Steps to reproduce the issue

I guess any extension using the FTPClient class is broken with J3.8.0.
I have an example one, but it needs an FTP account on the german DWD server. Unfortunately they don't give them out anymore since the service is going to shut down soon ?

Expected result

Works

Actual result

White page of death due to missing wrapper for "buffer"

System information (as much as possible)

J3.8.0

Additional comments

The failing code line is https://github.com/joomla/joomla-cms/blob/staging/libraries/src/Client/FtpClient.php#L858:
$tmp = fopen('buffer://tmp', 'br+');

Since J3.8.0 it returns false.

The responsible PR is #16934 which renamed JBuffer.
Thus pinging @laoneo

According to Michael it worked before due to the way JLoader loads the classes into memory. With the namespaced classes and its autoloader, that is no longer the case and we need to somehow register the wrapper in a better way.

Adding stream_wrapper_register('buffer', '\\Joomla\\CMS\\Utility\\BufferStreamHandler'); to the constructor of FtpClient did get it back to work, but I got a warning "stream_wrapper_register(): Protocol buffer:// is already defined." so that's not right.