reftable: memory optimizations for reflog iteration

Hi,

this patch series does the same as all the preceding patch series that optimized how the reftable library iterates through refs, but for reflogs instead.

The goal of this patch series is to arrive at a constant number of allocations when iterating refs. This is achieved in mostly the same way we did it for ref iteration, namely by reusing already-allocated memory. Overall, this brings us down from 8 allocations per reflog record to essentially 0 allocations per reflog. Iterating through 1 million reflogs with git reflog list thus goes down from 8.068m allocations to only around 68.5k.

This series is built on top of "master" at b387623c12 (The third batch, 2024-03-01) with Junio's "ps/reftable-iteration-perf-part2" at 43f70eaea0 (refs/reftable: precompute prefix length, 2024-03-04) merged into it.

Patrick

Patrick Steinhardt (7):
refs/reftable: reload correct stack when creating reflog iter reftable/record: convert old and new object IDs to arrays reftable/record: avoid copying author info reftable/record: reuse refnames when decoding log records reftable/record: reuse message when decoding log records reftable/record: use scratch buffer when decoding records refs/reftable: track last log record name via strbuf

 refs/reftable-backend.c    |  52 +++++----------
 reftable/block.c           |   4 +-
 reftable/block.h           |   2 +
 reftable/merged_test.c     |  11 ++--
 reftable/readwrite_test.c  |  62 +++++++-----------
 reftable/record.c          | 129 ++++++++++++++-----------------------
 reftable/record.h          |   5 +-
 reftable/record_test.c     |  68 ++++++++++---------
 reftable/reftable-record.h |   6 +-
 reftable/stack_test.c      |  26 ++++----
 10 files changed, 154 insertions(+), 211 deletions(-)

2.44.0