2018-07-13 - HAProxy Internal Buffer API


1. Background

HAProxy uses a "struct buffer" internally to store data received from external
agents, as well as data to be sent to external agents. These buffers are also
used during data transformation such as compression, header insertion or
defragmentation, and are used to carry intermediary representations between the
various internal layers. They support wrapping at the end, and they carry their
own size information so that in theory it would be possible to use different
buffer sizes in parallel even though this is not currently implemented.

The format of this structure has evolved over time, to reach a point where it
is convenient and versatile enough to have permitted to make several internal
types converge into a single one (specifically the struct chunk disappeared).


2. Representation as of 1.9-dev1

The current buffer representation consists in a linear storage area of known
size, with a head position indicating the oldest data, and a total data count
expressed in bytes. The head position, data count and size are expressed as
integers and are positive or null. By convention, the head position is strictly
smaller than the buffer size and the data count is smaller than or equal to the
size, so that wrapping can be resolved with a single subtract. A buffer not
respecting these rules is said to be degenerate. Unless specified otherwise,
the various API functions will adopt an undefined behaviour when passed such a
degenerate buffer.

 Buffer declaration :

    struct buffer {
        size_t size;     // size of the storage area (wrapping point)
        char  *area;     // start of the storage area
        size_t data;     // contents length after head
        size_t head;     // start offset of remaining data relative to area
    };


 Linear buffer representation :

    area
      |
      V<--------------------------------------------------------->| size
      +-----------+---------------------------------+-------------+
      |           |/////////////////////////////////|             |
      +-----------+---------------------------------+-------------+
      |<--------->|<------------------------------->|
           head                 data                ^
                                                    |
                                                   tail


 Wrapping buffer representation :

    area
      |
      V<--------------------------------------------------------->| size
      +---------------+------------------------+------------------+
      |///////////////|                        |//////////////////|
      +---------------+------------------------+------------------+
      |<-------------------------------------->| head
      |-------------->| ...data         data...|<-----------------|
                      ^
                      |
                     tail


3. Terminology

Manipulating a buffer just based on a head and a wrapping data count is not
very convenient, so we define a certain number of terms for important elements
characterizing a buffer :

   - origin    : pointer to relative position 0 in the storage area. Undefined
                 when the buffer is not allocated.

   - size      : the allocated size of the storage area starting at the origin,
                 expressed in bytes. A buffer whose size is zero is said not to
                 be allocated, and its origin in this case is undefined.

   - data      : the amount of data the buffer contains, in bytes. It is always
                 lower than or equal to the buffer's size, hence it is always 0
                 for an unallocated buffer.

   - emptiness : a buffer is said to be empty when it contains no data, hence
                 data == 0. It is possible for such buffers not to be allocated
                 and to have size == 0 as well.

   - room      : the available space in the buffer. This is its size minus data.

   - head      : position relative to origin where the oldest data byte is found
                 (it typically is what send() uses to pick outgoing data). The
                 head is strictly smaller than the size.

   - tail      : position relative to origin where the first spare byte is found
                 (it typically is what recv() uses to store incoming data). It
                 is always equal to the buffer's data added to its head modulo
                 the buffer's size.

   - wrapping  : the byte following the last one of the storage area loops back
                 to position 0. This is called wrapping. The wrapping point is
                 the first position relative to origin which doesn't belong to
                 the storage area. There is no wrapping when a buffer is not
                 allocated. Wrapping requires special care and means that the
                 regular string manipulation functions are not usable on most
                 buffers, unless it is known that no wrapping happens. Free
                 space may wrap as well if the buffer only contains data in the
                 middle.

   - alignment : a buffer is said to be aligned if its data do not wrap. That
                 is, its head is strictly before the tail, or the buffer is
                 empty and the head is null. Aligning a buffer may be required
                 to use regular string manipulation functions which have no
                 support for wrapping.


A buffer may be in three different states :
   - unallocated : size == 0, area == 0 (b_is_null() is true)
   - waiting     : size == 0, area != 0
   - allocated   : size  > 0, area  > 0

It is not permitted to have area == 0 with a non-null size. In addition, the
waiting state may also be used to indicate a read-only buffer which does not
wrap and which must not be freed (e.g. for use with error messages).

