A download is not served by PHP. PHP authorizes it and hands the web
server the path with X-Accel-Redirect, so the web server has to open a
file PHP wrote. Where those are different users — cPanel and Plesk
commonly arrange it that way — it cannot: uploads land 0600 inside a 0700
directory, and traversing 0700 means being its owner. Nothing else on the
site shows a symptom. Uploading works, the library lists everything, and
only downloads fail, as ERR_INVALID_RESPONSE in the browser and
`open() ... failed (13: Permission denied)` in the web server's log.
FILES_WEB_SERVER_READABLE writes uploads 0644/0755 instead. Opt-in and
spread into the disk configuration rather than switched by a ternary, so
an install that does not set it keeps byte-for-byte the configuration it
had: the relaxed modes are readable by every account on the machine,
which is the wrong trade wherever the web server and PHP are one user, as
in the image and on most self-administered servers.
The two halves are not enforced alike, which is the part worth knowing.
`visibility` has Flysystem chmod each file after writing it, so 0644
holds under any umask. A directory is created by mkdir(), which masks its
mode argument, so 0755 is a ceiling: a pool at umask 0077 still produces
0700 and still cannot be traversed. That cannot be fixed from config, so
INSTALL.md carries it — how to tell the two users apart, the one-time
chmod for files already on disk, and the pool setting for the umask.
FilePermissionsTest asserts all three modes, umask cases included, since
the asymmetry is invisible from the configuration.
Reported by @denkfabrik-li (#1668), who diagnosed it and verified the
remedy on the affected host.
ignacionelson