Found by Coverity (CID 1662016).

The ssl_certificate_compression directive allows to send compressed
server certificates.  In OpenSSL, they are pre-compressed on startup.
To simplify configuration, the SSL_OP_NO_TX_CERTIFICATE_COMPRESSION
option is automatically cleared if certificates were pre-compressed.

SSL_CTX_compress_certs() may return an error in legitimate cases,
e.g., when none of compression algorithms is available or if the
resulting compressed size is larger than the original one, thus it
is silently ignored.

Certificate compression is supported in Chrome with brotli only,
in Safari with zlib only, and in Firefox with all listed algorithms.
It is supported since Ubuntu 24.10, which has OpenSSL with enabled
zlib and zstd support.

The actual list of algorithms supported in OpenSSL depends on how
the library was configured; it can be brotli, zlib, zstd as listed
in RFC 8879.

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>