How to fork properly within an nginx/php-fpm environment?

Question

I have a very simple php socket server that runs on my machine. I created a convinience class with simple methods like "restart" and "stop" and etc. to control the server once it is already running.

What the restart function does is it sends the server the command to stop and then it forks the process and starts a new instance of the socket server within the child process while the parent process returns to the caller.

This works great on the command line, however I am trying to make an admin webpage which restarts the socket server and the forking is causing problems in php-fpm. Basically, what it appears is happening is that the life of the "page loading" is not ending when the parent process ends and nginx/php-fpm are not reassigning the process to new connections until the forked process also ends.

In my case, I do not want the forked process to ever end, and I end up with a completely debilitated server. (in my test environment, for simplicity i have the worker pool set to only 1, in a production environment this will be higher, but this issue would lead to one worker slot being permanently occupied).

I have tried a few things including calling fastcgi_finish_request() just prior to the forking, however this had no affect.

How can I restart my service from a php-fpm worker process without locking up an assignment in the nginx connection pool?

Answer 1

The solution was simple and elementary.

My child processes were not redirecting STDOUT and STDERR to /dev/null so therefore even though the parent process finished, the fact that the child process still had active file descriptors was causing php-fpm to consider that connection in its pool as still active, and therefore it would never be re-assigned to new connections on the basis that the child process would run continually.

Redirecting STDERR and STDOUT to /dev/null caused php-fpm to correctly reassign connections while simultaneously allowing the child process (the socket server) to run forever. Case closed.

./some/command >/dev/null 2>&1

Should have seen this one a mile off...

(I solved this problem months ago, but haven't signed into this account in a long time ... it didn't take me 7 months to figure out I need to direct output to /dev/null!)

Answer 2

Sounds like you have your protocol design wrong. The server should be capabele of restarting itself. There's quite a few examples of how to do that in the internet. The client (your webpage) should not need to do any forking. The server should also not run inside php-fpm, but be a console application that uses a daemon(3) type interface to detach itself from the console.