Re: Incremental 'git fetch' downloaded everything again

SZEDER Gábor [off-list ref] writes:

I usually try to fetch from https://github.com/git/git daily, and
today morning something unusual happened:

  $ git fetch
  remote: Enumerating objects: 406099, done.
  remote: Counting objects: 100% (1229/1229), done.
  remote: Compressing objects: 100% (1044/1044), done.
  remote: Total 406093 (delta 207), reused 1189 (delta 185), pack-reused 404864 (from 2)
  Receiving objects: 100% (406093/406093), 292.22 MiB | 4.62 MiB/s, done.
  Resolving deltas: 100% (308943/308943), done.
  From https://github.com/git/git
   + ffe2b816f5...106a830b98 jch        -> origin/jch  (forced update)
     e9019fcafe..f85a7e6620  master     -> origin/master
   + c42f45431d...00534a21ce next       -> origin/next  (forced update)
   + f6884212b2...73452939f9 seen       -> origin/seen  (forced update)

The only difference I can think of is that 'next' has been rewound
recently, but because 'seen' and 'jch' are constantly reound, it
would be very strange that it made such a big difference.

So, sorry, no idea.  If I were bug-hunting this, I would first try
to eliminate whatever GitHub runs on their server end from the
picture.

Note that it downloaded over 400k objects in an almost 300MB packfile.

Looking at the objects that I already had and the objects in the newly
downloaded packfile:

  $ git rev-list --objects origin/master@{1} origin/next@{1} origin/jch@{1} origin/seen@{1} | cut -d' ' -f1 | sort >existing-objects
  $ git verify-pack -v .git/objects/pack/pack-080fedc9c19f711dd1b22103b382ede8925b90a6.idx | sed -n -E -e 's/^([0-9a-f]{40}) .*/\1/p' | sort >received-objects
  $ wc -l existing-objects received-objects 
    406954 existing-objects
    406093 received-objects
    813047 total
  $ git diff --no-index --stat existing-objects received-objects
   existing-objects => received-objects | 3615 +++++++++++++---------------------
   1 file changed, 1377 insertions(+), 2238 deletions(-)

The vast majority of objects were already available locally.

What's going on?!

This might be a recurring issue: I remember a similar large download
from 2 or 3 weeks ago, but back then I didn't have time to investigate
or to report.
I tried to reproduce this issue by attempting to recreate the state of
my git repository from yesterday in a new repo, but no luck, 'git
fetch' only downloads what's necessary.

I use a Git version based on next, with a bunch of my own patches on
top, but none of them has anything to do with object transfer and I've
been using most of them for years.  I don't have any config set under
'fetch.*' or 'transfer.*'.