[bug#58405] [PATCH] services: nginx: Add reload action

[EuAndreh]

> With the NGinx service currently, you need to restart it to change the
> NGinx binary or configuration file.

It is true that you need to restart to change the NGINX binary, but this
is not true for changing the configuration file.

NGINX's master process reloads the configuration file, which could have
an "include" line that points to ad-hoc files in /etc.  So even though
the NGINX service is using the immutable file inside /gnu/store,
reloading it can have it change its runtime behaviour.

The same behaviour is relied upon for certbot certificates: the current
certificate lives in /etc/letsencrypt/live, but it is a symlink that
points to /etc/letsencrypt/archive.  When a certificate is renewed, a
SIGHUP ought to be sent to NGINX in order to reload the configuration
file, so that the certificates themselves can be reloaded, even though
neither the NGINX binary nor the configuration file changed, but only
what they point to did.


> What's the purpose of the reload action here given that neither the
> binary or configuration file being used will change?

I'm doing blue/green deployments on a web service.  I have the
equivalent of /etc/my-service/{blue,green,active}.conf files, and an
"include" line in the main NGINX configuration that includes the
"active" one.  Doing a deploy from blue to green is done by changing the
`active.conf` symlink to point to `green.conf` instead, and sending a
SIGHUP to NGINX.