Unable to start PHP-FPM if no domains are assigned to it: No pool defined. at least one pool section must be specified in config file

Symptoms

• Unable to start PHP-FPM if no domains are using it:

  # systemctl restart plesk-php81-fpm
  Job for plesk-php81-fpm.service failed because the control process exited with error code. See "systemctl status plesk-php81-fpm.service" and "journalctl -xe" for details.

  # systemctl status plesk-php81-fpm.service -o cat
  WARNING: Nothing matches the include pattern '/opt/plesk/php/8.1/etc/php-fpm.d/*.conf' from /opt/plesk/php/8.1/etc/php-fpm.conf at line 119.
  [15-Nov-2023 09:45:40] ERROR: No pool defined. at least one pool section must be specified in config file
  [15-Nov-2023 09:45:40] ERROR: failed to post process the configuration
  [15-Nov-2023 09:45:40] ERROR: FPM initialization failed

• The command below reports that PHP-FPM is not in use by any domain:

  # plesk bin php_handler --get-usage -id plesk-php81-fpm
  The PHP handler with ID "plesk-php81-fpm" is not used.

• There are no (zero) configuration files inside a PHP-FPM pool directory:

  # ls /opt/plesk/php/8.1/etc/php-fpm.d/* | wc -l
  ls: cannot access /opt/plesk/php/8.1/etc/php-fpm.d/*: No such file or directory
  0

• The following error message appear in /var/log/plesk-php81-fpm/error.log:

  ERROR: No pool defined. at least one pool section must be specified in config file

Cause

This is expected behavior: If a PHP-FPM handler is not in use by any of domains, it will not start.

When a domain will be switched to this PHP-FPM handler, its PHP-FPM service will start automatically.

Resolution

1. Log into Plesk
2. To start using a particular PHP-FPM version, just assign it to any domain at Domains > example.com > PHP Settings. The service will be started automatically.