Unable to see messages logged to STDERR in docker logs

[siddjain]

We are using the wordpress:php7.4-fpm-alpine image. We are logging to STDERR using following as example:

error_log($msg, 3, '/dev/stderr');

but do not see the message in Docker logs. If we replace /dev/stderr with a local filename such as /var/www/html/wp-debug.log the message does appear there.

Things we tried (none of which worked):

• replace /dev/stderr with /proc/self/fd/2
• replace /dev/stderr with /proc/1/fd/2

we have verified catch_workers_output is set to yes in /usr/local/etc/php-fpm.d/docker.conf.

below are the different kinds of error messages in wordpress logs:

[23-Mar-2021 18:48:55 UTC] PHP Warning:  error_log(/dev/stderr): failed to open stream: No such file or directory in /var/www/html/wp-content/logging.php on line 11
[23-Mar-2021 20:26:06 UTC] PHP Warning:  error_log(/proc/1/fd/2): failed to open stream: Permission denied in /var/www/html/wp-content/logging.php on line 11
[23-Mar-2021 21:13:15 UTC] PHP Warning:  error_log(/proc/self/fd/2): failed to open stream: No such file or directory in /var/www/html/wp-content/logging.php on line 11

the /dev/stderr does clearly exist:

bash-5.0# ls -al /dev/stderr
lrwxrwxrwx    1 root     root            15 Mar 23 20:04 /dev/stderr -> /proc/self/fd/2
bash-5.0# ls -al /proc/self/fd/2
lrwx------    1 root     root            64 Mar 23 21:18 /proc/self/fd/2 -> /dev/pts/0
bash-5.0# ls -al /dev/pts/0
crw--w----    1 root     tty       136,   0 Mar 23 21:18 /dev/pts/0

[wglambert]

The /proc/1/fd/2 should be the stderr of PID 1 so anything written there should then output in docker log
docker-library/php#811 (comment)

I think the error_log(/proc/self/fd/2): failed to open stream: No such file or directory is because /proc/self/fd/ is redirected to /dev/pts/0 which only exists when you're "attached" to the container (with docker exec -it)

[siddjain]

So what's the solution here? Is there something that can be done to prevent permission denied when trying to open /proc/1/fd/2? Can one chmod this file? is it safe?

bash-5.0# ls -al /proc/1/fd/2
l-wx------    1 root     root            64 Mar 23 20:04 /proc/1/fd/2 -> pipe:[33197]

[siddjain]

this answer has 28 upvotes and says to set:

access.log = /proc/self/fd/2
error_log = /proc/self/fd/2

which is also the setting in /usr/local/etc/php-fpm.d/docker.conf that comes with the Docker image. But logging to this doesn't work...

[tianon]

I think the conclusion in docker-library/php#878 is that this is complicated and often doesn't work at all -- I don't think anyone's found a good generally applicable workaround for it.

[fullbl]

what about this?

https://github.com/nginxinc/docker-nginx/blob/8921999083def7ba43a06fabd5f80e4406651353/mainline/jessie/Dockerfile#L21-L23