Once the USR2 signal is received the master process forks, the child process inherits the parents file descriptors including listen().
One process stops accepting connections creating a queue in the kernel. The new process takes over and starts accepting connections.
You can follow the trail by searching for ngx_exec_new_binary in the nginx repo.
Correct but to clarify, only the master process binds to the ports. The master process creates socketpairs to the workers for interprocess communication. The workers accept connections over the shared socket.
There’s an ioctl for this on FreeBSD and Linux — SO_REUSEPORT. You could also just leave the listening socket open when exec’ing the new httpd, or send it with a unix domain socket.
Is there any restrictions on this option? Eg only children of the same parent process are allowed to bind to same port. Otherwise how does the packet distribution work? And how does the response from that port work?
>So long as the first server sets this option before binding its socket, then any number of other servers can also bind to the same port if they also set the option beforehand. [...] To prevent unwanted processes from hijacking a port that has already been bound by a server using SO_REUSEPORT, all of the servers that later bind to that port must have an effective user ID that matches the effective user ID used to perform the first bind on the socket.
