commit 909af1a43b5a7fed8b5a4ca145e39f46b2f50325
author Stefan Eissing <stefan@eissing.org> Tue Mar 25 09:47:40 2025 +0100
committer Daniel Stenberg <daniel@haxx.se> Thu Apr 17 17:28:38 2025 +0200
tree eb438711e9b5c123a90d5947903eb5e196b8a125
parent 02e9690c3ee2feb7c78c93c1c544c898df733383

multi: do transfer book keeping using mid

Change multi's book keeping of transfers to no longer use lists, but a
special table and bitsets for unsigned int values.

`multi-xfers` is the `uint_tbl` where `multi_add_handle()` inserts a new
transfer which assigns it a unique identifier `mid`. Use bitsets to keep
track of transfers that are in state "process" or "pending" or
"msgsent".

Use sparse bitsets to replace `conn->easyq` and event handlings tracking
of transfers per socket. Instead of pointers, keep the mids involved.

Provide base data structures and document them in docs/internal:
* `uint_tbl`: a table of transfers with `mid` as lookup key,
   handing out a mid for adds between 0 - capacity.
* `uint_bset`: a bitset keeping unsigned ints from 0 - capacity.
* `uint_spbset`: a sparse bitset for keeping a small number of
  unsigned int values
* `uint_hash`: for associating `mid`s with a pointer.

This makes the `mid` the recommended way to refer to transfers inside
the same multi without risk of running into a UAF.

Modifying table and bitsets is safe while iterating over them. Overall
memory requirements are lower as with the double linked list apprach.

Closes #16761