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reftable/stack: use geometric table compaction
To reduce the number of on-disk reftables, compaction is performed. Contiguous tables with the same binary log value of size are grouped into segments. The segment that has both the lowest binary log value and contains more than one table is set as the starting point when identifying the compaction segment. Since segments containing a single table are not initially considered for compaction, if the table appended to the list does not match the previous table log value, no compaction occurs for the new table. It is therefore possible for unbounded growth of the table list. This can be demonstrated by repeating the following sequence: git branch -f foo git branch -d foo Each operation results in a new table being written with no compaction occurring until a separate operation produces a table matching the previous table log value. Instead, to avoid unbounded growth of the table list, the compaction strategy is updated to ensure tables follow a geometric sequence after each operation by individually evaluating each table in reverse index order. This strategy results in a much simpler and more robust algorithm compared to the previous one while also maintaining a minimal ordered set of tables on-disk. When creating 10 thousand references, the new strategy has no performance impact: Benchmark 1: update-ref: create refs sequentially (revision = HEAD~) Time (mean ± σ): 26.516 s ± 0.047 s [User: 17.864 s, System: 8.491 s] Range (min … max): 26.447 s … 26.569 s 10 runs Benchmark 2: update-ref: create refs sequentially (revision = HEAD) Time (mean ± σ): 26.417 s ± 0.028 s [User: 17.738 s, System: 8.500 s] Range (min … max): 26.366 s … 26.444 s 10 runs Summary update-ref: create refs sequentially (revision = HEAD) ran 1.00 ± 0.00 times faster than update-ref: create refs sequentially (revision = HEAD~) Some tests in `t0610-reftable-basics.sh` assert the on-disk state of tables and are therefore updated to specify the correct new table count. Since compaction is more aggressive in ensuring tables maintain a geometric sequence, the expected table count is reduced in these tests. In `reftable/stack_test.c` tests related to `sizes_to_segments()` are removed because the function is no longer needed. Also, the `test_suggest_compaction_segment()` test is updated to better showcase and reflect the new geometric compaction behavior. Signed-off-by: Justin Tobler <jltobler@gmail.com>
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