The basic API only covers allocated buffers. Switching to/from the other states
is covered by the management API since it requires specific allocation and free
calls.


4. Using buffers

Buffers are defined in a few files :
   - include/common/buf.h    : structure definition, and manipulation functions
   - include/common/buffer.h : resource management (alloc/free/wait lists)
   - include/common/istbuf.h : advanced string manipulation


4.1. Basic API

The basic API is made of the functions which abstract accesses to the buffers
and which help calculating their state, free space or used space.

====================+==================+=======================================
Function            | Arguments/Return | Description
--------------------+------------------+---------------------------------------
b_is_null()         | const buffer *buf| returns true if (and only if) the
                    | ret: int         | buffer is not yet allocated and thus
                    |                  | points to a NULL area
--------------------+------------------+---------------------------------------
b_orig()            | const buffer *buf| returns the pointer to the origin of
                    | ret: char *      | the storage, which is the location of
                    |                  | byte at offset zero. This is mostly
                    |                  | used by functions which handle the
                    |                  | wrapping by themselves
--------------------+------------------+---------------------------------------
b_size()            | const buffer *buf| returns the size of the buffer
                    | ret: size_t      |
--------------------+------------------+---------------------------------------
b_wrap()            | const buffer *buf| returns the pointer to the wrapping
                    | ret: char *      | position of the buffer area, which is
                    |                  | by definition the first byte not part
                    |                  | of the buffer
--------------------+------------------+---------------------------------------
b_data()            | const buffer *buf| returns the number of bytes present in
                    | ret: size_t      | the buffer
--------------------+------------------+---------------------------------------
b_room()            | const buffer *buf| returns the amount of room left in the
                    | ret: size_t      | buffer
--------------------+------------------+---------------------------------------
b_full()            | const buffer *buf| returns true if the buffer is full
                    | ret: int         |
--------------------+------------------+---------------------------------------
__b_stop()          | const buffer *buf| returns a pointer to the byte
                    | ret: char *      | following the end of the buffer, which
                    |                  | may be out of the buffer if the buffer
                    |                  | ends on the last byte of the area. It
                    |                  | is the caller's responsibility to
                    |                  | either know that the buffer does not
                    |                  | wrap or to check that the result does
                    |                  | not wrap
--------------------+------------------+---------------------------------------
__b_stop_ofs()      | const buffer *buf| returns an origin-relative offset
                    | ret: size_t      | pointing to the byte following the end
                    |                  | of the buffer, which may be out of the
                    |                  | buffer if the buffer ends on the last
                    |                  | byte of the area. It's the caller's
                    |                  | responsibility to either know that the
                    |                  | buffer does not wrap or to check that
                    |                  | the result does not wrap
--------------------+------------------+---------------------------------------
b_stop()            | const buffer *buf| returns the pointer to the byte
                    | ret: char *      | following the end of the buffer, which
                    |                  | may be out of the buffer if the buffer
                    |                  | ends on the last byte of the area
--------------------+------------------+---------------------------------------
b_stop_ofs()        | const buffer *buf| returns an origin-relative offset
                    | ret: size_t      | pointing to the byte following the end
                    |                  | of the buffer, which may be out of the
                    |                  | buffer if the buffer ends on the last
                    |                  | byte of the area
--------------------+------------------+---------------------------------------
__b_peek()          | const buffer *buf| returns a pointer to the data at
                    | size_t ofs       | position <ofs> relative to the head of
                    | ret: char *      | the buffer. Will typically point to
                    |                  | input data if called with the amount
                    |                  | of output data. It's the caller's
                    |                  | responsibility to either know that the
                    |                  | buffer does not wrap or to check that
                    |                  | the result does not wrap
--------------------+------------------+---------------------------------------
__b_peek_ofs()      | const buffer *buf| returns an origin-relative offset
                    | size_t ofs       | pointing to the data at position <ofs>
                    | ret: size_t      | relative to the head of the
                    |                  | buffer. Will typically point to input
                    |                  | data if called with the amount of
                    |                  | output data. It's the caller's
                    |                  | responsibility to either know that the
                    |                  | buffer does not wrap or to check that
                    |                  | the result does not wrap
--------------------+------------------+---------------------------------------
