CONTRIBUTING

snapshot: search for unrelated but reusable full-snapshot

# New Strategy Step: Reusing Snapshot Outside Of Parents' Chain.

If no suitable bases were found in the parent's chains, see if we could reuse
a full snapshot not directly related to the current revision. Such search can
be expensive, so we only search for snapshots appended to the revlog *after*
the bases used by the parents of the current revision (the one we just
tested). We assume the parent's bases were created because the previous
snapshots were unsuitable, so there are low odds they would be useful now.

This search gives a chance to reuse a delta chain unrelated to the current
revision. Without this re-use, topological branches would keep reopening new
full chains. Creating more and more snapshots as the repository grow.

In repositories with many topological branches, the lack of delta reuse can
create too many snapshots reducing overall compression to nothing. This
results in a very large repository and other usability issues.

For now, we still focus on creating level-1 snapshots. However, this principle
will play a large part in how we avoid snapshot explosion once we have more
snapshot levels.

# Effects On The Test Repository

In the test repository we created, we can see the beneficial effect of such
reuse. We need very few level-0 snapshots and the overall revlog size has
decreased.

The `hg debugrevlog` call, show a "lvl-2" snapshot. It comes from the existing
delta logic using the `prev` revision (revlog's tip) as the base. In this
specific case, it turns out the tip was a level-1 snapshot. This is a
coincidence that can be ignored.

Finding and testing against all these unrelated snapshots can have a
performance impact at write time. We currently focus on building good deltas
chain we build. Performance concern will be dealt with later in another
series.

Our full contribution guidelines are in our wiki, please see:

https://www.mercurial-scm.org/wiki/ContributingChanges

If you just want a checklist to follow, you can go straight to

https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist

If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).