Description:
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Trying to start PHP-FPM with the --allow-to-run-as-root flag will not work when the user directive is not given in the FPM worker pool configuration. Parsing the config will fail. Consequently, FPM cannot start.

The offending code is in /master/sapi/fpm/fpm/fpm_conf.c:

766		/* alert if user is not set only if we are not root*/
767		if (!wp->config->user && !geteuid()) {
768			zlog(ZLOG_ALERT, "[pool %s] user has not been defined", wp->config->name);
769			return -1;
770		}

On a side note: the comment is wrong (should be "if we are root").

Related https://bugs.php.net/bug.php?id=61295

Test script:
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Comment the user directive in a worker pool confing. Then try to start php-fpm as root.

Actual result:
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root@4d9ea34ff691:/var/www/html# php-fpm -D
[11-Jul-2016 08:27:46] ALERT: [pool www] user has not been defined
[11-Jul-2016 08:27:46] ERROR: failed to post process the configuration
[11-Jul-2016 08:27:46] ERROR: FPM initialization failed