Re: [PATCH v3 4/4] clone: open a shortcut for connectivity check

> In order to make sure the cloned repository is good, we run "rev-list
> --objects --not --all $new_refs" on the repository. This is expensive
> on large repositories. This patch attempts to mitigate the impact in
> this special case.
>
> In the "good" clone case, we only have one pack. If all of the
> following are met, we can be sure that all objects reachable from the
> new refs exist, which is the intention of running "rev-list ...":
>
>  - all refs point to an object in the pack
>  - there are no dangling pointers in any object in the pack
>  - no objects in the pack point to objects outside the pack
>
> The second and third checks can be done with the help of index-pack as
> a slight variation of --strict check (which introduces a new condition
> for the shortcut: pack transfer must be used and the number of objects
> large enough to call index-pack). The first is checked in
> check_everything_connected after we get an "ok" from index-pack.
>
> "index-pack + new checks" is still faster than the current "index-pack
> + rev-list", which is the whole point of this patch. If any of the
> conditions fails, we fall back to the good old but expensive "rev-list

> ..". In that case it's even more expensive because we have to pay for
> the new checks in index-pack. But that should only happen when the
> other side is either buggy or malicious.
>
> Cloning linux-2.6 over file://
>
>         before         after
> real    3m25.693s      2m53.050s
> user    5m2.037s       4m42.396s
> sys     0m13.750s      0m16.574s
>
> A more realistic test with ssh:// over wireless
>
>         before         after
> real    11m26.629s     10m4.213s
> user    5m43.196s      5m19.444s
> sys     0m35.812s      0m37.630s
>
> This shortcut is not applied to shallow clones, partly because shallow
> clones should have no more objects than a usual fetch and the cost of
> rev-list is acceptable, partly to avoid dealing with corner cases when
> grafting is involved.
>
> Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>