Make the priority for picking the storage driver configurable

This fixes #1457

Signed-off-by: Dan Čermák <dcermak@suse.com>

drivers/driver.go

@@ -329,15 +329,22 @@ type Options struct {
 }
 
 // New creates the driver and initializes it at the specified root.
-func New(name string, config Options) (Driver, error) {
+func New(name string, driver_priority []string, config Options) (Driver, error) {
 	if name != "" {
 		logrus.Debugf("[graphdriver] trying provided driver %q", name) // so the logs show specified driver
 		return GetDriver(name, config)
 	}
 
 	// Guess for prior driver
 	driversMap := scanPriorDrivers(config.Root)
-	for _, name := range priority {
+
+	// use the supplied priority list unless it is empty
+	prio_list := driver_priority
+	if len(prio_list) == 0 {
+		prio_list = priority
+	}
+
+	for _, name := range prio_list {
 		if name == "vfs" {
 			// don't use vfs even if there is state present.
 			continue
@@ -372,7 +379,7 @@ func New(name string, config Options) (Driver, error) {
 	}
 
 	// Check for priority drivers first
-	for _, name := range priority {
+	for _, name := range prio_list {
 		driver, err := getBuiltinDriver(name, config.Root, config)
 		if err != nil {
 			if isDriverNotSupported(err) {

store.go

@@ -591,8 +591,9 @@ type store struct {
 
 	// The following fields are only set when constructing store, and must never be modified afterwards.
 	// They are safe to access without any other locking.
-	runRoot         string
-	graphDriverName string // Initially set to the user-requested value, possibly ""; updated during store construction, and does not change afterwards.
+	runRoot             string
+	graphDriverName     string // Initially set to the user-requested value, possibly ""; updated during store construction, and does not change afterwards.
+	graphDriverPriority []string
 	// graphLock:
 	// - Ensures that we always reload graphDriver, and the primary layer store, after any process does store.Shutdown. This is necessary
 	//   because (??) the Shutdown may forcibly unmount and clean up, affecting graph driver state in a way only a graph driver
@@ -723,20 +724,21 @@ func GetStore(options types.StoreOptions) (Store, error) {
 		autoNsMaxSize = AutoUserNsMaxSize
 	}
 	s := &store{
-		runRoot:         options.RunRoot,
-		graphDriverName: options.GraphDriverName,
-		graphLock:       graphLock,
-		usernsLock:      usernsLock,
-		graphRoot:       options.GraphRoot,
-		graphOptions:    options.GraphDriverOptions,
-		pullOptions:     options.PullOptions,
-		uidMap:          copyIDMap(options.UIDMap),
-		gidMap:          copyIDMap(options.GIDMap),
-		autoUsernsUser:  options.RootAutoNsUser,
-		autoNsMinSize:   autoNsMinSize,
-		autoNsMaxSize:   autoNsMaxSize,
-		disableVolatile: options.DisableVolatile,
-		transientStore:  options.TransientStore,
+		runRoot:             options.RunRoot,
+		graphDriverName:     options.GraphDriverName,
+		graphDriverPriority: options.GraphDriverPriority,
+		graphLock:           graphLock,
+		usernsLock:          usernsLock,
+		graphRoot:           options.GraphRoot,
+		graphOptions:        options.GraphDriverOptions,
+		pullOptions:         options.PullOptions,
+		uidMap:              copyIDMap(options.UIDMap),
+		gidMap:              copyIDMap(options.GIDMap),
+		autoUsernsUser:      options.RootAutoNsUser,
+		autoNsMinSize:       autoNsMinSize,
+		autoNsMaxSize:       autoNsMaxSize,
+		disableVolatile:     options.DisableVolatile,
+		transientStore:      options.TransientStore,
 
 		additionalUIDs: nil,
 		additionalGIDs: nil,
@@ -940,7 +942,7 @@ func (s *store) createGraphDriverLocked() (drivers.Driver, error) {
 		UIDMaps:       s.uidMap,
 		GIDMaps:       s.gidMap,
 	}
-	return drivers.New(s.graphDriverName, config)
+	return drivers.New(s.graphDriverName, s.graphDriverPriority, config)
 }
 
 func (s *store) GraphDriver() (drivers.Driver, error) {

types/options.go

@@ -19,6 +19,7 @@ import (
 type TomlConfig struct {
 	Storage struct {
 		Driver              string            `toml:"driver,omitempty"`
+		DriverPriority      []string          `toml:"driver_priority,omitempty"`
 		RunRoot             string            `toml:"runroot,omitempty"`
 		GraphRoot           string            `toml:"graphroot,omitempty"`
 		RootlessStoragePath string            `toml:"rootless_storage_path,omitempty"`
@@ -213,10 +214,16 @@ type StoreOptions struct {
 	// RootlessStoragePath is the storage path for rootless users
 	// default $HOME/.local/share/containers/storage
 	RootlessStoragePath string `toml:"rootless_storage_path"`
-	// GraphDriverName is the underlying storage driver that we'll be
-	// using.  It only needs to be specified the first time a Store is
-	// initialized for a given RunRoot and GraphRoot.
+	// If the driver is not specified, the best suited driver will be picked
+	// either from GraphDriverPriority or from the platform dependent
+	// priority list (in that order).
 	GraphDriverName string `json:"driver,omitempty"`
+	// GraphDriverPriority is a list of storage drivers that will be tried
+	// to initialize the Store for a given RunRoot and GraphRoot unless a
+	// GraphDriverName is set.
+	// This list can be used to define a custom order in which the drivers
+	// will be tried.
+	GraphDriverPriority []string `json:"driver-priority,omitempty"`
 	// GraphDriverOptions are driver-specific options.
 	GraphDriverOptions []string `json:"driver-options,omitempty"`
 	// UIDMap and GIDMap are used for setting up a container's root filesystem
@@ -383,6 +390,7 @@ func ReloadConfigurationFile(configFile string, storeOptions *StoreOptions) erro
 	if storeOptions.GraphDriverName == "" {
 		logrus.Errorf("The storage 'driver' option must be set in %s to guarantee proper operation", configFile)
 	}
+	storeOptions.GraphDriverPriority = config.Storage.DriverPriority
 	if config.Storage.RunRoot != "" {
 		storeOptions.RunRoot = config.Storage.RunRoot
 	}