tree ff4da2793b9df6a2b05014120a5b1cafbb29374c
parent 50fe9fba4bc622de338d464188b82d093e45eb4c
author Christopher Faulet <cfaulet@haproxy.com> 1560242443 +0200
committer Christopher Faulet <cfaulet@haproxy.com> 1560254725 +0200

MAJOR: htx: Rework how free rooms are tracked in an HTX message

In an HTX message, it may have 2 available rooms to store a new block. The first
one is between the blocks and their payload. Blocks are added starting from the
end of the buffer and their payloads are added starting from the begining. So
the first free room is between these 2 edges. The second one is at the begining
of the buffer, when we start to wrap to add new payloads. Once we start to use
this one, the other one is ignored until the next defragmentation of the HTX
message.

In theory, there is no problem. But in practice, some lacks in the HTX structure
force us to defragment too often HTX messages to always be in a known state. The
second free room is not tracked as it should do and the first one may be easily
corrupted when rewrites happen.

So to fix the problem and avoid unecessary defragmentation, the HTX structure
has been refactored. The front (the block's position of the first payload before
the blocks) is no more stored. Instead we keep the relative addresses of 3 edges:

 * tail_addr : The start address of the free space in front of the the blocks
               table
 * head_addr : The start address of the free space at the beginning
 * end_addr  : The end address of the free space at the beginning

Here is the general view of the HTX message now:

           head_addr     end_addr    tail_addr
               |            |            |
               V            V            V
  +------------+------------+------------+------------+------------------+
  |            |            |            |            |                  |
  |  PAYLOAD   | Free space |  PAYLOAD   | Free space |    Blocks area   |
  |    ==>     |     1      |    ==>     |     2      |        <==       |
  +------------+------------+------------+------------+------------------+

<head_addr> is always lower or equal to <end_addr> and <tail_addr>. <end_addr>
is always lower or equal to <tail_addr>.

In addition;, to simplify everything, the blocks area are now contiguous. It
doesn't wrap anymore. So the head is always the block with the lowest position,
and the tail is always the one with the highest position.