The support first appeared in OS X Mavericks 10.9 and documented since
OS X Yosemite 10.10.

It has a subtle implementation difference from other operating systems
in that the TCP_KEEPALIVE socket option (used in place of TCP_KEEPIDLE)
isn't inherited from a listening socket to an accepted socket.

An apparent reason for this behaviour is that it might be preserved for
the sake of backward compatibility.  The TCP_KEEPALIVE socket option is
not inherited since appearance in OS X Panther 10.3, which long predates
two other TCP_KEEPINTVL and TCP_KEEPCNT socket options.

Thanks to Andy Pan for initial work.

Linux with EPOLLEXCLUSIVE usually notifies only the process which was first
to add the listening socket to the epoll instance.  As a result most of the
connections are handled by the first worker process (ticket #2285).  To fix
this, we re-add the socket periodically, so other workers will get a chance
to accept connections.

Similar to lingering_time, it limits total connection lifetime before
keepalive is switched off.  The default is 1 hour, which is close to
the total maximum connection lifetime possible with default
keepalive_requests and keepalive_timeout.

Previously, ngx_event_recvmsg() got remote socket addresses after creating
the connection object.  In preparation to handling multiple UDP packets in a
single session, this code was moved up.

On Linux recvmsg() syscall may return a zero-length client address when
receiving a datagram from an unbound unix datagram socket.  It is usually
assumed that socket address has at least the sa_family member.  Zero-length
socket address caused buffer over-read in functions which receive socket
address, for example ngx_sock_ntop().  Typically the over-read resulted in
unexpected socket family followed by session close.  Now a fake socket address
is allocated instead of a zero-length client address.

Some OSes (notably macOS, NetBSD, and Solaris) allow unix socket addresses
larger than struct sockaddr_un.  Moreover, some of them (macOS, Solaris)
return socklen of the socket address before it was truncated to fit the
buffer provided.  As such, on these systems socklen must not be used without
additional check that it is within the buffer provided.

Appropriate checks added to ngx_event_accept() (after accept()),
ngx_event_recvmsg() (after recvmsg()), and ngx_set_inherited_sockets()
(after getsockname()).

We also obtain socket addresses via getsockname() in
ngx_connection_local_sockaddr(), but it does not need any checks as
it is only used for INET and INET6 sockets (as there can be no
wildcard unix sockets).

Previously, the read event of the accepted connection was marked ready, but not
available.  This made EPOLLRDHUP-related code (for example, in ngx_unix_recv())
expect more data from the socket, leading to unexpected behavior.

For example, if SSL, PROXY protocol and deferred accept were enabled on a listen
socket, the client connection was aborted due to unexpected return value of
c->recv().

It's properly aligned and can hold any supported sockaddr.