b_peek()            | const buffer *buf| returns a pointer to the data at
                    | size_t ofs       | position <ofs> relative to the head of
                    | ret: char *      | the buffer. Will typically point to
                    |                  | input data if called with the amount
                    |                  | of output data. If applying <ofs> to
                    |                  | the buffers' head results in a
                    |                  | position between <size> and 2*>size>-1
                    |                  | included, a wrapping compensation is
                    |                  | applied to the result
--------------------+------------------+---------------------------------------
b_peek_ofs()        | const buffer *buf| returns an origin-relative offset
                    | size_t ofs       | pointing to the data at position <ofs>
                    | ret: size_t      | relative to the head of the
                    |                  | buffer. Will typically point to input
                    |                  | data if called with the amount of
                    |                  | output data. If applying <ofs> to the
                    |                  | buffers' head results in a position
                    |                  | between <size> and 2*>size>-1
                    |                  | included, a wrapping compensation is
                    |                  | applied to the result
--------------------+------------------+---------------------------------------
__b_head()          | const buffer *buf| returns the pointer to the buffer's
                    | ret: char *      | head, which is the location of the
                    |                  | next byte to be dequeued. The result
                    |                  | is undefined for unallocated buffers
--------------------+------------------+---------------------------------------
__b_head_ofs()      | const buffer *buf| returns an origin-relative offset
                    | ret: size_t      | pointing to the buffer's head, which
                    |                  | is the location of the next byte to be
                    |                  | dequeued. The result is undefined for
                    |                  | unallocated buffers
--------------------+------------------+---------------------------------------
b_head()            | const buffer *buf| returns the pointer to the buffer's
                    | ret: char *      | head, which is the location of the
                    |                  | next byte to be dequeued. The result
                    |                  | is undefined for unallocated
                    |                  | buffers. If applying <ofs> to the
                    |                  | buffers' head results in a position
                    |                  | between <size> and 2*>size>-1
                    |                  | included, a wrapping compensation is
                    |                  | applied to the result
--------------------+------------------+---------------------------------------
b_head_ofs()        | const buffer *buf| returns an origin-relative offset
                    | ret: size_t      | pointing to the buffer's head, which
                    |                  | is the location of the next byte to be
                    |                  | dequeued. The result is undefined for
                    |                  | unallocated buffers.  If applying
                    |                  | <ofs> to the buffers' head results in
                    |                  | a position between <size> and
                    |                  | 2*>size>-1 included, a wrapping
                    |                  | compensation is applied to the result
--------------------+------------------+---------------------------------------
__b_tail()          | const buffer *buf| returns the pointer to the tail of the
                    | ret: char *      | buffer, which is the location of the
                    |                  | first byte where it is possible to
                    |                  | enqueue new data. The result is
                    |                  | undefined for unallocated buffers
--------------------+------------------+---------------------------------------
__b_tail_ofs()      | const buffer *buf| returns an origin-relative offset
                    | ret: size_t      | pointing to the tail of the buffer,
                    |                  | which is the location of the first
                    |                  | byte where it is possible to enqueue
                    |                  | new data. The result is undefined for
                    |                  | unallocated buffers
--------------------+------------------+---------------------------------------
b_tail()            | const buffer *buf| returns the pointer to the tail of the
                    | ret: char *      | buffer, which is the location of the
                    |                  | first byte where it is possible to
                    |                  | enqueue new data. The result is
                    |                  | undefined for unallocated buffers
--------------------+------------------+---------------------------------------
b_tail_ofs()        | const buffer *buf| returns an origin-relative offset
                    | ret: size_t      | pointing to the tail of the buffer,
                    |                  | which is the location of the first
                    |                  | byte where it is possible to enqueue
                    |                  | new data. The result is undefined for
                    |                  | unallocated buffers
--------------------+------------------+---------------------------------------
b_next()            | const buffer *buf| for an absolute pointer <p> pointing
                    | const char *p    | to a valid location within buffer <b>,
                    | ret: char *      | returns the absolute pointer to the
                    |                  | next byte, which usually is at (p + 1)
                    |                  | unless p reaches the wrapping point
