doc: update documentation for systemd + PrivateTmp users

The PrivateTmp feature of systemd breaks the usage of /tmp for the
shared Unix domain socket between nginx and unicorn, so discourage the
use of /tmp in that case.

While we're at it, use consistent paths for everything and use an
obviously intended-for-user-customization "/path/to" prefix instead
of "/tmp"

ML-Ref: CAKLVLx_t+9zWMhquMWDfStrxS7xrNoGmN0ZDsjSCUE=VxU+oyQ@mail.gmail.com
Reported-by: David Wilkins <dwilkins@conecuh.com>

examples/nginx.conf

@@ -24,8 +24,8 @@ user nobody nogroup; # for systems with a "nogroup"
 # user nobody nobody; # for systems with "nobody" as a group instead
 
 # Feel free to change all paths to suite your needs here, of course
-pid /tmp/nginx.pid;
-error_log /tmp/nginx.error.log;
+pid /path/to/nginx.pid;
+error_log /path/to/nginx.error.log;
 
 events {
   worker_connections 1024; # increase if you have lots of clients
@@ -42,7 +42,7 @@ http {
   default_type application/octet-stream;
 
   # click tracking!
-  access_log /tmp/nginx.access.log combined;
+  access_log /path/to/nginx.access.log combined;
 
   # you generally want to serve static files with nginx since neither
   # Unicorn nor Rainbows! is optimized for it at the moment
@@ -74,7 +74,7 @@ http {
     # single worker for timing out).
 
     # for UNIX domain socket setups:
-    server unix:/tmp/.sock fail_timeout=0;
+    server unix:/path/to/.unicorn.sock fail_timeout=0;
 
     # for TCP setups, point these to your backend servers
     # server 192.168.0.7:8080 fail_timeout=0;

examples/unicorn.conf.rb

@@ -25,7 +25,7 @@
 
 # listen on both a Unix domain socket and a TCP port,
 # we use a shorter backlog for quicker failover when busy
-listen "/tmp/.sock", :backlog => 64
+listen "/path/to/.unicorn.sock", :backlog => 64
 listen 8080, :tcp_nopush => true
 
 # nuke workers after 30 seconds instead of 60 seconds (the default)

lib/unicorn/configurator.rb

@@ -188,7 +188,7 @@ def before_exec(*args, &block)
   #    # on nginx upstream configuration:
   #    upstream unicorn_backend {
   #      # for UNIX domain socket setups:
-  #      server unix:/path/to/unicorn.sock fail_timeout=0;
+  #      server unix:/path/to/.unicorn.sock fail_timeout=0;
   #
   #      # for TCP setups
   #      server 192.168.0.7:8080 fail_timeout=0;
@@ -229,9 +229,15 @@ def listeners(addresses) # :nodoc:
   #
   #   listen 3000 # listen to port 3000 on all TCP interfaces
   #   listen "127.0.0.1:3000"  # listen to port 3000 on the loopback interface
-  #   listen "/tmp/.unicorn.sock" # listen on the given Unix domain socket
+  #   listen "/path/to/.unicorn.sock" # listen on the given Unix domain socket
   #   listen "[::1]:3000" # listen to port 3000 on the IPv6 loopback interface
   #
+  # When using Unix domain sockets, be sure:
+  # 1) the path matches the one used by nginx
+  # 2) uses the same filesystem namespace as the nginx process
+  # For systemd users using PrivateTmp=true (for either nginx or unicorn),
+  # this means Unix domain sockets must not be placed in /tmp
+  #
   # The following options may be specified (but are generally not needed):
   #
   # [:backlog => number of clients]