Ignore missing config.v2.json files on migration

We have seen some cases in which the file
`/var/lib/docker/containers/<UUID>/config.v2.json` was not found for
certain UUIDs. This causes the whole migration to fail and, worse,
causes our migration cleanup code to fail.

A missing `config.v2.json` indicates that directory does not contain a
valid container, so we should not even try to migrate them. That's what
this commit does: it makes the migration ignore directories with a
missing `config.v2.json`.

I don't think we should have directories like this in the first place,
this is probably the side effect of some other issue. That said, Docker
itself ignores these directories (after logging a warning) during
startup, so here are just bringing the Engine in line with the standard
behavior.

Signed-off-by: Leandro Motta Barros <[email protected]>
Change-type: patch

pkg/storagemigration/migrate.go

@@ -336,10 +336,14 @@ func SwitchAllContainersStorageDriver(root, newStorageDriver string) error {
 	logrus.Debugf("migrating %v container(s) to %s", len(containerIDs), newStorageDriver)
 	for _, containerID := range containerIDs {
 		err := switchContainerStorageDriver(root, containerID, newStorageDriver)
-		if err != nil {
+		switch err {
+		case nil:
+			logrus.WithField("container_id", containerID).Debugf("reconfigured storage-driver to %s", newStorageDriver)
+		case errNoConfigV2JSON:
+			logrus.WithField("container_id", containerID).Warning("ignoring container without config.v2.json")
+		default:
 			return fmt.Errorf("Error rewriting container config for %s: %v", containerID, err)
 		}
-		logrus.WithField("container_id", containerID).Debugf("reconfigured storage-driver to %s", newStorageDriver)
 	}
 	return nil
 }

pkg/storagemigration/utils.go

@@ -2,6 +2,7 @@ package storagemigration
 
 import (
 	"encoding/json"
+	"errors"
 	"os"
 	"path/filepath"
 
@@ -11,6 +12,8 @@ import (
 	"golang.org/x/sys/unix"
 )
 
+var errNoConfigV2JSON = errors.New("config.v2.json not found for container")
+
 // exists checks if a file  (or if isDir is set to "true" a directory) exists
 func exists(path string, isDir bool) (bool, error) {
 	fi, err := os.Stat(path)
@@ -60,6 +63,10 @@ func replicate(sourceDir, targetDir string) error {
 // this is the only change needed to make it work after the migration
 func switchContainerStorageDriver(root, containerID, newStorageDriver string) error {
 	containerConfigPath := filepath.Join(root, "containers", containerID, "config.v2.json")
+	if ok, _ := exists(containerConfigPath, false); !ok {
+		return errNoConfigV2JSON
+	}
+
 	f, err := os.OpenFile(containerConfigPath, os.O_RDWR, os.ModePerm)
 	if err != nil {
 		return err