2014/04/16 - Pointer assignments during processing of the HTTP body

In HAProxy, a struct http_msg is a descriptor for an HTTP message, which stores
the state of an HTTP parser at any given instant, relative to a buffer which
contains part of the message being inspected.

Currently, an http_msg holds a few pointers and offsets to some important
locations in a message depending on the state the parser is in. Some of these
pointers and offsets may move when data are inserted into or removed from the
buffer, others won't move.

An important point is that the state of the parser only translates what the
parser is reading, and not at all what is being done on the message (eg:
forwarding).

For an HTTP message <msg> and a buffer <buf>, we have the following elements
to work with :


Buffer :
--------

buf.size : the allocated size of the buffer. A message cannot be larger than
           this size. In general, a message will even be smaller because the
           size is almost always reduced by global.maxrewrite bytes.

buf.data : memory area containing the part of the message being worked on. This
           area is exactly <buf.size> bytes long. It should be seen as a sliding
           window over the message, but in terms of implementation, it's closer
           to a wrapping window. For ease of processing, new messages (requests
           or responses) are aligned to the beginning of the buffer so that they
           never wrap and common string processing functions can be used.

buf.p    : memory pointer (char *) to the beginning of the buffer as the parser
           understands it. It commonly refers to the first character of an HTTP
           request or response, but during forwarding, it can point to other
           locations. This pointer always points to a location in <buf.data>.

buf.i    : number of bytes after <buf.p> that are available in the buffer. If
           <buf.p + buf.i> exceeds <buf.data + buf.size>, then the pending data
           wrap at the end of the buffer and continue at <buf.data>.

buf.o    : number of bytes already processed before <buf.p> that are pending
           for departure. These bytes may leave at any instant once a connection
           is established. These ones may wrap before <buf.data> to start before
           <buf.data + buf.size>.

It's common to call the part between buf.p and buf.p+buf.i the input buffer, and
the part between buf.p-buf.o and buf.p the output buffer. This design permits
efficient forwarding without copies. As a result, forwarding one byte from the
input buffer to the output buffer only consists in :
        - incrementing buf.p
        - incrementing buf.o
        - decrementing buf.i


Message :
---------
Unless stated otherwise, all values are relative to <buf.p>, and are always
comprised between 0 and <buf.i>. These values are relative offsets and they do
not need to take wrapping into account, they are used as if the buffer was an
infinite length sliding window. The buffer management functions handle the
wrapping automatically.

msg.next : points to the next byte to inspect. This offset is automatically
           adjusted when inserting/removing some headers. In data states, it is
           automatically adjusted to the number of bytes already inspected.

msg.sov  : start of value. First character of the header's value in the header
           states, start of the body in the data states. Strictly positive
           values indicate that headers were not forwarded yet (<buf.p> is
           before the start of the body), and null or negative values are seen
           after headers are forwarded (<buf.p> is at or past the start of the
           body). The value stops changing when data start to leave the buffer
           (in order to avoid integer overflows). So the maximum possible range
           is -<buf.size> to +<buf.size>. This offset is automatically adjusted
           when inserting or removing some headers. It is useful to rewind the
           request buffer to the beginning of the body at any phase. The
           response buffer does not really use it since it is immediately
           forwarded to the client.

msg.sol  : start of line. Points to the beginning of the current header line
           while parsing headers. It is cleared to zero in the BODY state,
           and contains exactly the number of bytes comprising the preceeding
           chunk size in the DATA state (which can be zero), so that the sum of
           msg.sov + msg.sol always points to the beginning of data for all
           states starting with DATA. For chunked encoded messages, this sum
           always corresponds to the beginning of the current chunk of data as
           it appears in the buffer, or to be more precise, it corresponds to
           the first of the remaining bytes of chunked data to be inspected. In
           TRAILERS state, it contains the length of the last parsed part of
           the trailer headers.

msg.eoh  : end of headers. Points to the CRLF (or LF) preceeding the body and
           marking the end of headers. It is where new headers are appended.
           This offset is automatically adjusted when inserting/removing some
           headers. It always contains the size of the headers excluding the
           trailing CRLF even after headers have been forwarded.

msg.eol  : end of line. Points to the CRLF or LF of the current header line
           being inspected during the various header states. In data states, it
           holds the trailing CRLF length (1 or 2) so that  msg.eoh + msg.eol
           always equals the exact header length. It is not affected during data
           states nor by forwarding.

The beginning of the message headers can always be found this way even after
headers or data have been forwarded, provided that everything is still present
in the buffer :

            headers = buf.p + msg->sov - msg->eoh - msg->eol


Message length :
----------------
msg.chunk_len : amount of bytes of the current chunk or total message body
                remaining to be inspected after msg.next. It is automatically
                incremented when parsing a chunk size, and decremented as data
                are forwarded.

msg.body_len  : total message body length, for logging. Equals Content-Length
                when used, otherwise is the sum of all correctly parsed chunks.


Message state :
---------------
msg.msg_state contains the current parser state, one of HTTP_MSG_*. The state
indicates what byte is expected at msg->next.

HTTP_MSG_BODY       : all headers have been parsed, parsing of body has not
                      started yet.

HTTP_MSG_100_SENT   : parsing of body has started. If a 100-Continue was needed
                      it has already been sent.

HTTP_MSG_DATA       : some bytes are remaining for either the whole body when
                      the message size is determined by Content-Length, or for
                      the current chunk in chunked-encoded mode.

HTTP_MSG_CHUNK_CRLF : msg->next points to the CRLF after the current data chunk.

HTTP_MSG_TRAILERS   : msg->next points to the beginning of a possibly empty
                      trailer line after the final empty chunk.

HTTP_MSG_DONE       : all the Content-Length data has been inspected, or the
                      final CRLF after trailers has been met.


Message forwarding :
--------------------
Forwarding part of a message consists in advancing buf.p up to the point where
it points to the byte following the last one to be forwarded. This can be done
inline if enough bytes are present in the buffer, or in multiple steps if more
buffers need to be forwarded (possibly including splicing). Thus by definition,
after a block has been scheduled for being forwarded, msg->next and msg->sov
must be reset.

The communication channel between the producer and the consumer holds a counter
of extra bytes remaining to be forwarded directly without consulting analysers,
after buf.p. This counter is called to_forward. It commonly holds the advertised
chunk length or content-length that does not fit in the buffer. For example, if
2000 bytes are to be forwarded, and 10 bytes are present after buf.p as reported
by buf.i, then both buf.o and buf.p will advance by 10, buf.i will be reset, and
to_forward will be set to 1990 so that in total, 2000 bytes will be forwarded.
At the end of the forwarding, buf.p will point to the first byte to be inspected
after the 2000 forwarded bytes.