Previously, connections returned from keepalive cache had c->data
pointing to the keepalive cache item.  While this shouldn't be a problem
for correct code, as c->data is not expected to be used before it is set,
explicitly clearing it might help to avoid confusion.

Previously only an rbtree was associated with a limit_conn.  To make it
possible to associate more data with a limit_conn, shared context is introduced
similar to limit_req.  Also, shared pool pointer is kept in a way similar to
limit_req.

The variable takes one of the values: PASSED, REJECTED or REJECTED_DRY_RUN.

A new directive limit_conn_dry_run allows enabling the dry run mode.  In this
mode connections are not rejected, but reject status is logged as usual.

The variable takes one of the values: PASSED, DELAYED, REJECTED,
DELAYED_DRY_RUN or REJECTED_DRY_RUN.

New variables $proxy_protocol_server_addr and $proxy_protocol_server_port are
added both to HTTP and Stream.

Now a new structure ngx_proxy_protocol_t holds these fields.  This allows
to add more PROXY protocol fields in the future without modifying the
connection structure.

With MinGW-w64, building 64-bit nginx binary with GCC 8 and above
results in warning due to cast of GetProcAddress() result to ngx_wsapoll_pt,
which GCC thinks is incorrect.  Added intermediate cast to "void *" to
silence the warning.

FormatMessage() seems to return many errors which essentially indicate that
the language in question is not available.  At least the following were
observed in the wild and during testing: ERROR_MUI_FILE_NOT_FOUND (15100)
(ticket #1868), ERROR_RESOURCE_TYPE_NOT_FOUND (1813).  While documentation
says it should be ERROR_RESOURCE_LANG_NOT_FOUND (1815), this doesn't seem
to be the case.

As such, checking error code was removed, and as long as FormatMessage()
returns an error, we now always try the default language.

Added code to track number of bytes available in the socket.
This makes it possible to avoid looping for a long time while
working with fast enough peer when data are added to the socket buffer
faster than we are able to read and process data.

When kernel does not provide number of bytes available, it is
retrieved using ioctl(FIONREAD) as long as a buffer is filled by
SSL_read().

It is assumed that number of bytes returned by SSL_read() is close
to the number of bytes read from the socket, as we do not use
SSL compression.  But even if it is not true for some reason, this
is not important, as we post an additional reading event anyway.

Note that data can be buffered at SSL layer, and it is not possible
to simply stop reading at some point and wait till the event will
be reported by the kernel again.  This can be only done when there
are no data in SSL buffers, and there is no good way to find out if
it's the case.

Instead of trying to figure out if SSL buffers are empty, this patch
introduces events posted for the next event loop iteration - such
events will be processed only on the next event loop iteration,
after going into the kernel and retrieving additional events.  This
seems to be simple and reliable approach.

This makes it possible to avoid looping for a long time while working
with a fast enough peer when data are added to the socket buffer faster
than we are able to read and process them (ticket #1431).  This is
basically what we already do on FreeBSD with kqueue, where information
about the number of bytes in the socket buffer is returned by
the kevent() call.

With other event methods rev->available is now set to -1 when the socket
is ready for reading.  Later in ngx_recv() and ngx_recv_chain(), if
full buffer is received, real number of bytes in the socket buffer is
retrieved using ioctl(FIONREAD).  Reading more than this number of bytes
ensures that even with edge-triggered event methods the event will be
triggered again, so it is safe to stop processing of the socket and
switch to other connections.

Using ioctl(FIONREAD) only after reading a full buffer is an optimization.
With this approach we only call ioctl(FIONREAD) when there are at least
two recv()/readv() calls.

As long as there are data to read in the socket, yet the amount of data
is less than total size of the buffers in the chain, this saves one
unneeded read() syscall.  Before this change, reading only stopped if
ngx_ssl_recv() returned no data, that is, two read() syscalls in a row
returned EAGAIN.

In SSL connections, data can be buffered by the SSL layer, and it is
wrong to avoid doing c->recv_chain() if c->read->available is 0 and
c->read->pending_eof is set.  And tests show that the optimization in
question indeed can result in incorrect detection of premature connection
close if upstream closes the connection without sending a close notify
alert at the same time.  Fix is to disable c->read->available optimization
for SSL connections.

The parsing was broken when the first character of the header name was invalid.

Based on a patch by Alan Kemp.

Previously, "/foo///../bar" was normalized into "/foo/bar"
instead of "/foo//bar".

The result of ngx_rbtree_min() is always a node with the left child equal to
sentinel, thus the check is unnecessary.

This could happen when graceful shutdown configured by worker_shutdown_timeout
times out and is then followed by another timeout such as proxy_read_timeout.
In this case, the HEADERS frame is added to the output queue, but attempt to
send it fails (due to c->error forcibly set during graceful shutdown timeout).
This triggers request finalization which attempts to close the stream.  But the
stream cannot be closed because there is a frame in the output queue, and the
connection cannot be finalized.  This leaves the connection open without any
timer events leading to alert.

The fix is to post write event when sending output queue fails on c->error.
That will finalize the connection.

With this patch, all traffic over an HTTP/2 connection is counted in
the h2c->total_bytes field, and payload traffic is counted in
the h2c->payload_bytes field.  As long as total traffic is many times
larger than payload traffic, we consider this to be a flood.

In 8df664ebe037, we've switched to maximizing stream window instead
of sending RST_STREAM.  Since then handling of RST_STREAM with NO_ERROR
was fixed at least in Chrome, hence we switch back to using RST_STREAM.

This allows more effective rejecting of large bodies, and also minimizes
non-payload traffic to be accounted in the next patch.

Previously, if a response to the PTR request was cached, and ngx_resolver_dup()
failed to allocate memory for the resulting name, then the original node was
freed but left in expire_queue.  A subsequent address resolving would end up
in a use-after-free memory access of the node either in ngx_resolver_expire()
or ngx_resolver_process_ptr(), when accessing it through expire_queue.

The fix is to leave the resolver node intact.

Don't waste server resources by sending RST_STREAM frames.  Instead,
reject WINDOW_UPDATE frames with invalid zero increment by closing
connection with PROTOCOL_ERROR.

Don't waste server resources by sending RST_STREAM frames.  Instead,
reject HEADERS and PRIORITY frames with self-dependency by closing
connection with PROTOCOL_ERROR.

When ngx_http_discard_request_body() call was added to ngx_http_send_response(),
there were no return codes other than NGX_OK and NGX_HTTP_INTERNAL_SERVER_ERROR.
Now it can also return NGX_HTTP_BAD_REQUEST, but ngx_http_send_response() still
incorrectly transforms it to NGX_HTTP_INTERNAL_SERVER_ERROR.

The fix is to propagate ngx_http_discard_request_body() errors.

As defined in HTTP/1.1, body chunks have the following ABNF:

   chunk = chunk-size [ chunk-ext ] CRLF chunk-data CRLF

where chunk-data is a sequence of chunk-size octets.

With this change, chunk-data that doesn't end up with CRLF at chunk-size
offset will be treated as invalid, such as in the example provided below:

4
SEE-THIS-AND-
4
THAT
0

Previously, if unbuffered request body reading wasn't finished before
the request was redirected to a different location using error_page
or X-Accel-Redirect, and the request body is read again, this could
lead to disastrous effects, such as a duplicate post_handler call or
"http request count is zero" alert followed by a segmentation fault.

This happened in the following configuration (ticket #1819):

    location / {
        proxy_request_buffering off;
        proxy_pass http://bad;
        proxy_intercept_errors on;
        error_page 502 = /error;
    }

    location /error {
        proxy_pass http://backend;
    }