options: keep using prior drivers if found

There is no need for `vfs` to be the default storage driver since kernel
>= 5.13 supports `overlay` natively however there is use-case for users
who don't had any configs and they started using `vfs` in a default
manner following check is a hack to keep `buildah` and `podman` working
for such users.

See: #1571 for prior
discussions.

Signed-off-by: Aditya R <arajan@redhat.com>

drivers/driver.go

@@ -342,12 +342,12 @@ func New(name string, config Options) (Driver, error) {
 	}
 
 	// Guess for prior driver
-	driversMap := scanPriorDrivers(config.Root)
+	driversMap := ScanPriorDrivers(config.Root)
 
 	// use the supplied priority list unless it is empty
 	prioList := config.DriverPriority
 	if len(prioList) == 0 {
-		prioList = priority
+		prioList = Priority
 	}
 
 	for _, name := range prioList {
@@ -419,12 +419,12 @@ func isDriverNotSupported(err error) bool {
 }
 
 // scanPriorDrivers returns an un-ordered scan of directories of prior storage drivers
-func scanPriorDrivers(root string) map[string]bool {
+func ScanPriorDrivers(root string) map[string]bool {
 	driversMap := make(map[string]bool)
 
 	for driver := range drivers {
 		p := filepath.Join(root, driver)
-		if _, err := os.Stat(p); err == nil && driver != "vfs" {
+		if _, err := os.Stat(p); err == nil {
 			driversMap[driver] = true
 		}
 	}

drivers/driver_darwin.go

@@ -2,7 +2,7 @@ package graphdriver
 
 var (
 	// Slice of drivers that should be used in order
-	priority = []string{
+	Priority = []string{
 		"vfs",
 	}
 )

drivers/driver_freebsd.go

@@ -13,7 +13,7 @@ const (
 
 var (
 	// Slice of drivers that should be used in an order
-	priority = []string{
+	Priority = []string{
 		"zfs",
 		"vfs",
 	}

drivers/driver_linux.go

@@ -90,7 +90,7 @@ const (
 
 var (
 	// Slice of drivers that should be used in an order
-	priority = []string{
+	Priority = []string{
 		"overlay",
 		// We don't support devicemapper without configuration
 		// "devicemapper",

drivers/driver_solaris.go

@@ -31,7 +31,7 @@ const (
 
 var (
 	// Slice of drivers that should be used in an order
-	priority = []string{
+	Priority = []string{
 		"zfs",
 	}
 

drivers/driver_unsupported.go

@@ -5,7 +5,7 @@ package graphdriver
 
 var (
 	// Slice of drivers that should be used in an order
-	priority = []string{
+	Priority = []string{
 		"unsupported",
 	}
 )

drivers/driver_windows.go

@@ -2,7 +2,7 @@ package graphdriver
 
 var (
 	// Slice of drivers that should be used in order
-	priority = []string{
+	Priority = []string{
 		"windowsfilter",
 	}
 )

types/options.go

@@ -10,6 +10,8 @@ import (
 	"time"
 
 	"github.com/BurntSushi/toml"
+	drivers "github.com/containers/storage/drivers"
+	_ "github.com/containers/storage/drivers/register"
 	cfg "github.com/containers/storage/pkg/config"
 	"github.com/containers/storage/pkg/idtools"
 	"github.com/sirupsen/logrus"
@@ -310,10 +312,21 @@ func getRootlessStorageOpts(rootlessUID int, systemOpts StoreOptions) (StoreOpti
 	}
 	if opts.GraphDriverName == "" {
 		if len(systemOpts.GraphDriverPriority) == 0 {
-			if canUseRootlessOverlay(opts.GraphRoot, opts.RunRoot) {
-				opts.GraphDriverName = overlayDriver
-			} else {
-				opts.GraphDriverName = "vfs"
+			driversMap := drivers.ScanPriorDrivers(opts.GraphRoot)
+
+			for _, name := range drivers.Priority {
+				if _, prior := driversMap[name]; prior {
+					opts.GraphDriverName = name
+					break
+				}
+			}
+
+			if opts.GraphDriverName == "" {
+				if canUseRootlessOverlay(opts.GraphRoot, opts.RunRoot) {
+					opts.GraphDriverName = overlayDriver
+				} else {
+					opts.GraphDriverName = "vfs"
+				}
 			}
 		} else {
 			opts.GraphDriverPriority = systemOpts.GraphDriverPriority