Add an update_index to every reference in a reftable, storing the
exact transaction that last modified the reference. This is necessary
to fix some merge race conditions.
Consider updates at T1, T3 are present in two reftables. Compacting
these will create a table with range [T1,T3]. If T2 arrives during
or after the compaction its impossible for readers to know how to
merge the [T1,T3] table with the T2 table.
With an explicit update_index per reference, MergedReftable is able to
individually sort each reference, merging individual entries at T3
from [T1,T3] ahead of identically named entries appearing in T2.
Change-Id: Ie4065d4176a5a0207dcab9696ae05d086e042140
Some repositories contain a lot of references (e.g. android at 866k,
rails at 31k). The reftable format provides:
- Near constant time lookup for any single reference, even when the
repository is cold and not in process or kernel cache.
- Near constant time verification a SHA-1 is referred to by at least
one reference (for allow-tip-sha1-in-want).
- Efficient lookup of an entire namespace, such as `refs/tags/`.
- Support atomic push `O(size_of_update)` operations.
- Combine reflog storage with ref storage.
Change-Id: I29d0ff1eee475845660ac9173413e1407adcfbf2
Han-Wen Nienhuys