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                                   HTX API
                                  Version 1.0
                          ( Last update: 2019-06-20 )
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                          Author : Christopher Faulet
                      Contact : cfaulet at haproxy dot com

1. Background

Historically, HAProxy stored HTTP messages in a raw fashion in buffers, keeping
parsing information separately in a "struct http_msg" owned by the stream. It was
optimized to the data transfer, but not so much for rewrites. It was also HTTP/1
centered. While it was the only HTTP version supported, it was not a
problem. But with the rise of HTTP/2, it starts to be hard to still use this
representation.

At the first age of the HTTP/2 in HAProxy, H2 messages were converted into
H1. This was terribly unefficient because it required two parsing passes, a
first one in H2 and a second one in H1, with a conversion in the middle. And of
course, the same was also true in the opposite direction. outgoing H1 messages
had to be converted back in H2 to be sent. Even worse, because the H2->H1
conversion, only client H2 connections were supported.

So, to address all these problems, we decided to replace the old raw
representation by a version-agnostic and self-structured internal HTTP
representation, the HTX. As an additional benefit, with this new representation,
the message parsing and its processing are now separated, making all the HTTP
analysis simpler and cleaner. The parsing of HTTP messages is now handled by
the multiplexers (h1 or h2).


2. The HTX message

The HTX is a structure containing useful information about an HTTP message
followed by a contiguous array with some parts of the message. These parts are
called blocks. A block is composed of metadata (htx_blk) and an associated
payload. Blocks' metadata are stored starting from the end of the array while
their payload are stored at the beginning. Blocks' metadata are often simply
called blocks. it is a misuse of language that's simplify explanations.

Internally, this structure is "hidden" in a buffer. This way, there are few
changes into intermediate layers (stream-interface and channels). They still
manipulate buffers. Only the multiplexer and the stream have to know how data
are really stored. From the HTX perspective, a buffer is just a memory
area. When an HTX message is stored in a buffer, this one appears as full.

  * General view of an HTX message :


  buffer->area
    |
    |<------------ buffer->size == buffer->data ----------------------|
    |                                                                 |
    |     |<------------- Blocks array (htx->size) ------------------>|
    V     |                                                           |
    +-----+-----------------+-------------------------+---------------+
    | HTX |   PAYLOADS ==>  |                         |  <== HTX_BLKs |
    +-----+-----------------+-------------------------+---------------+
          |                 |                         |               |
          |<-payloads part->|<----- free space ------>|<-blocks part->|
              (htx->data)


The blocks part remains linear and sorted. You may think about it as an array
with negative indexes. But, instead of using negative indexes, we use positive
positions to identify a block. This position is then converted to an address
relatively to the beginning of the blocks array.

                tail                                 head
                 |                                     |
                 V                                     V
     .....--+----+-----------------------+------+------+
            | Bn |       ...             |  B1  |  B0  |
     .....--+----+-----------------------+------+------+
                 ^                       ^      ^
 Addr of the block       Addr of the block      Addr of the block
 at the position N       at the position 1      at the position 0


In the HTX structure, 3 "special" positions are stored:

    - tail  : Position of the newest inserted block
    - head  : Position of the oldest inserted block
    - first : Position of the first block to (re)start the analyse

The blocks part never wrap. If we have no space to allocate a new block and if
there is a hole at the beginning of the blocks part (so at the end of the blocks
array), we move back all blocks.


      tail           head                                tail           head
       |              |                                   |              |
       V              V                                   V              V
    ...+--------------+---------+    blocks  ...----------+--------------+
       | X== HTX_BLKS |         |    defrag               | <== HTX_BLKS |
    ...+--------------+---------+    =====>  ...----------+--------------+


The payloads part is a raw space that may wrap. You never access to a block's
payload directly. Instead you get a block to retrieve the address of its
payload.


          +------------------------( B0.addr )--------------------------+
          |    +-------------------( B1.addr )----------------------+   |
          |    |       +-----------( B2.addr )----------------+     |   |
          V    V       V                                      |     |   |
    +-----+----+-------+----+--------+-------------+-------+----+----+----+
    | HTX | P0 |   P1  | P2 | ...==> |             | <=... | B2 | B1 | B0 |
    +-----+----+-------+----+--------+-------------+-------+----+----+----+


Because the payloads part may wrap, there are 2 usable free spaces:

    - The free space in front of the blocks part. This one is used if and only if
      the other one was not used yet.

    - The free space at the beginning of the message. Once this one is used, the
      other one is never used again, until a message defragmentation.


