Sticky sessions allow to route the same client to the same upstream server.

- upstream structures are extended to keep session-related information

- existing balancing modules are updated to provide an id of the selected
  server (SID) in pc->sid, and to select the server, given it's SID.

- other balancing modules are allowed to set the pc->hint value to choose
  the desired peer.  The sticky module will not change the hint if it's
  already set.

- the feature is enabled by default and can be disabled with the
  "--without-http_upstream_sticky" switch of the configure script.

The following configuration can be used to enable sticky sessions for
supported balancing modules:

    upstream u1 {
        server 127.0.0.1:8080;
        server 127.0.0.1:8081;

        sticky cookie server_id expires=1h domain=.example.com path=/;
    }

Co-authored-by: Ruslan Ermilov <ru@nginx.com>
Co-authored-by: Roman Arutyunyan <arut@nginx.com>
Co-authored-by: Maxim Dounin <mdounin@mdounin.ru>

Previously ngx_quic_compat_create_record() could try to encrypt a TLS
record even if encryption context was missing, which resulted in a NULL
pointer dereference.

The context is created by ngx_quic_compat_set_encryption_secret() called
from the OpenSSL keylog callback.  If an error occurred in that function,
the context could remain missing.  This could happen under memory pressure,
if an allocation failed inside this function.

The fix is to handle errors from ngx_quic_compat_set_encryption_secret()
and set qc->error to trigger an error after SSL_do_handshake() return.
Also, a check for context is added to ngx_quic_compat_create_record()
to avoid other similar issues.

Previously, it was possible to obtain a stateless reset token for a
connection by routing its packet to a wrong worker.  This allowed to
terminate the connection.

The fix is to bind stateless reset token to the worker number.

QUIC bpf program previously used struct bpf_map_def which was
deprecated in [1] (kernel version 5.18) and removed in [2] (kernel 6.1).
New-style BTF map definitions were added in [3] (linux kernel 5.3).

Switching the program to BTF is however not necessary since nginx has
its own relocation procedure which allows referencing the real map
structure by its file descriptor allocated earlier.  In particular,
bpf instruction BPF_LD_IMM64 (0x18/0x0) is substituted with instruction
BPF_LD_MAP_FD (0x18/0x1) and map_fd is stored in the imm field, see [4]
and [5] for details.

To fix compilation, struct bpf_map_def is changed to a known type (int)
and "extern" is added to indicate external linkage and reduce object
file size.

[1] https://github.com/torvalds/linux/commit/93b8952d223af03c51fba0c6258173d2ffbd2cb7
[2] https://github.com/torvalds/linux/commit/dc567045f1590f6460d3e9a6ea6ad5e600b58b84
[3] https://github.com/torvalds/linux/commit/abd29c9314595b1ee5ec6c61d7c49a497ffb30a3
[4] https://github.com/torvalds/linux/blob/master/include/linux/filter.h
[5] https://datatracker.ietf.org/doc/rfc9669/

It uses a bloom filter to limit sending Stateless Reset packets no more
than once per second in average for the given address.  This allows to
address resource asymmetry from precomputed packets, as well as to limit
potential Stateless Reset exchange.

Now it accepts an optional salt, to be used in a subsequent change.

Made sure to send packets smaller than the triggering packet,
following RFC 9000, Section 10.3.3.

Reported-by: cyberspace61

Now to be valid, it also assumes the Connection ID we require from a client.

The alert is send by a client after its ECH configuration was rejected by
a server.