                    |                  | and wrapping is needed
--------------------+------------------+---------------------------------------
b_next_ofs()        | const buffer *buf| for an origin-relative offset <o>
                    | size_t o         | pointing to a valid location within
                    | ret: size_t      | buffer <b>, returns either the
                    |                  | relative offset pointing to the next
                    |                  | byte, which usually is at (o + 1)
                    |                  | unless o reaches the wrapping point
                    |                  | and wrapping is needed
--------------------+------------------+---------------------------------------
b_dist()            | const buffer *buf| returns the distance between two
                    | const char *from | pointers, taking into account the
                    | const char *to   | ability to wrap around the buffer's
                    | ret: size_t      | end. The operation is not defined if
                    |                  | either of the pointers does not belong
                    |                  | to the buffer or if their distance is
                    |                  | greater than the buffer's size
--------------------+------------------+---------------------------------------
b_almost_full()     | const buffer *buf| returns 1 if the buffer uses at least
                    | ret: int         | 3/4 of its capacity, otherwise
                    |                  | zero. Buffers of size zero are
                    |                  | considered full
--------------------+------------------+---------------------------------------
b_space_wraps()     | const buffer *buf| returns non-zero only if the buffer's
                    | ret: int         | free space wraps, which means that the
                    |                  | buffer contains data that are not
                    |                  | touching at least one edge
--------------------+------------------+---------------------------------------
b_contig_data()     | const buffer *buf| returns the amount of data that can
                    | size_t start     | contiguously be read at once starting
                    | ret: size_t      | from a relative offset <start> (which
                    |                  | allows to easily pre-compute blocks
                    |                  | for memcpy). The start point will
                    |                  | typically contain the amount of past
                    |                  | data already returned by a previous
                    |                  | call to this function
--------------------+------------------+---------------------------------------
b_contig_space()    | const buffer *buf| returns the amount of bytes that can
                    | ret: size_t      | be appended to the buffer at once
--------------------+------------------+---------------------------------------
b_getblk()          | const buffer *buf| gets one full block of data at once
                    | char *blk        | from a buffer, starting from offset
                    | size_t len       | <offset> after the buffer's head, and
                    | size_t offset    | limited to no more than <len> bytes.
                    | ret: size_t      | The caller is responsible for ensuring
                    |                  | that neither <offset> nor <offset> +
                    |                  | <len> exceed the total number of bytes
                    |                  | available in the buffer.  Return zero
                    |                  | if not enough data was available, in
                    |                  | which case blk is left undefined, or
                    |                  | the number of bytes read which is
                    |                  | equal to the requested size
--------------------+------------------+---------------------------------------
b_getblk_nc()       | const buffer *buf| gets one or two blocks of data at once
                    | const char **blk1| from a buffer, starting from offset
                    | size_t *len1     | <ofs> after the beginning of its
                    | const char **blk2| output, and limited to no more than
                    | size_t *len2     | <max> bytes. The caller is responsible
                    | size_t ofs       | for ensuring that neither <ofs> nor
                    | size_t max       | <ofs>+<max> exceed the total number of
                    | ret: int         | bytes available in the buffer. Returns
                    |                  | 0 if not enough data were available,
                    |                  | or the number of blocks filled (1 or
                    |                  | 2). <blk1> is always filled before
                    |                  | <blk2>. The unused blocks are left
                    |                  | undefined, and the buffer is left
                    |                  | unaffected. Unused buffers are left in
                    |                  | an undefined state
--------------------+------------------+---------------------------------------
b_reset()           | buffer *buf      | resets a buffer. The size is not
                    | ret: void        | touched. In practice it resets the
                    |                  | head and the data length
--------------------+------------------+---------------------------------------
b_sub()             | buffer *buf      | decreases the buffer length by <count>
                    | size_t count     | without touching the head position
                    | ret: void        | (only the tail moves). this may mostly
                    |                  | be used to trim pending data before
                    |                  | reusing a buffer. The caller is
                    |                  | responsible for not removing more than
                    |                  | the available data
--------------------+------------------+---------------------------------------
b_add()             | buffer *buf      | increase the buffer length by <count>
                    | size_t count     | without touching the head position
                    | ret: void        | (only the tail moves). This is used
