socket: Use a default listen backlog of -1 on Linux

[ac000]

On FreeBSD, OpenBSD & macOS we use a default listen(2) backlog of -1 which means use the OS's default value.

On Linux (and others) we used a hard coded value of 511, presumably due to this comment

  /* Linux, Solaris, and NetBSD treat negative value as 0. */

On Linux (at least since 2.4), this is wrong, Linux treats -1 (and so on) as use the OS's default (net.core.somaxconn). See this code in net/socket.c::__sys_listen()

                if ((unsigned int)backlog > somaxconn)
                        backlog = somaxconn;

On Linux prior to 5.4 somaxconn defaulted to 128, since 5.4 it defaults to 4096.

We've had complaints that a listen backlog of 511 is too small. This would help in those cases.

Unless they are on an old Kernel, in which case it's worse, but then the plan is to also make this configurable. This would effect RHEL 8, which is based on 4.10, however they seem to set somaxconn to 2048, so that's fine.

Another advantage of using -1 is that we will automatically keep up to date with the kernels default value.

Before this change

  $ ss -tunxlp | grep unit
  Netid State  Recv-Q Send-Q                                                  Local Address:Port     Peer Address:Port           Process
  u_str LISTEN 0      511                                   /opt/unit/control.unit.sock.tmp 4302333            * 0    users:(("unitd",pid=18290,fd=6),("unitd",pid=18289,fd=6),("unitd",pid=18287,fd=6))
  tcp   LISTEN 0      511                                                         127.0.0.1:8080         0.0.0.0:*    users:(("unitd",pid=18290,fd=12))
  tcp   LISTEN 0      511                                                             [::1]:8080            [::]:*    users:(("unitd",pid=18290,fd=11))

After

  $ ss -tunxlp | grep unit
  Netid State  Recv-Q Send-Q                                                  Local Address:Port     Peer Address:Port           Process
  u_str LISTEN 0      4096                                  /opt/unit/control.unit.sock.tmp 5408464            * 0    users:(("unitd",pid=132442,fd=6),("unitd",pid=132441,fd=6),("unitd",pid=132439,fd=6))
  tcp   LISTEN 0      4096                                                        127.0.0.1:8080         0.0.0.0:*    users:(("unitd",pid=132442,fd=12))
  tcp   LISTEN 0      4096                                                            [::1]:8080            [::]:*    users:(("unitd",pid=132442,fd=11))

[ac000]

Add link to related GH issue...

$ git range-diff 46334aa2...5819e3ea
1:  46334aa2 ! 1:  5819e3ea socket: Use a default listen backlog of -1 on Linux
    @@ Commit message
           tcp   LISTEN 0      4096                                                        127.0.0.1:8080         0.0.0.0:*    users:(("unitd",pid=132442,fd=12))
           tcp   LISTEN 0      4096                                                            [::1]:8080            [::]:*    users:(("unitd",pid=132442,fd=11))
     
    +    Link: <https://github.com/nginx/unit/issues/1384>
         Link: <https://lore.kernel.org/netdev/[email protected]/>
         Signed-off-by: Andrew Clayton <[email protected]>
     

[ac000]

Rebased with master