Fix nested resource check for publicly shared space root

The space root's path is always set to ".", which makes the simple
'HasPrefix' check for whether a file a ancestor fail.

Co-Authored-By: Christian Richter <crichter@owncloud.com>

changelog/unreleased/fix_public_share_space_root.md

@@ -0,0 +1,7 @@
+Bugfix: Fix access to files withing a public link targetting a space root
+
+We fixed an issue that prevented users from opening documents within a public share
+that targets a space root.
+
+https://github.com/cs3org/reva/pull/4632/
+https://github.com/owncloud/ocis/issues/8691

internal/grpc/interceptors/auth/scope.go

@@ -265,6 +265,11 @@ func checkIfNestedResource(ctx context.Context, ref *provider.Reference, parent
  return false, statuspkg.NewErrorFromCode(statResponse.Status.Code, "auth interceptor")
  }
  parentPath := statResponse.Info.Path
+ if utils.IsSpaceRoot(statResponse.Info) && parentPath == "." {
+ // The space root's path is always ".". We explicitly set it to "/" here to
+ // make the HasPrefix check further down work correctly.
+ parentPath = "/"
+ }
 
  childPath := ref.GetPath()
  if childPath != "" && childPath != "." && strings.HasPrefix(childPath, parentPath) {