comparison mercurial/revlogutils/deltas.py @ 39511:e72130f58f5d

snapshot: consider all snapshots in the parents' chains

There are no reasons to only consider full snapshot as a possible base for an
intermediate snapshot.  Now that the basic principles have been set, we can
start adding more levels of snapshots.

We now consider all snapshots in the parent's chains (full or intermediate).
This creates a chain of intermediate snapshots, each smaller than the previous
one.

# Effect On The Test Repository

In the test repository, we can see a decrease in the revlog size and slightly
shorter delta chain.

However, that approach creates snapshots more frequently, increasing the risk
of ending into problematic cases in very branchy repositories (not triggered
by the test repository). The next changesets will remove that risk by adding
logic that increases deltas reuse.

39510:3ca144f1c8dd

        elif len(parents) > 0:
            # Test all parents (1 or 2), and keep the best candidate
            yield parents

    if sparse and parents:
        # See if we can use an existing snapshot in the parent chains to use as
        # a base for a new intermediate-snapshot
        bases = []
        for p in parents:
            bases.append(deltachain(p)[0])
        yield tuple(sorted(bases))
        # No suitable base found in the parent chain, search if any full
        # snapshots emitted since parent's base would be a suitable base for an
        # intermediate snapshot.
        #
        # It give a chance to reuse a delta chain unrelated to the current
        # revisions instead of starting our own. Without such re-use,
        # topological branches would keep reopening new full chains. Creating
        # more and more snapshot as the repository grow.
        snapfloor = min(bases) + 1
        snapshots = collections.defaultdict(list)
        _findsnapshots(revlog, snapshots, snapfloor)
        yield tuple(snapshots[nullrev])

    # other approach failed try against prev to hopefully save us a
    # fulltext.

39511:e72130f58f5d

        elif len(parents) > 0:
            # Test all parents (1 or 2), and keep the best candidate
            yield parents

    if sparse and parents:
        snapshots = collections.defaultdict(list) # map: base-rev: snapshot-rev
        # See if we can use an existing snapshot in the parent chains to use as
        # a base for a new intermediate-snapshot
        #
        # search for snapshot in parents delta chain
        # map: snapshot-level: snapshot-rev
        parents_snaps = collections.defaultdict(set)
        for p in parents:
            for idx, s in enumerate(deltachain(p)):
                if not revlog.issnapshot(s):
                    break
                parents_snaps[idx].add(s)
        # Test them as possible intermediate snapshot base
        # We test them from highest to lowest level. High level one are more
        # likely to result in small delta
        for idx, snaps in sorted(parents_snaps.items(), reverse=True):
            yield tuple(sorted(snaps))
        # No suitable base found in the parent chain, search if any full
        # snapshots emitted since parent's base would be a suitable base for an
        # intermediate snapshot.
        #
        # It give a chance to reuse a delta chain unrelated to the current
        # revisions instead of starting our own. Without such re-use,
        # topological branches would keep reopening new full chains. Creating
        # more and more snapshot as the repository grow.
        snapfloor = min(parents_snaps[0]) + 1
        _findsnapshots(revlog, snapshots, snapfloor)
        yield tuple(snapshots[nullrev])

    # other approach failed try against prev to hopefully save us a
    # fulltext.