2021-11-09 - List API


1. Background
-------------

HAProxy's lists are almost all doubly-linked and circular so that it is always
possible to insert at the beginning, append at the end, scan them in any order
and delete any element without having to scan to search the predecessor nor the
successor.

A list's head is just a regular list element, and an element always points to
another list element. Such elements only have two pointers, the next and the
previous elements. The object being pointed to is retrieved by subtracting the
list element's offset in its structure from the list element's pointer. This
way there is no need for any separate allocation for the list element, for a
pointer to the object in the list, nor for a pointer to the list element from
the object, as the list is embedded into the object.

All basic operations are provided, as well as some iterators. Some iterators
are safe for removal of the current element within the loop, others not. In any
case a list cannot be freely modified while iterating over it (e.g. the current
element's successor cannot not be freed if it's saved as the restart point).

Extreme care is taken nowadays in HAProxy to make sure that no dangling
pointers are left in elements, so it is important to always initialize list
heads and list elements, as well as elements that are removed from a list if
they are not immediately freed, so that their deletion is idempotent. A rule of
thumb is that a list pointer's validity never has to be checked, it is always
valid to dereference it. A lot of complex bugs have been caused in the past by
incorrect list manipulation, such as an element being deleted twice, resulting
in damaging previously adjacent elements' neighbours. This usually has serious
consequences at locations that are totally different from the one of the bug,
and that are only detected much later, so it is required to be particularly
strict on using lists safely.

The lists are not thread-safe, but mt_lists may be used instead.


2. API description
------------------

A list is defined like this, both for the list's head, and for any other
element:

    struct list {
        struct list *n;    /* next */
        struct list *p;    /* prev */
    };

An empty list points to itself for both pointers. I.e. a list's head is both
its own successor and its own predecessor. This guarantees that insertions
and deletions can be done without any check and that deletion is idempotent.
For this reason and by convention, a detached element ought to be represented
like an empty head.

Lists are manipulated using a set of macros which are used to initialize, add,
remove, or iterate over elements. Most of these macros are extremely simple and
are not even protected against multiple evaluation, so it is fundamentally
important that the expressions used in the arguments are idempotent and that
the result does not depend on the evaluation order of the arguments.

Macro   Description

ILH
        Initialized List Head : this is a non-NULL, non-empty list element used
        to prevent the compiler from moving an empty list head declaration to
        BSS, typically when it appears in an array of keywords Without this,
        some older versions of gcc tend to trim all the array and cause
        corruption.

LIST_INIT(l)
        Initialize the list as an empty list head

LIST_HEAD_INIT(l)
        Return a valid initialized empty list head pointing to this
        element. Essentially used with assignments in declarations.

LIST_INSERT(l, e)
        Add an element at the beginning of a list and return it

LIST_APPEND(l, e)
        Add an element at the end of a list and return it

LIST_SPLICE(n, o)
        Add the contents of a list <o> at the beginning of another list <n>.
        The old list head remains untouched.

LIST_SPLICE_END_DETACHED(n, o)
        Add the contents of a list whose first element is is <o> and last one
        is <o->p> at the end of another list <n>. The old list DOES NOT have
        any head here.

LIST_DELETE(e)
        Remove an element from a list and return it. Safe to call on
        initialized elements, but will not change the element itself so it is
        not idempotent. Consider using LIST_DEL_INIT() instead unless called
        immediately after a free().

LIST_DEL_INIT(e)
        Remove an element from a list, initialize it and return it so that a
        subsequent LIST_DELETE() is safe. This is faster than performing a
        LIST_DELETE() followed by a LIST_INIT() as pointers are not reloaded.

LIST_ELEM(l, t, m)
        Return a pointer of type <t> to a structure containing a list head
        member called <m> at address <l>. Note that <l> can be the result of a
        function or macro since it's used only once.

LIST_ISEMPTY(l)
        Check if the list head <l> is empty (=initialized) or not, and return
        non-zero only if so.

LIST_INLIST(e)
        Check if the list element <e> was added to a list or not, thus return
        true unless the element was initialized.

LIST_INLIST_ATOMIC(e)
        Atomically check if the list element's next pointer points to anything
        different from itself, implying the element should be part of a
        list. This usually is similar to LIST_INLIST() except that while that
        one might be instrumented using debugging code to perform further
        consistency checks, the macro below guarantees to always perform a
        single atomic test and is safe to use with barriers.

LIST_NEXT(l, t, m)
        Return a pointer of type <t> to a structure following the element which
	contains list head <l>, which is known as member <m> in struct <t>.

LIST_PREV(l, t, m)
        Return a pointer of type <t> to a structure preceding the element which
        contains list head <l>, which is known as member <m> in struct <t>.
        Note that this macro is first undefined as it happened to already exist
        on some old OSes.

list_for_each_entry(i, l, m)
        Iterate local variable <i> through a list of items of type "typeof(*i)"
        which are linked via a "struct list" member named <m>. A pointer to the
        head of the list is passed in <l>. No temporary variable is needed.
        Note that <i> must not be modified during the loop.

list_for_each_entry_from(i, l, m)
        Same as list_for_each_entry() but starting from current value of <i>
        instead of the list's head.

list_for_each_entry_from_rev(i, l, m)
        Same as list_for_each_entry_rev() but starting from current value of <i>
        instead of the list's head.

list_for_each_entry_rev(i, l, m)
       Iterate backwards local variable <i> through a list of items of type
        "typeof(*i)" which are linked via a "struct list" member named <m>. A
        pointer to the head of the list is passed in <l>. No temporary variable
        is needed. Note that <i> must not be modified during the loop.

list_for_each_entry_safe(i, b, l, m)
        Iterate variable <i> through a list of items of type "typeof(*i)" which
        are linked via a "struct list" member named <m>. A pointer to the head
        of the list is passed in <l>. A temporary backup variable <b> of same
        type as <i> is needed so that <i> may safely be deleted if needed. Note
        that it is only permitted to delete <i> and no other element during
        this operation!

list_for_each_entry_safe_from(i, b, l, m)
        Same as list_for_each_entry_safe() but starting from current value of
        <i> instead of the list's head.

list_for_each_entry_safe_from_rev(i, b, l, m)
        Same as list_for_each_entry_safe_rev() but starting from current value
        of <i> instead of the list's head.

list_for_each_entry_safe_rev(i, b, l, m)
        Iterate backwards local variable <i> through a list of items of type
        "typeof(*i)" which are linked via a "struct list" member named <m>. A
        pointer to the head of the list is passed in <l>. A temporary variable
        <b> of same type as <i> is needed so that <i> may safely be deleted if
        needed. Note that it is only permitted to delete <i> and no other
        element during this operation!

3. Notes
--------

- This API is quite old and some macros are missing. For example there's still
  no list_first() so it's common to use LIST_ELEM(head->n, ...) instead. Some
  older parts of the code also used to rely on list_for_each() followed by a
  break to stop on the first element.

- Some parts were recently renamed because LIST_ADD() used to do what
  LIST_INSERT() currently does and was often mistaken with LIST_ADDQ() which is
  what LIST_APPEND() now is. As such it is not totally impossible that some
  places use a LIST_INSERT() where a LIST_APPEND() would be desired.

- The structure must not be modified at all (even to add debug info). Some
  parts of the code assume that its layout is exactly this one, particularly
  the parts ensuring the casting between MT lists and lists.