In particular, it was not possible to obtain SSLv2 protocol version.

In TLSv1.3, NewSessionTicket messages arrive after the handshake and
can come at any time.  Therefore we use a callback to save the session
when we know about it.  This approach works for < TLSv1.3 as well.
The callback function is set once per location on merge phase.

Since SSL_get_session() in BoringSSL returns an unresumable session for
TLSv1.3, peer save_session() methods have been updated as well to use a
session supplied within the callback.  To preserve API, the session is
cached in c->ssl->session.  It is preferably accessed in save_session()
methods by ngx_ssl_get_session() and ngx_ssl_get0_session() wrappers.

Previously, listenings sockets were not cloned if the worker_processes
directive was specified after "listen ... reuseport".

This also simplifies upcoming configuration check on the number
of worker connections, as it needs to know the number of listening
sockets before cloning.

The variable keeps the latest SSL protocol version supported by the client.
The variable has the same format as $ssl_protocol.

The version is read from the client_version field of ClientHello.  If the
supported_versions extension is present in the ClientHello, then the version
is set to TLSv1.3.

The module implements random load-balancing algorithm with optional second
choice.  In the latter case, the best of two servers is chosen, accounting
number of connections and server weight.

Example:

upstream u {
    random [two [least_conn]];

    server 127.0.0.1:8080;
    server 127.0.0.1:8081;
    server 127.0.0.1:8082;
    server 127.0.0.1:8083;
}

Previously, only one client packet could be processed in a udp stream session
even though multiple response packets were supported.  Now multiple packets
coming from the same client address and port are delivered to the same stream
session.

If it's required to maintain a single stream of data, nginx should be
configured in a way that all packets from a client are delivered to the same
worker.  On Linux and DragonFly BSD the "reuseport" parameter should be
specified for this.  Other systems do not currently provide appropriate
mechanisms.  For these systems a single stream of udp packets is only
guaranteed in single-worker configurations.

The proxy_response directive now specifies how many packets are expected in
response to a single client packet.

Cast to intermediate "void *" to lose compiler knowledge about the original
type and pass the warning.  This is not a real fix but rather a workaround.

Found by gcc8.

In mail and stream modules, no certificate provided is a fatal condition,
much like with the "ssl" and "starttls" directives.

In http, "listen ... ssl" can be used in a non-default server without
certificates as long as there is a certificate in the default one, so
missing certificate is only fatal for default servers.

Now it's clear from log error message if the error occurred on client or
upstream side.