  * Linear payloads part :


      head_addr             end_addr     tail_addr
          |                    |             |
          V                    V             V
    +-----+--------------------+-------------+--------------------+-------...
    | HTX |                    |   PAYLOADS  |                    | HTX_BLKs
    +-----+--------------------+-------------+--------------------+-------...
          |<-- free space 2 -->|             |<-- free space 1 -->|
     (used if the other is too small)          (used in priority)


  * Wrapping payloads part :


                            head_addr end_addr        tail_addr
                                |        |                |
                                V        V                V
    +-----+----+----------------+--------+----------------+-------+-------...
    | HTX |    | PAYLOADS part2 |        | PAYLOADS part1 |       | HTX_BLKs
    +-----+----+----------------+--------+----------------+-------+-------...
          |<-->|                |<------>|                |<----->|
         unusable               free space                 unusable
        free space                                        free space


Finally, when the usable free space is not enough to store a new block, unusable
parts may be get back with a full defragmentation. The payloads part is then
realigned at the beginning of the blocks array and the free space becomes
continuous again.


3. The HTX blocks

An HTX block can be as well a start-line as a header, a body part or a
trailer. For all these types of block, a payload is attached to the block. It
can also be a marker, the end-of-headers, end-of-trailers or end-of-message. For
these blocks, there is no payload but it counts for a byte. It is important to
not skip it when data are forwarded.

As already said, a block is composed of metadata and a payload. Metadata are
stored in the blocks part and are composed of 2 fields :

    - info : It a 32 bits field containing the block's type on 4 bits followed
             by the payload length. See below for details.

    - addr : The payload's address, if any, relatively to the beginning the
             array used to store part of the HTTP message itself.


  * Block's info representation :

    0b 0000 0000 0000 0000 0000 0000 0000 0000
       ---- ------------------------ ---------
       type     value (1 MB max)     name length (header/trailer - 256B max)
            ----------------------------------
                 data length (256 MB max)
     (body, method, path, version, status, reason)


Supported types are :

    - 0000  (0) : The request start-line
    - 0001  (1) : The response start-line
    - 0010  (2) : A header block
    - 0011  (3) : The end-of-headers marker
    - 0100  (4) : A data block
    - 0101  (5) : A trailer block
    - 0110  (6) : The end-of-trailers marker
    - 0111  (7) : The end-of-message marker
    - 1111 (15) : An unused block

Other types are unused for now and reserved for futur extensions.

An HTX message is typically composed of following blocks, in this order :

    - a start-line
    - zero or more header blocks
    - an end-of-headers marker
    - zero or more data blocks
    - zero or more trailer blocks (optional)
    - an end-of-trailers marker (optional but always set if there is at least
      one trailer block)
    - an end-of-message marker.

Only one HTTP request at a time can be stored in an HTX message. For HTTP
response, it is more complicated. Only one "final" response can be stored in an
HTX message. It is a response with status-code 101 or greater or equal to
200. But it may be preceded by several 1xx informational responses. Such
responses are part of the same HTX message, so there is no end-of-message marker
for them.


3.1. The start-line

Every HTX message starts with a start-line. Its payload is a "struct htx_sl". In
addition to the parts of the HTTP start-line, this structure contains some
information about the represented HTTP message, mainly in the form of flags
(HTX_SL_F_*). For instance, if an HTTP message contains the header
"conten-length", then the flag HTX_SL_F_CLEN is set.

Each HTTP message has its own start-line. So an HTX request has one and only one
start-line because it must contain only one HTTP request at a time. But an HTX
response may have more than one start-line if the final HTTP response is
precedeed by some 1xx informational responses.

In HTTP/2, there is no start-line. So the H2 multiplexer must create one when it
converts an H2 message to HTX :

    - For the request, it uses the pseudo headers ":method", ":path" or
      ":authority" depending on the method and the hardcoded version "HTTP/2.0".

    - For the response, it used the hardcoded version "HTTP/2.0", the
      pseudo-header ":status" and an empty reason.


3.2. The headers and trailers

HTX Headers and trailers are quite similar. Different types are used to simplify
headers processing. But from the HTX point of view, there is no real difference,
except their position in the HTX message. The header blocks always follow an HTX
start-line while trailer blocks come after the data. If there is no data, they
follow the end-of-headers marker.

Headers and trailers are the only blocks containing a Key/Value payload. The
corresponding end-of marker must always be placed after each group to mark, as
it name suggests, the end.

In HTTP/1, trailers are only present on chunked messages. But chunked messages
do not always have trailers. In this case, the end-of-trailers block may or may
not be present. Multiplexers must be able to handle both situations. In HTTP/2,
trailers are only present if a HEADERS frame is sent after DATA frames.


3.3. The data

The payload body of an HTTP message is stored as DATA blocks in the HTX
message. For HTTP/1 messages, it is the message body without the chunks
formatting, if any. For HTTP/2, it is the payload of DATA frames.

The DATA blocks are the only HTX blocks that may be partially processed (copied
or removed). All other types of block must be entierly processed. This means
DATA blocks can be resized.


