Plesk never loads or fails with 502 Bad Gateway/504 Gateway errors

Symptoms

1. Plesk never loads or fails with 502 Bad Gateway/504 Gateway errors. One of the following error messages appears in /var/log/sw-cp-server/error_log:

   CONFIG_TEXT: [error] 8074#0: *1046 upstream timed out (110: Connection timed out) while reading response header from upstream, client: 203.0.113.2, server: , request: "GET /login_up.php3 HTTP/1.1", upstream: "fastcgi://unix:/var/run/sw-engine.sock", host: "example.com:8443"

   OR

   CONFIG_TEXT: [error] 2550#0: *521 recv() failed (104: Connection reset by peer) while reading response header from upstream, client:203.0.113.2, server: , request: "GET /login_up.php3 HTTP/1.1", upstream: "fastcgi://unix:/var/run/sw-engine.sock:", host: "203.0.113.2:8443"

2. Restarting Plesk services has no effect:

   # service sw-cp-server restart && service sw-engine restart

3. There are sw-engine processes even when the sw-engine service is stopped:

   # service sw-engine status | grep active
   Active: inactive (dead) since Wed 2018-06-20 20:46:50 +07; 39s ago
   
   # ps aux | grep sw-engine
   root 23173 0.0 0.3 383796 37504 ? SN 09:19 0:00 /usr/bin/sw-engine -c /usr/local/psa/admin/conf/php.ini /usr/local/psa/admin/plib/DailyMaintainance/script.php
   root 30148 0.0 0.3 398044 45916 ? SN 09:20 0:00 /usr/bin/sw-engine -c /usr/local/psa/admin/conf/php.ini /usr/local/psa/admin/plib/DailyMaintainance/task-script.php PleskUsage --period=daily
   root 30164 0.0 0.3 391748 38820 ? SN 09:20 0:04 /usr/bin/sw-engine -c /usr/local/psa/admin/conf/php.ini /usr/local/psa/bin/php_handler --list -json true

Cause

Plesk services hanged.

Resolution

1. Connect to a Plesk server via SSH.

2. Stop the sw-engine and sw-cp-services services:

   # service sw-engine stop && service sw-cp-service stop

3. Find hanged sw-engine processes:

   # ps aux | grep sw-engine
   root 23173 0.0 0.3 383796 37504 ? SN 09:19 0:00 /usr/bin/sw-engine -c /usr/local/psa/admin/conf/php.ini /usr/local/psa/admin/plib/DailyMaintainance/script.php
   root 30148 0.0 0.3 398044 45916 ? SN 09:20 0:00 /usr/bin/sw-engine -c /usr/local/psa/admin/conf/php.ini /usr/local/psa/admin/plib/DailyMaintainance/task-script.php PleskUsage --period=daily
   root 30164 0.0 0.3 391748 38820 ? SN 09:20 0:04 /usr/bin/sw-engine -c /usr/local/psa/admin/conf/php.ini /usr/local/psa/bin/php_handler --list -json true

4. Kill all found hanged processes using their PID:

   # kill -9 23173 30148 30164

5. Start Plesk services:

   # service sw-cp-server start && service sw-engine start

6. Log in to Plesk.