MediaWiki's GlobalIdGenerator class creates two files in $wgTmpDirectory, i.e. mw-GlobalIdGenerator-UID-nodeid and mw-GlobalIdGenerator-UUID-128. The default value for $wgTmpDirectory is the system directory for temporary files (e.g. /tmp on Linux). When a system hosts multiple MediaWiki installations (for example on shared hosting) with the default settings, this causes trouble when one installation has already created /tmp/mw-GlobalIdGenerator-UID-nodeid and /tmp/mw-GlobalIdGenerator-UUID-128 and another installation attempts to access or create them. This is similar to T161453 .

One consequence of this, is that the VisualEditor returns a HTTP 500 error because of a permission error on the files in /tmp because another user has already created those files:

[2020-11-09 03:51:20.876788] [proxy_fcgi:error] [R:X6iup-BKHJAtBifWDmWMbQAAABQ] AH01071: Got error 'PHP message: PHP Warning:  fopen(/tmp/mw-GlobalIdGenerator-UUID-128): failed to open stream: Permission denied in /home/boss/thomasd/www/includes/libs/uuid/GlobalIdGenerator.php on line 449' web2

If one user hosts multiple MediaWiki installations, or if a shared hosting service would run installations from different users all as www-data, then one installation would be messing with the files of another.

There might be other consequences depending on where GlobalIdGenerator gets used, though I have only seen issues with VisualEditor so far. This does mean that it's impossible to use VisualEditor on shared hosting with the default settings.

My suggestion is to either ensure that the file names will be unique (such as by using a prefix), or by implementing T179901 as long as the created directory is unique per installation and not system-wide.