[PATCH v2 0/6] reftable: a couple of improvements for libgit2

Hi,

this small patch series contains a couple of improvements I required for libgit2. With those changes libgit2 is now able to run its full test suite with reftable-enabled repositories. I still need to invest a bit of work to make it memory-leak free and compile on Windows, but overall I think that support for reftables is almost ready.

Changes in v2:
  - Another commit that fixes handling of outdated stacks when doing
    compaction. This issue is hit in libgit2, which has a test that
    performs writes with a couple dozen concurrent threads.
  - Add a link to past discussions around `{0}` vs `{{0}}` as provided
    by Eric.
  - Link to v1: https://lore.kernel.org/r/20250801-pks-reftable-fixes-for-libgit2-v1-0-f446e1c33cb9@pks.im

Thanks!

Patrick

---
Patrick Steinhardt (6):
      reftable/writer: fix type used for number of records
      reftable/writer: drop Git-specific `QSORT()` macro
      reftable/stack: fix compiler warning due to missing braces
      reftable/stack: reorder code to avoid forward declarations
      reftable/stack: allow passing flags to `reftable_stack_add()`
      reftable/stack: handle outdated stacks when compacting

 refs/reftable-backend.c         |   8 +-
 reftable/reftable-stack.h       |   9 +-
 reftable/reftable-writer.h      |   4 +-
 reftable/stack.c                | 398 ++++++++++++++++++++--------------------
 reftable/writer.c               |  23 ++-
 t/unit-tests/t-reftable-stack.c |  50 ++---
 6 files changed, 253 insertions(+), 239 deletions(-)

Range-diff versus v1:

1:  e8aacccd51 = 1:  c8bba1f88b reftable/writer: fix type used for number of records
2:  b1a331bc78 = 2:  6c20e9a874 reftable/writer: drop Git-specific `QSORT()` macro
3:  1b3556685d ! 3:  c2ed4eab6a reftable/stack: fix compiler warning due to missing braces
    @@ Commit message
     
         Silence this warning by using `{{0}}` instead.
     
    +    Note that we had the discussion around whether or not we want to handle
    +    such errors in the past already [1], where we basically decided that we
    +    do not care about such old-and-buggy compilers. But the reftable library
    +    is a special case because it is used by projects other than Git, and
    +    libgit2 for example hits the above issue in its pipeline. As there is
    +    only a single problematic instance of this issue we do the pragmatic
    +    thing and simply make the compiler happy.
    +
    +    [1]: https://lore.kernel.org/git/20220710081135.74964-1-sunshine@sunshineco.com/T/
    +
         Signed-off-by: Patrick Steinhardt <ps@pks.im>
     
      ## reftable/stack.c ##
4:  c1fb437781 = 4:  0f1bb446ce reftable/stack: reorder code to avoid forward declarations
5:  33dcf7ca70 = 5:  5aaff713dd reftable/stack: allow passing flags to `reftable_stack_add()`
-:  ---------- > 6:  ccfa5b59a0 reftable/stack: handle outdated stacks when compacting

--- base-commit: e813a0200a7121b97fec535f0d0b460b0a33356c change-id: 20250801-pks-reftable-fixes-for-libgit2-562b959a5603