3.4. The end-of markers

These blocks are used to delimit parts of an HTX message. It exists three
markers:

    - end-of-headers (EOH)
    - end-of-trailers (EOT)
    - end-of-message (EOM)

EOH and EOM are always present in an HTX message. EOT is optional.


4. The HTX API


4.1. Get/set HTX message from/to the underlying buffer

The first thing to do to process an HTX message is to get it from the underlying
buffer. There are 2 functions to do so, the second one relying on the first:

    - htxbuf() returns an HTX message from a buffer. It does not modify the
      buffer. It only initialize the HTX message if the buffer is empty.

    - htx_from_buf() uses htxbuf(). But it also updates the underlying buffer so
      that it appears as full.

Both functions return a "zero-sized" HTX message if the buffer is null. This
way, you are sure to always have a valid HTX message. The first function is the
default function to use. The second one is only useful when some content will be
added. For instance, it used by the HTX analyzers when HAproxy generates a
response. This way, the buffer is in a right state and you don't need to take
care of it anymore outside the possible error paths.

Once the processing done, if the HTX message has been modified, the underlying
buffer must be also updated, except you uses htx_from_buf() and you only add
data. For all other cases, the function htx_to_buf() must be called.

Finally, the function htx_reset() may be called at any time to reset an HTX
message. And the function buf_room_for_htx_data() may be called to know if a raw
buffer is full from the HTX perspective. It is used during conversion from/to
the HTX.


4.2. Helpers to deal with free space in an HTX message

Once you have an HTX message, following functions may help you to process it :

    - htx_used_space() and htx_meta_space() return, respectively, the total
      space used in an HTX message and the space used by block's metadata only.

    - htx_free_space() and htx_free_data_space() return, respectively, the total
      free space in an HTX message and the free space available for the payload
      if a new HTX block is stored (so it is the total free space minus the size
      of an HTX block).

    - htx_is_empty() and htx_is_not_empty() are boolean functions to know if an
      HTX message is empty or not.

    - htx_get_max_blksz() returns the maximum size available for the payload,
      not exceeding a maximum, metadata included.

    - htx_almost_full() should be used to know if an HTX message uses at least
      3/4 of its capacity.


4.3. HTX Blocks manipulations

Once you know how much space is available in an HTX message, the next step is to
add HTX blocks. First of all the function htx_nbblks() returns the number of
blocks allocated in an HTX message. Then, there is an add function per block's
type:

    - htx_add_stline() adds a start-line. The type (request or response) and the
      flags of the start-line must be provided, as well as its three parts
      (method,uri,version or version,status-code,reason).

    - htx_add_header() and htx_add_trailers() are similar. The name and the
      value must be provided. The inserted HTX block is returned on success or
      NULL if an error occurred.

    - htx_add_endof() must be used to add any end-of marker. The block's type
      (EOH, EOT or EOM) must be specified. The inserted HTX block is returned on
      success or NULL if an error occurred.

    - htx_add_all_headers() and htx_add_all_trailers() add, respectively, a list
      of headers and a list of trailers, followed by the appropriate end-of
      marker. On success, this marker is returned. Otherwise, NULL is
      returned. Note there is no rollback on the HTX message when an error
      occurred. Some headers or trailers may have been added. So it is the
      caller responsibility to take care of that.

    - htx_add_data() must be used to add a DATA block. Unlike previous
      functions, this one returns the number of bytes copied or 0 if nothing was
      copied. If possible, the data are appended to the last DATA block, if
      any. Only a part of the payload may be copied because this function will
      try to limit the message defragmentation and the wrapping of blocks as far
      as possible. If you really need to add all data or nothing, the function
      htx_add_data_atonce() must be used instead. Because it tries to insert all
      the payload, this function returns the inserted block on success.
      Otherwise it returns NULL.

When an HTX block is added, it is always the last one (the tail). But, if you
need to add a block at a specific place, it is not really handy. 2 functions may
help you (others could be added) :

    - htx_add_last_data() adds a DATA block just after all other DATA blocks and
      before any trailers and EOT or EOM markers. It relies on
      htx_add_data_atonce(), so a defragmentation may be performed.

    - htx_move_blk_before() moves a specific block just after another one. Both
      blocks must already be in the HTX message and the block to move must
      always be placed after the "pivot".

Once added, there are three functions to update the block's payload :

    - htx_replace_stline() updates a start-line. The HTX block must be passed as
      argument. Only string parts of the start-line are updated by this
      function. On success, it returns the new start-line. So it is pretty easy
      to update its flags. NULL is returned if an error occurred.

    - htx_replace_header() fully replaces a header (its name and its value) by a
      new one. The HTX block must be passed a argument, as well as its new name
      and its new value. The new header can be smaller or larger than the old
      one. This function returns the new HTX block on success, or NULL is an
      error occurred.

