LibraryStream::getTarget() does not return expected string|bool

Problem/Motivation

When experimenting with the CDN module, encountered an fatal error while visiting the module's config page due to LibraryStream::getTarget() return an array.

It's unclear if this is just an incompatibility with the CDN module, or an unexpected implementation of LibraryStream::getTarget().

LibraryStream extends Drupal\Core\StreamWrapper\PublicStream, which also inherits Drupal\Core\StreamWrapper\LocalStream. When reviewing LocalStream::getTarget(), its dockblock states the following. Note, the interface does not declare

* @return string|bool
* Returns a string representing a location suitable for writing of a file,
* or FALSE if unable to write to the file such as with read-only streams

Steps to reproduce

• Install and enable latest CDN module version, enable CDN UI module
• Go to UI config form path /admin/config/services/cdn
• Observe error, check logs

Proposed resolution

TBD

Remaining tasks

TBD

User interface changes

TBD

API changes

TBD

Data model changes

TBD