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>

Previously, request might be left in inconsistent state in case of error,
which manifested in "http request count is zero" alerts when used by SSI
filter.

The fix is to reshuffle initialization order to postpone committing state
changes until after any potentially failing parts.

Found by bad memory allocator simulation.

The directive sets a timeout during which a keepalive connection will
not be closed by nginx for connection reuse or graceful shutdown.

The change allows clients that send multiple requests over the same
connection without delay or with a small delay between them, to avoid
receiving a TCP RST in response to one of them.  This excludes network
issues and non-graceful shutdown.  As a side-effect, it also addresses
the TCP reset problem described in RFC 9112, Section 9.6, when the last
sent HTTP response could be damaged by a followup TCP RST.  It is important
for non-idempotent requests, which cannot be retried by client.

It is not recommended to set keepalive_min_timeout to large values as
this can introduce an additional delay during graceful shutdown and may
restrict nginx from effective connection reuse.

Now "fastopen", "backlog", "accept_filter", "deferred", and "so_keepalive"
parameters are not allowed with "quic" in the "listen" directive.

Reported by Izorkin.

The directive enables HTTP/2 in the current server.  The previous way to
enable HTTP/2 via "listen ... http2" is now deprecated.  The new approach
allows to share HTTP/2 and HTTP/0.9-1.1 on the same port.

For SSL connections, HTTP/2 is now selected by ALPN callback based on whether
the protocol is enabled in the virtual server chosen by SNI.  This however only
works since OpenSSL 1.0.2h, where ALPN callback is invoked after SNI callback.
For older versions of OpenSSL, HTTP/2 is enabled based on the default virtual
server configuration.

For plain TCP connections, HTTP/2 is now auto-detected by HTTP/2 preface, if
HTTP/2 is enabled in the default virtual server.  If preface is not matched,
HTTP/0.9-1.1 is assumed.

The parameter has been deprecated since c851a2ed5ce8.

Since 4611:2b6cb7528409 responses from the gzip static, flv, and mp4 modules
can be used with subrequests, though empty files were not properly handled.
Empty gzipped, flv, and mp4 files thus resulted in "zero size buf in output"
alerts.  While valid corresponding files are not expected to be empty, such
files shouldn't result in alerts.

Fix is to set b->sync on such empty subrequest responses, similarly to what
ngx_http_send_special() does.

Additionally, the static module, the ngx_http_send_response() function, and
file cache are modified to do the same instead of not sending the response
body at all in such cases, since not sending the response body at all is
believed to be at least questionable, and might break various filters
which do not expect such behaviour.

Now "ssl", "proxy_protocol" and "http2" are not allowed with "quic" in "listen"
directive.  Previously, only "ssl" was not allowed.

Now "listen" directve has a new "quic" parameter which enables QUIC protocol
for the address.  Further, to enable HTTP/3, a new directive "http3" is
introduced.  The hq-interop protocol is enabled by "http3_hq" as before.
Now application protocol is chosen by ALPN.

Previously used "http3" parameter of "listen" is deprecated.