devmapper: Drop devices lock before returning from function

cleanupDeleted() takes devices.Lock() but does not drop it if there are
no deleted devices. Hence docker deadlocks if one is using deferred
device deletion feature. (--storage-opt dm.use_deferred_deletion=true).

Fix it. Drop the lock before returning.

Also added a unit test case to make sure in future this can be easily
detected if somebody changes the function.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>

daemon/graphdriver/devmapper/deviceset.go

@@ -599,6 +599,7 @@ func (devices *DeviceSet) cleanupDeletedDevices() error {
 
 	// If there are no deleted devices, there is nothing to do.
 	if devices.nrDeletedDevices == 0 {
+		devices.Unlock()
 		return nil
 	}
 

daemon/graphdriver/devmapper/devmapper_test.go

@@ -5,6 +5,7 @@ package devmapper
 import (
 	"fmt"
 	"testing"
+	"time"
 
 	"github.com/docker/docker/daemon/graphdriver"
 	"github.com/docker/docker/daemon/graphdriver/graphtest"
@@ -79,3 +80,31 @@ func testChangeLoopBackSize(t *testing.T, delta, expectDataSize, expectMetaDataS
 		t.Fatal(err)
 	}
 }
+
+// Make sure devices.Lock() has been release upon return from cleanupDeletedDevices() function
+func TestDevmapperLockReleasedDeviceDeletion(t *testing.T) {
+	driver := graphtest.GetDriver(t, "devicemapper").(*graphtest.Driver).Driver.(*graphdriver.NaiveDiffDriver).ProtoDriver.(*Driver)
+	defer graphtest.PutDriver(t)
+
+	// Call cleanupDeletedDevices() and after the call take and release
+	// DeviceSet Lock. If lock has not been released, this will hang.
+	driver.DeviceSet.cleanupDeletedDevices()
+
+	doneChan := make(chan bool)
+
+	go func() {
+		driver.DeviceSet.Lock()
+		defer driver.DeviceSet.Unlock()
+		doneChan <- true
+	}()
+
+	select {
+	case <-time.After(time.Second * 5):
+		// Timer expired. That means lock was not released upon
+		// function return and we are deadlocked. Release lock
+		// here so that cleanup could succeed and fail the test.
+		driver.DeviceSet.Unlock()
+		t.Fatalf("Could not acquire devices lock after call to cleanupDeletedDevices()")
+	case <-doneChan:
+	}
+}