The gRPC module context has connection specific state, which can be lost
after request reinitialization when it comes to processing early hints.

The fix is to do only a portion of u->reinit_request() implementation
required after processing early hints, now inlined in modules.

Now NGX_HTTP_UPSTREAM_EARLY_HINTS is returned from u->process_header()
for early hints.  When reading a cached response, this code is mapped
to NGX_HTTP_UPSTREAM_INVALID_HEADER to indicate invalid header format.

There were a few random places where 0 was being used as a null pointer
constant.

We have a NULL macro for this very purpose, use it.

There is also some interest in actually deprecating the use of 0 as a
null pointer constant in C.

This was found with -Wzero-as-null-pointer-constant which was enabled
for C in GCC 15 (not enabled with Wall or Wextra... yet).

Link: <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117059>

The functions ngx_http_merge_types() & ngx_conf_merge_path_value()
return either NGX_CONF_OK aka NULL aka ((void *)0) (probably) or
NGX_CONF_ERROR aka ((void *)-1).

They don't return an integer constant which is what NGX_OK aka (0) is.

Lets use the right thing in the function return check.

This was found with -Wzero-as-null-pointer-constant which was enabled
for C in GCC 15 (not enabled with Wall or Wextra... yet).

Link: <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=117059>

The change implements processing upstream early hints response in
ngx_http_proxy_module and ngx_http_grpc_module.  A new directive
"early_hints" enables sending early hints to the client.  By default,
sending early hints is disabled.

Example:

    map $http_sec_fetch_mode $early_hints {
        navigate $http2$http3;
    }

    early_hints $early_hints;

    proxy_pass http://example.com;

Passwords were not preserved in optimized SSL contexts, the bug had
appeared in d791b4aab (1.23.1), as in the following configuration:

    server {
        proxy_ssl_password_file password;
        proxy_ssl_certificate $ssl_server_name.crt;
        proxy_ssl_certificate_key $ssl_server_name.key;

        location /original/ {
            proxy_pass https://u1/;
        }

        location /optimized/ {
            proxy_pass https://u2/;
        }
    }

The fix is to always preserve passwords, by copying to the configuration
pool, if dynamic certificates are used.  This is done as part of merging
"ssl_passwords" configuration.

To minimize the number of copies, a preserved version is then used for
inheritance.  A notable exception is inheritance of preserved empty
passwords to the context with statically configured certificates:

    server {
        proxy_ssl_certificate $ssl_server_name.crt;
        proxy_ssl_certificate_key $ssl_server_name.key;

        location / {
            proxy_pass ...;

            proxy_ssl_certificate example.com.crt;
            proxy_ssl_certificate_key example.com.key;
        }
    }

In this case, an unmodified version (NULL) of empty passwords is set,
to allow reading them from the password prompt on nginx startup.

As an additional optimization, a preserved instance of inherited
configured passwords is set to the previous level, to inherit it
to other contexts:

    server {
        proxy_ssl_password_file password;

        location /1/ {
            proxy_pass https://u1/;
            proxy_ssl_certificate $ssl_server_name.crt;
            proxy_ssl_certificate_key $ssl_server_name.key;
        }

        location /2/ {
            proxy_pass https://u2/;
            proxy_ssl_certificate $ssl_server_name.crt;
            proxy_ssl_certificate_key $ssl_server_name.key;
        }
    }

It was possible to write outside of the buffer used to keep UTF-8
decoded values when parsing conversion table configuration.

Since this happened before UTF-8 decoding, the fix is to check in
advance if character codes are of more than 3-byte sequence.  Note
that this is already enforced by a later check for ngx_utf8_decode()
decoded values for 0xffff, which corresponds to the maximum value
encoded as a valid 3-byte sequence, so the fix does not affect the
valid values.

Found with AddressSanitizer.
Fixes GitHub issue #529.

As uncovered by recent addition in slice.t, a partially initialized
context, coupled with HTTP 206 response from stub backend, might be
accessed in the next slice subrequest.

Found by bad memory allocator simulation.

Caching is enabled with proxy_ssl_certificate_cache and friends.

Co-authored-by: Aleksei Bavshin <a.bavshin@nginx.com>

A new directive "ssl_certificate_cache max=N [valid=time] [inactive=time]"
enables caching of SSL certificate chain and secret key objects specified
by "ssl_certificate" and "ssl_certificate_key" directives with variables.

Co-authored-by: Aleksei Bavshin <a.bavshin@nginx.com>