                    |                  | when adding data at the tail of a
                    |                  | buffer. The caller is responsible for
                    |                  | not adding more than the available
                    |                  | room
--------------------+------------------+---------------------------------------
b_set_data()        | buffer *buf      | sets the buffer's length, by adjusting
                    | size_t len       | the buffer's tail only. The caller is
                    | ret: void        | responsible for passing a valid length
--------------------+------------------+---------------------------------------
b_del()             | buffer *buf      | deletes <del> bytes at the head of
                    | size_t del       | buffer <b> and updates the head. The
                    | ret: void        | caller is responsible for not removing
                    |                  | more than the available data. This is
                    |                  | used after sending data from the
                    |                  | buffer
--------------------+------------------+---------------------------------------
b_realign_if_empty()| buffer *buf      | realigns a buffer if it's empty, does
                    | ret: void        | nothing otherwise. This is mostly used
                    |                  | after b_del() to make an empty
                    |                  | buffer's free space contiguous
--------------------+------------------+---------------------------------------
b_slow_realign()    | buffer *buf      | realigns a possibly wrapping buffer so
                    | size_t output    | that the part remaining to be parsed
                    | ret: void        | is contiguous and starts at the
                    |                  | beginning of the buffer and the
                    |                  | already parsed output part ends at the
                    |                  | end of the buffer. This provides the
                    |                  | best conditions since it allows the
                    |                  | largest inputs to be processed at once
                    |                  | and ensures that once the output data
                    |                  | leaves, the whole buffer is available
                    |                  | at once. The number of output bytes
                    |                  | supposedly present at the beginning of
                    |                  | the buffer and which need to be moved
                    |                  | to the end must be passed in <output>.
                    |                  | It will effectively make this offset
                    |                  | the new wrapping point. A temporary
                    |                  | swap area at least as large as b->size
                    |                  | must be provided in <swap>.  It's up
                    |                  | to the caller to ensure <output> is no
                    |                  | larger than the difference between the
                    |                  | whole buffer's length and its input
--------------------+------------------+---------------------------------------
b_putchar()         | buffer *buf      | tries to append char <c> at the end of
                    | char c           | buffer <b>. Supports wrapping. New
                    | ret: void        | data are silently discarded if the
                    |                  | buffer is already full
--------------------+------------------+---------------------------------------
b_putblk()          | buffer *buf      | tries to append block <blk> at the end
                    | const char *blk  | of buffer <b>. Supports wrapping. Data
                    | size_t len       | are truncated if the buffer is too
                    | ret: size_t      | short or if not enough space is
                    |                  | available. It returns the number of
                    |                  | bytes really copied
--------------------+------------------+---------------------------------------
b_move()            | buffer *buf      | moves block (src,len) left or right
                    | size_t src       | by <shift> bytes, supporting wrapping
                    | size_t len       | and overlapping.
                    | size_t shift     |
--------------------+------------------+---------------------------------------
b_rep_blk()         | buffer *buf      | writes the block <blk> at position
                    | char *pos        | <pos> which must be in buffer <b>, and
                    | char *end        | moves the part between <end> and the
                    | const char *blk  | buffer's tail just after the end of
                    | size_t len       | the copy of <blk>. This effectively
                    | ret: int         | replaces the part located between
                    |                  | <pos> and <end> with a copy of <blk>
                    |                  | of length <len>. The buffer's length
                    |                  | is automatically updated. This is used
                    |                  | to replace a block with another one
                    |                  | inside a buffer. The shift value
                    |                  | (positive or negative) is returned. If
                    |                  | there's no space left, the move is not
                    |                  | done. If <len> is null, the <blk>
                    |                  | pointer is allowed to be null, in
                    |                  | order to erase a block
--------------------+------------------+---------------------------------------
b_xfer()            | buffer *src      | transfers at most <count> bytes from
                    | buffer *dst      | buffer <src> to buffer <dst> and
                    | size_t cout      | returns the number of bytes copied.
                    | ret: size_t      | The bytes are removed from <src> and
                    |                  | added to <dst>. The caller guarantees
                    |                  | that <count> is <= b_room(dst)
====================+==================+=======================================


