feat: self-update of master and agents (#15)

internal/agent/updater/override.go

@@ -177,20 +177,35 @@ var errNoRoots = errors.New("refused: no stack directory is mounted into the age
 // path inside the container. Compose labels hold host paths, so only those
 // mounts make a label path usable as is.
 func MountRoots(ctx context.Context, cli SelfInspector, selfID string) ([]string, error) {
+	insp, err := inspectSelf(ctx, cli, selfID)
+	if err != nil {
+		return nil, err
+	}
+	return rootsOf(insp), nil
+}
+
+// inspectSelf inspects the agent's own container.
+func inspectSelf(ctx context.Context, cli SelfInspector, selfID string) (container.InspectResponse, error) {
 	if selfID == "" {
-		return nil, errNoRoots
+		return container.InspectResponse{}, errNoRoots
 	}
 	insp, err := cli.ContainerInspect(ctx, selfID)
 	if err != nil {
-		return nil, fmt.Errorf("inspect own container %s: %w", selfID, err)
+		return container.InspectResponse{}, fmt.Errorf("inspect own container %s: %w", selfID, err)
 	}
+	return insp, nil
+}
+
+// rootsOf returns the directories MountRoots describes, from an inspected
+// container.
+func rootsOf(insp container.InspectResponse) []string {
 	var roots []string
 	for _, m := range insp.Mounts {
 		if m.Type == mount.TypeBind && m.RW && filepath.Clean(m.Source) == filepath.Clean(m.Destination) {
 			roots = append(roots, filepath.Clean(m.Destination))
 		}
 	}
-	return roots, nil
+	return roots
 }
 
 // composeFiles returns the -f arguments for a job: the stack's config files