delta-find: use a single snapshot cache when applying a group to an object This will avoid walking the revlog over and over again in some situations. The difference is hard to show in our current benchmark suite, as the gain is lower than their overall instability.

delta-find: make sure we only use newer full snapshot as candidate The current code does not needs to protect against this, as there are no older snapshot in the current cache. However as we are getting ready to reuse this cache from one revision to another, we need the code to protect itself about what's coming.

delta-find: use sets instead of list in the snapshot cache This seems more appropriate.

delta-find: use a smarter object for snapshot caching This open the way for a longer lived cache.

find-delta: pass the cache-delta usage policy alongside the cache-delta The idea is to give higher level code more control to what will happens with the cache delta passed. This should help with controling how we treat delta's from different sources. The final goal of this change is to allow for server modes where the client can blindly accept any server delta without regards to any local constraints. This will be implemented in later changesets.

find-delta: move most of the debug-find-delta code in the debug module Lets us that module more. It will help us to keep revlog implementation details close to each other.

find-delta: minor preparatory change We are about to add more item in the cachedelta object, so lets access the item by index instead of doing a full extensions.

find-delta: rename _isgooddeltainfo Lets move to a more readable name now that we are allowed to. This cannot hurt.

test-revlog-raw: drop the overwrite of dead code The revlog class no longer have a _isgooddeltainfo method for a long time. So overwriting it does not get us anything. The test have been wrapping the right code since then anyway.

emitrevision: consider ancestors revision to emit as available base This should make more delta base valid. This notably affects: * case where we skipped some parent with empty delta to directly delta against an ancestors * case where an intermediate snapshots is stored. This change means we could sent largish intermediate snapshots over the wire. However this is actually a sub goal here. Sending snapshots over the wire means the client have a high odd of simply storing the pre-computed delta instead of doing a lengthy process that will… end up doing the same intermediate snapshot. In addition the overall size of snapshot (or any level) is "only" some or the overall delta size. (0.17% for my mercurial clone, 20% for my clone of Mozilla try). So Sending them other the wire is unlikely to change large impact on the bandwidth used. If we decide that minimising the bandwidth is an explicit goal, we should introduce new logic to filter-out snapshot as delta. The current code has no notion explicite of snapshot so far, they just tended to fall into the wobbly filtering options. In some cases, this patch can yield large improvement to the bundling time: ### data-env-vars.name = mozilla-try-2019-02-18-zstd-sparse-revlog # benchmark.name = perf-bundle # benchmark.variants.revs = last-100000 before: 68.787066 seconds after: 47.552677 seconds (-30.87%) That translate to large improvement to the pull time : ### data-env-vars.name = mozilla-try-2019-02-18-zstd-sparse-revlog # benchmark.name = pull # benchmark.variants.issue6528 = disabled # benchmark.variants.revs = last-100000 before: 142.186625 seconds after: 75.897745 seconds (-46.62%) No significant negative impact have been observed.