4.2. String API

The string API aims at providing both convenient and efficient ways to read and
write to/from buffers using indirect strings (ist). These strings and some
associated functions are defined in ist.h.

====================+==================+=======================================
Function            | Arguments/Return | Description
--------------------+------------------+---------------------------------------
b_isteq()           | const buffer *b  | b_isteq() : returns > 0 if the first
                    | size_t o         | <n> characters of buffer <b> starting
                    | size_t n         | at offset <o> relative to the buffer's
                    | const ist ist    | head match <ist>. (empty strings do
                    | ret: int         | match). It is designed to be used with
                    |                  | reasonably small strings (it matches a
                    |                  | single byte per loop iteration). It is
                    |                  | expected to be used with an offset to
                    |                  | skip old data. Return value number of
                    |                  | matching bytes if >0, not enough bytes
                    |                  | or empty string if 0, or non-matching
                    |                  | byte found if <0.
--------------------+------------------+---------------------------------------
b_isteat            | struct buffer *b | b_isteat() : "eats" string <ist> from
                    | const ist ist    | the head of buffer <b>. Wrapping data
                    | ret: ssize_t     | is explicitly supported. It matches a
                    |                  | single byte per iteration so strings
                    |                  | should remain reasonably small.
                    |                  | Returns the number of bytes matched
                    |                  | and eaten if >0, not enough bytes or
                    |                  | matched empty string if 0, or non
                    |                  | matching byte found if <0.
--------------------+------------------+---------------------------------------
b_istput            | struct buffer *b | b_istput() : injects string <ist> at
                    | const ist ist    | the tail of output buffer <b> provided
                    | ret: ssize_t     | that it fits. Wrapping is supported.
                    |                  | It's designed for small strings as it
                    |                  | only writes a single byte per
                    |                  | iteration. Returns the number of
                    |                  | characters copied (ist.len), 0 if it
                    |                  | temporarily does not fit, or -1 if it
                    |                  | will never fit. It will only modify
                    |                  | the buffer upon success. In all cases,
                    |                  | the contents are copied prior to
                    |                  | reporting an error, so that the
                    |                  | destination at least contains a valid
                    |                  | but truncated string.
--------------------+------------------+---------------------------------------
b_putist            | struct buffer *b | b_putist() : tries to copy as much as
                    | const ist ist    | possible of string <ist> into buffer
                    | ret: size_t      | <b> and returns the number of bytes
                    |                  | copied (truncation is possible). It
                    |                  | uses b_putblk() and is suitable for
                    |                  | large blocks.
====================+==================+=======================================


4.3. Management API

The management API makes a distinction between an empty buffer, which by
definition is not allocated but is ready to be allocated at any time, and a
buffer which failed an allocation and is waiting for an available area to be
offered. The functions allow to register on a list to be notified about buffer
availability, to notify others of a number of buffers just released, and to be
and to be notified of buffer availability. All allocations are made through the
standard buffer pools.

====================+==================+=======================================
Function            | Arguments/Return | Description
--------------------+------------------+---------------------------------------
buffer_almost_full  | const buffer *buf| returns true if the buffer is not null
                    | ret: int         | and at least 3/4 of the buffer's space
                    |                  | are used. A waiting buffer will match.
--------------------+------------------+---------------------------------------
b_alloc             | buffer *buf      | allocates a buffer and assigns it to
                    | ret: buffer *    | *buf. If no memory is available, (1)
                    |                  | is assigned instead with a zero size.
                    |                  | No control is made to check if *buf
                    |                  | already pointed to another buffer. The
                    |                  | allocated buffer is returned, or NULL
                    |                  | in case no memory is available
--------------------+------------------+---------------------------------------
b_alloc_fast        | buffer *buf      | allocates a buffer and assigns it to
                    | ret: buffer *    | *buf. If no memory is available, (1)
                    |                  | is assigned instead with a zero size.
                    |                  | No control is made to check if *buf
                    |                  | already pointed to another buffer. The
                    |                  | allocated buffer is returned, or NULL
                    |                  | in case no memory is available. The
                    |                  | difference with b_alloc() is that this
                    |                  | function only picks from the pool and
                    |                  | never calls malloc(), so it can fail
                    |                  | even if some memory is available
--------------------+------------------+---------------------------------------
__b_drop            | buffer *buf      | releases <buf> which must be allocated
                    | ret: void        | and marks it empty
--------------------+------------------+---------------------------------------
b_drop              | buffer *buf      | releases <buf> only if it is allocated
                    | ret: void        | and marks it empty
--------------------+------------------+---------------------------------------
b_free              | buffer *buf      | releases <buf> only if it is allocated
                    | ret: void        | and marks it empty
--------------------+------------------+---------------------------------------
b_alloc_margin      | buffer *buf      | ensures that <buf> is allocated. If an
                    | int margin       | allocation is needed, it ensures that
                    | ret: buffer *    | there are still at least <margin>
                    |                  | buffers available in the pool after
                    |                  | this allocation so that we don't leave
                    |                  | the pool in a condition where a
                    |                  | session or a response buffer could not
                    |                  | be allocated anymore, resulting in a
                    |                  | deadlock. This means that we sometimes
                    |                  | need to try to allocate extra entries
                    |                  | even if only one buffer is needed
--------------------+------------------+---------------------------------------
offer_buffers()     | void *from       | offer a buffer currently belonging to
                    | uint threshold   | target <from> to whoever needs
                    | ret: void        | one. Any pointer is valid for <from>,
                    |                  | including NULL. Its purpose is to
                    |                  | avoid passing a buffer to oneself in
                    |                  | case of failed allocations (e.g. need
                    |                  | two buffers, get one, fail, release it
                    |                  | and wake up self again). In case of
                    |                  | normal buffer release where it is
                    |                  | expected that the caller is not
                    |                  | waiting for a buffer, NULL is fine
====================+==================+=======================================


