I was trying to use ‘call-process’ in a thread in order to not have to deal
with process buffers and the like while still being able to run an external
program in the background.

As it turns out, ‘call-process’ hangs all of Emacs while the external
program is run.

To reproduce, simply type M-: (make-thread (lambda () (call-process "sleep"
nil nil nil "2")))

This will hang Emacs for 2 seconds.

Regards,
Elias