
+		return nil
 	}
 
 	mount.count--
 	if mount.count > 0 {
-		return
+		return nil
 	}
 
+	defer delete(d.active, id)
 	if mount.mounted {
-		if err := syscall.Unmount(mount.path, 0); err != nil {
+		err := syscall.Unmount(mount.path, 0)
+		if err != nil {
 			log.Debugf("Failed to unmount %s overlay: %v", id, err)
 		}
+		return err
 	}
-
-	delete(d.active, id)
+	return nil
 }
 
 func (d *Driver) ApplyDiff(id string, parent string, diff archive.ArchiveReader) (size int64, err error) {

daemon/graphdriver/vfs/driver.go

@@ -83,9 +83,10 @@ func (d *Driver) Get(id, mountLabel string) (string, error) {
 	return dir, nil
 }
 
-func (d *Driver) Put(id string) {
+func (d *Driver) Put(id string) error {
 	// The vfs driver has no runtime resources (e.g. mounts)
 	// to clean up, so we don't need anything here
+	return nil
 }
 
 func (d *Driver) Exists(id string) bool {