I run under Drupal 8.7.6.
My default / files is a symbolic link to a read-only mount:
files -> /mnt/files_ro
Drupal needs to write in a few folders because it aggregates and caches assets such as css, js, php / twig, ….
So I created a read and write montage. And created a number of symlinks from / mnt / files_ro:
css -> /mnt/files_rw/css/ js -> /mnt/files_rw/js/ styles -> /mnt/files_rw/styles/ ...
Now Drupal is generating the same errors it would have if the / default files had incorrect file permissions. but my files_rw directory belongs to the Apache user and is writable.
I've done enough tests to make sure the problem is not a simple file authorization.
I also know that in D7, you had to patch Core if you wanted Drupal to follow symlinks.
How to make D8 follow symbolic links in the / files / directory? Has anyone else done that?