    - htx_replace_blk_value() replaces a part of a block's payload or its
      totality. It works for HEADERS, TRAILERS or DATA blocks. The HTX block
      must be provided with the part to remove and the new one. The new part can
      be smaller or larger than the old one. This function returns the new HTX
      block on success, or NULL is an error occurred.

Finally, You may remove a block using the function htx_remove_blk(). This
function returns the block following the one removed or NULL if it is the tail
block.


4.4. The HTX start-line

Unlike other HTX blocks, the start-line is a bit special because its payload is
a structure followed by its three parts :

        +--------+-------+-------+-------+
        | HTX_SL | PART1 | PART2 | PART3 |
        +--------+-------+-------+-------+

Some macros and functions may help to manipulate these parts :

    - HTX_SL_P{N}_LEN() and HTX_SL_P{N}_PTR() are macros to get the length of a
      part and a pointer on it. {N} should be 1, 2 or 3.

    - HTX_SL_REQ_MLEN(), HTX_SL_REQ_ULEN(), HTX_SL_REQ_VLEN(),
      HTX_SL_REQ_MPTR(), HTX_SL_REQ_UPTR() and HTX_SL_REQ_VPTR() are macros to
      get info about a request start-line. These macros only wrap HTX_SL_P*
      ones.

    - HTX_SL_RES_VLEN(), HTX_SL_RES_CLEN(), HTX_SL_RES_RLEN(),
      HTX_SL_RES_VPTR(), HTX_SL_RES_CPTR() and HTX_SL_RES_RPTR() are macros to
      get info about a response start-line. These macros only wrap HTX_SL_P*
      ones.

    - htx_sl_p1(), htx_sl_p2() and htx_sl_p2() are functions to get the ist
      corresponding to the right part of a start-line.

    - htx_sl_req_meth(), htx_sl_req_uri() and htx_sl_req_vsn() get the ist
      corresponding to the right part of a request start-line.

    - htx_sl_res_vsn(), htx_sl_res_code() and htx_sl_res_reason() get the ist
      corresponding to the right part of a response start-line.


4.5. Iterate on the HTX message

To iterate on an HTX message, the first thing to do is to get the HTX block to
start the loop. There are three special blocks in an HTX message that may be
good candidates to start a loop :

  * the head block. It is the oldest inserted block. Multiplexers always start
    to consume an HTX message from this block. The function htx_get_head()
    returns its position and htx_get_head_blk() returns the blocks itself. In
    addition, the function htx_get_head_type() returns its block's type.

  * the tail block. It is the newest inserted block. The function htx_get_tail()
    returns its position and htx_get_tail_blk() returns the blocks itself. In
    addition, the function htx_get_tail_type() returns its block's type.

  * the first block. It is the block where to (re)start the analyse. It is used
    as start point by HTX analyzers. The function htx_get_first() returns its
    position and htx_get_first_blk() returns the blocks itself. In addition, the
    function htx_get_first_type() returns its block's type.

For all these functions, if the HTX message is empty, -1 is returned for the
block's position, NULL instead of a block and HTX_BLK_UNUSED for its type.

Then to iterate on blocks, you may move foreword or backward :

  * htx_get_prev() and htx_get_next() return, respectively, the position of the
    previous block or the next block, given a specific position. Or -1 if an edge
    is reached.

  * htx_get_prev_blk() and htx_get_next_blk() return, respectively, the previous
    block or the next one, given a specific block. Or NULL if an edge is
    reached.


4.6. Advanced functions

Some more advanced functions may be used to do complex processing on the HTX
message. These functions are used by HTX analyzers or by multiplexers.

  * htx_truncate() removes all blocks after the one containing a specific offset
    relatively to the head block of the HTX message. If the offset is inside a
    DATA block, it is truncated. For all other blocks, the removal starts to the
    next block.

  * htx_drain() tries to remove a specific amount of bytes of payload. If the
    last block is a DATA block, it may be truncated if necessary. All other
    block are removed at once or kept. This function returns a mixed value, with
    the first block not removed, or NULL if everything was removed, and the
    amount of data drained.

  * htx_xfer_blks() transfers HTX blocks from an HTX message to another,
    stopping on the first block of a specified type or when a specific amount of
    bytes, including meta-data, was moved. If the last block is a DATA block, it
    may be partially moved. All other block are transferred at once or
    kept. This function returns a mixed value, with the last block moved, or
    NULL if nothing was moved, and the amount of data transferred. When HEADERS
    or TRAILERS blocks must be transferred, this function transfers all of
    them. Otherwise, if it is not possible, it triggers an error. It is the
    caller responsibility to transfer all headers or trailers at once.