ostree-deploy: fix umount error

Sometimes umount fails after ostree-deploy with following error:
2019/08/26 14:08:51 ==== Deploying ostree onto image ====
2019/08/26 14:08:57 ==== List files on ostreetestarmhf ====
2019/08/26 14:08:59 Warning: Failed to get unmount /: device or resource busy
2019/08/26 14:08:59 Unmount failure can cause images being incomplete!

libostree keeps some information, like repo lock file descriptor, in thread
specific variables.
As Go Garbage Collector can be run in a thread different from the one
which created the libostree object, it may not been able to access the
object thread specific variables, preventing to free them correctly.
To prevent this, explicitly dereference libostree objects.

Signed-off-by: Frédéric Danis <frederic.danis@collabora.com>

actions/ostree_deploy_action.go

@@ -55,7 +55,6 @@ import (
 	"io"
 	"os"
 	"path"
-	"runtime"
 	"strings"
 
 	"github.com/go-debos/debos"
@@ -198,6 +197,11 @@ func (ot *OstreeDeployAction) Run(context *debos.DebosContext) error {
 		return err
 	}
 
-	runtime.GC()
+	/* libostree keeps some information, like repo lock file descriptor, in
+	 * thread specific variables. As GC can be run from another thread, it
+	 * may not been able to access this, preventing to free them correctly.
+	 * To prevent this, explicitly dereference libostree objects. */
+	dstRepo.Unref()
+	sysroot.Unref()
 	return nil
 }