5. Porting code from older versions

The previous buffer API introduced in 1.5-dev9 (May 2012) used to look like the
following (with the struct renamed to old_buffer here to avoid confusion during
quick lookups at the doc). It's worth noting that the "data" field used to be
part of the struct but with a different type and meaning. It's important to be
careful about potential code making use of &b->data as it will silently compile
but fail.

 Previous buffer declaration :

    struct old_buffer {
        char *p;                        /* buffer's start pointer, separates in and out data */
        unsigned int size;              /* buffer size in bytes */
        unsigned int i;                 /* number of input bytes pending for analysis in the buffer */
        unsigned int o;                 /* number of out bytes the sender can consume from this buffer */
        char data[0];                   /* <size> bytes */
    };

 Previous linear buffer representation :

    data                               p
      |                                |
      V                                V
      +-----------+--------------------+------------+-------------+
      |           |////////////////////|////////////|             |
      +-----------+--------------------+------------+-------------+
       <---------------------------------------------------------> size
                   <------------------> <---------->
                            o                i

There is this correspondance between old and new fields (some will involve a
knowledge of a channel when the output byte count is required) :

     Old    | New
    --------+----------------------------------------------------
     p      | data + head + co_data(channel) // ci_head(channel)
     size   | size
     i      | data - co_data(channel) // ci_data(channel)
     o      | co_data(channel) // channel->output
     data   | area
    --------+-----------------------------------------------------

Then some common expressions can be mapped like this :

     Old                   | New
    -----------------------+---------------------------------------
     b->data               | b_orig(b)
     &b->data              | b_orig(b)
     bi_ptr(b)             | ci_head(channel)
     bi_end(b)             | b_tail(b)
     bo_ptr(b)             | b_head(b)
     bo_end(b)             | co_tail(channel)
     bi_putblk(b,s,l)      | b_putblk(b,s,l)
     bo_getblk(b,s,l,o)    | b_getblk(b,s,l,o)
     bo_getblk_nc(b,s,l,o) | b_getblk_nc(b,s,l,o,0,co_data(channel))
     b->i + b->o           | b_data(b)
     b->data + b->size     | b_wrap(b)
     b->i += len           | b_add(b, len)
     b->i -= len           | b_sub(b, len)
     b->i = len            | b_set_data(b, co_data(channel) + len)
     b->o += len           | b_add(b, len); channel->output += len
     b->o -= len           | b_del(b, len); channel->output -= len
    -----------------------+---------------------------------------

The buffer modification functions are less straightforward and depend a lot on
the context where they are used. It is strongly advised to figure in the list
of functions above what is available based on what is attempted to be done in
the existing code.

Note that it is very likely that any out-of-tree code relying on buffers will
not use both ->i and ->o but instead will use exclusively ->i on the side
producing data and use exclusively ->o on the side consuming data (such as in a
mux or in an applet). In both cases, it should be assumed that the other side
is always zero and that either ->i or ->o is replaced with ->data, making the
remaining code much simpler (no more code duplication based on the data
direction).