[PATCH] odb: do not list our own refs as alternate refs in the quarantine

From: Kristofer Karlsson <krka@spotify.com>

Since 722ff7f876 (receive-pack: quarantine objects until pre-receive accepts, 2016-10-03), receive-pack writes incoming objects to a temporary object directory. Child processes see it as their primary object directory, and tmp-objdir passes the repository's own object directory to them via GIT_ALTERNATE_OBJECT_DIRECTORIES.

The connectivity check passes --alternate-refs to rev-list since 39b44ba771 (check_everything_connected: assume alternate ref tips are valid, 2019-07-01). Inside the quarantine, the repository's own object directory is one of the alternates, so --alternate-refs runs for-each-ref on the repository itself and lists all of its refs.

That includes hidden refs. bcec6780b2 (receive-pack: only use visible refs for connectivity check, 2022-11-17) added --exclude-hidden=receive to the check so that repositories with many hidden refs do not have to load all of them on each push. The hidden refs are excluded from --all, but they all come back through --alternate-refs.

Skip an alternate whose repository is our own when listing alternate refs. This is done by comparing paths, which is a hack: it is mainly here to measure the cost of the bug, and a proper fix would have the quarantine tell its children which directory is ours. It is safe, though: alternate refs only serve as negative tips that are assumed to be complete, so dropping some can only make the traversal walk further, never make it accept a missing object.

Add a test that our own refs are no longer listed, a test that the refs of a real alternate still are, and a perf test pushing a single commit into a repository with a visible branch and about 200,000 refs hidden by receive.hideRefs, and into a copy of it that hides nothing:

    Test                           HEAD~1            HEAD
    -----------------------------------------------------------------------
    5552.2: push, hidden refs      0.31(0.32+0.08)   0.10(0.06+0.03) -67.7%
    5552.3: push, no hidden refs   1.25(1.08+0.68)   1.03(0.84+0.62) -17.6%

With hidden refs, most of the time went to the for-each-ref child that listed the alternate refs. Without hidden refs, every ref was listed twice, once for --all and once for --alternate-refs.

Signed-off-by: Kristofer Karlsson <krka@spotify.com>
---
    receive-pack: connectivity check does not exclude hidden refs in the
    quarantine
    
    This is mainly a bug report, in patch form so that it comes with a test
    and numbers. I don't expect it to be merged, at least not in this form.
    
    When a repository hides refs with receive.hideRefs, every push still
    loads all of the hidden refs in the connectivity check, even though
    bcec6780b2 (receive-pack: only use visible refs for connectivity check,
    2022-11-17) excludes them from it. With many hidden refs, that is most
    of the time spent on a small push.
    
    I stumbled across this while working on the boundary walk of the
    connectivity check (the follow-up to kk/connectivity-check-incremental),
    when the walk loaded every ref during a push even though it should only
    have looked at a few of them.
    
    The cause is that receive-pack runs the check inside the object
    quarantine, and there the repository's own object directory is passed to
    rev-list as an alternate. --alternate-refs then lists all of the
    repository's own refs, hidden ones included. Without hidden refs, every
    ref is listed twice instead. Setting core.alternateRefsPrefixes to the
    visible namespaces works around it, though that also limits which refs
    of real alternates are used.
    
    The patch includes a small fix by comparing paths, mostly so that the
    perf test can show the difference (about 0.31s -> 0.10s for a push with
    200,000 hidden refs). I am not really happy with it as a proper fix
    though.
    
    I also tried resolving the object directories that --alternate-refs uses
    before the quarantine is created. That got a bit messy: it needed a new
    internal rev-list option, and receive-pack had to carry the list of
    directories through to each of its connectivity checks. And it only
    fixes the check itself; other processes running inside the quarantine,
    like pre-receive hooks, would still see the repository as its own
    alternate.
    
    So the question is really where this should be fixed: is there a better
    way for processes running inside the quarantine to tell the repository's
    own object directory apart from real alternates?
    
    I am likely not the best person to answer that. Patrick, maybe you could
    take a look, since you have recently reworked tmp-objdir and the odb
    sources?

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-2285%2Fspkrka%2Fquarantine-altrefs-bugreport-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-2285/spkrka/quarantine-altrefs-bugreport-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/2285

 odb.c                            | 16 ++++++++++++++
 t/perf/p5552-push-hidden-refs.sh | 37 ++++++++++++++++++++++++++++++++
 t/t5547-push-quarantine.sh       | 25 +++++++++++++++++++++
 3 files changed, 78 insertions(+)
 create mode 100755 t/perf/p5552-push-hidden-refs.sh