On Thu, Jul 25, 2019 at 07:18:57PM -0700, Gregory Szorc wrote:

I think I've found some undesirable behavior with regards to the
behavior of `git gc --auto`. The tl;dr is that a warning message written
to gc.log can result in `git gc --auto` effectively disabling itself for
gc.logExpiry. The problem is easier to trigger in 2.22 as a result of
enabling bitmap indices for bare repositories by default and the
behavior can easily result in performance degradation, especially on
servers.

Yuck, thanks for reporting this.

As you note, this is a special case of a much larger problem. The other
common case is the "oops, you still have a lot of loose objects after
repacking" warning. There's more discussion and some patches here:

  https://public-inbox.org/git/20180716172717.237373-1-jonathantanmy@google.com/

though I don't think any of the work that came out of that fundamentally
solves the issue.

I don't prescribe to know the best way to solve this problem. I just
know it is a footgun sitting in the default Git configuration. And the
footgun became a lot easier to fire with the introduction of warning
messages related to bitmap indices and again when bitmap indices were
enabled by default for bare repositories in Git 2.22.

IMHO one way to mitigate this is to simply warn less. In particular, if
we are auto-enabling bitmaps, then it doesn't necessarily make sense for
us to warn about them being disabled.

In the case of .keep files, we've already got 7328482253 (repack:
disable bitmaps-by-default if .keep files exist, 2019-06-29), which
should be in the next released version of Git. But I suspect that's
racy with respect to somebody creating .keep files, and as you note
there are other config options that might prevent us from generating
bitmaps.

Instead, it may make sense to turn the --write-bitmap-index option of
pack-objects into a tri-state: true/false/auto. Then pack-objects would
know that we are in best-effort mode, and would avoid warning in that
case. That would also let git-repack express its intentions better to
git-pack-objects, so we could replace 7328482253, and keep more of the
logic in pack-objects, which is ultimately what has to make the decision
about whether it can generate bitmaps.

-Peff