How do I preserve a UNIX socket across reboots (RasPi, Arch, chroot)

Question

I've set up nginx and php-fpm under Arch Linux on a RasPi. nginx was set up in a chroot as described in the ArchWiki. Everything runs fine when php-fpm.conf and nginx.conf are set up to listen to 127.0.0.1:9000 and php-fpm.conf doesn't declare a path to the pid.

Now I want to change the setup to use a UNIX socket as is suggested across the web. So I added to php-fpm.conf the following.

pid = /srv/http/run/php-fpm/php-fpm.pid
...
[www]
chroot = /srv/http;
listen = /srv/http/run/php-fpm/php-fpm.sock

Also replaced fastcgi_pass in nginx.conf with

fastcgi_pass   unix:/run/php-fpm/php-fpm.sock;

When I manually create the .sock file and restart both php-fpm and nginx, it works. But whenever I reboot, the php-fpm directory and .sock file are gone...

How can I preserve them across reboots? /etc/fstab includes

tmpfs           /srv/http/run   tmpfs   rw,noexec,relatime,size=1024k   0       0
tmpfs           /srv/http/tmp   tmpfs   rw,noexec,relatime,size=102400k 0       0

Answer 1

You can't. This would be the same thing as preserving a TCP connection over reboot. The socket is created by the process that listens on it (in your case php-fpm) and nginx connects to it. If you get problems because nginx can't open it, you most probably start your services in the wrong order (you'd have to start php-fpm before starting nginx, so that the socket exists when nginx starts).

Answer 2

tmpfiles (part of systemd) will create and remove temporary/volatile directories and files. Typically these are placed in /run or /tmp, but it will work anywhere.

To have the directory /srv/http/run/php-fpm create on startup, create a config file in /etc/tmpfiles.d like this:

d /srv/http/run/php-fpm 0755 http http -

This will create the directory with the permissions 0755 for the http user and group (Arch's default php-fpm / web user)

The file can be called whatever you like but needs to end in .conf. Example (php-fpm.conf)

The socket itself is created by php-fpm when it starts, so be sure it is enabled in systemd:

systemctl enable php-fpm