options: keep using vfs for old users

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>

types/options.go

@@ -310,10 +310,16 @@ 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 {
+			// Note: Stat check on `graphRoot/vfs` is a hack to prevent
+			// old users from breaking.
+			// Reason: There can be users who defaulted to `vfs` and have
+			// already generated working content with `vfs` driver in such
+			// case `storage` will not silently default to `overlay`.
+			_, err := os.Stat(filepath.Join(opts.GraphRoot, "vfs"))
+			if err == nil || !canUseRootlessOverlay(opts.GraphRoot, opts.RunRoot) {
 				opts.GraphDriverName = "vfs"
+			} else {
+				opts.GraphDriverName = overlayDriver
 			}
 		} else {
 			opts.GraphDriverPriority = systemOpts.GraphDriverPriority