katex: Replace subprocess call with minimal external service.

Replace a separate call to subprocess, starting `node` from scratch,
with an optional standalone node Express service which performs the
rendering.  In benchmarking, this reduces the overhead of a KaTeX call
from 120ms to 2.8ms.  This is notable because enough calls to KaTeX in
a single message would previously time out the whole message
rendering.

The service is optional because he majority of deployments do not use
enough LaTeX to merit the additional memory usage (60Mb).

Fixes: #17425.

puppet/zulip/manifests/app_frontend_base.pp

@@ -177,6 +177,9 @@ class zulip::app_frontend_base {
     include zulip::smokescreen
   }
 
+  $katex_server = zulipconf('application_server', 'katex_server', false)
+  $katex_server_port = zulipconf('application_server', 'katex_server_port', '9700')
+
   if $proxy_host != '' and $proxy_port != '' {
     $proxy = "http://${proxy_host}:${proxy_port}"
   } else {

puppet/zulip/templates/supervisor/zulip.conf.template.erb

@@ -106,6 +106,23 @@ programs=<% @queues.each_with_index do |queue, i| -%>zulip_events_<%= queue %><%
 programs=zulip_events
 <% end %>
 
+<% if @katex_server %>
+[program:zulip-katex]
+command=/usr/local/bin/secret-env-wrapper SHARED_SECRET=shared_secret /srv/zulip-node/bin/node /home/zulip/deployments/current/static/webpack-bundles/katex_server.js <%= @katex_server_port %>
+environment=NODE_ENV=production
+priority=200                   ; the relative start priority (default 999)
+autostart=true                 ; start at supervisord start (default: true)
+autorestart=true               ; whether/when to restart (default: unexpected)
+stopsignal=TERM                 ; signal used to kill process (default TERM)
+stopwaitsecs=30                ; max num secs to wait b4 SIGKILL (default 10)
+user=zulip                    ; setuid to this UNIX account to run the program
+redirect_stderr=true           ; redirect proc stderr to stdout (default false)
+stdout_logfile=/var/log/zulip/katex.log         ; stdout log path, NONE for none; default AUTO
+stdout_logfile_maxbytes=20MB   ; max # logfile bytes b4 rotation (default 50MB)
+stdout_logfile_backups=3     ; # of stdout logfile backups (default 10)
+directory=/home/zulip/deployments/current/
+<% end %>
+
 ; The [include] section can just contain the "files" setting.  This
 ; setting can list multiple files (separated by whitespace or
 ; newlines).  It can also contain wildcards.  The filenames are