Re: [BUG] `git push` sends unnecessary objects

On Wed, Sep 13, 2023 at 11:59:35PM +0100, Javier Mora wrote:

I came across this issue accidentally when trying to move a directory
containing a very large file, and deleting another file in that
directory while I was at it.
It seems to be caused by `pack.useSparse=true` being the default since
v2.27 (which I found out after spending quite a while manually
bisecting and compiling git since I noticed that this didn't happen in
v2.25; commit de3a864 introduces this regression).

* Expected:
    Pushing a commit that moves a file without modifying it shouldn't
require sending a blob object for that file, since the remote server
already has that blob object.
* Observed:
    Pushing a commit that moves a directory containing a file and also
adds/deletes other files in that directory will for some reason also
send blobs for all the files in that directory, even the ones that
were already in the remote.
* Consequences:
    This has a very big impact in push times for very small commits
that just move around files, if those files are very big (I had this
happen with a >100MB file over a problematic connection... yikes!)
* Note:
    The commit introducing the regression does warn about possible
scenarios involving a special arrangement of exact copies across
directories, but these are not "copies", I just moved a file, which
seems like a rather common operation.

Code snippet for reproduction:

mkdir TEST_git
cd TEST_git

mkdir -p local remote/origin.git
cd remote/origin.git
git init --bare
cd ../../local
git init
git remote add origin file://"${PWD%/*}"/remote/origin.git

mkdir zig
for i in a b c d e; do
    dd if=/dev/urandom of=zig/"$i" bs=1M count=1
done
git add .
git commit -m 'Add big files'
git push -u origin master
#>> Writing objects: 100% (8/8), 5.00 MiB | 13.27 MiB/s, done.
#^ makes sense: 1 commit + 2 trees (/ and /zig) + 5 files = 8;
#  5 MiB in total for the 5x 1 MiB binary files

git mv zig zag
git commit -m 'Move zig'
git push
#>> Writing objects: 100% (2/2), 233 bytes | 233.00 KiB/s, done.
#^ makes sense: 1 commit + 1 tree (/ renames /zig to /zag) = 2;
#  a,b,c,d,e objects already in remote

git mv zag zog
touch zog/f
git add zog/f
git commit -m 'For great justice'
git push
#>> Writing objects: 100% (9/9), 5.00 MiB | 24.63 MiB/s, done.
#^ It re-uploaded the 5x 1 MiB blobs
#  even though remote already had them.

Note that the latter doesn't happen if I use `git -c pack.useSparse=false push`.

I can reproduce this regression on v2.42.0 (self-compiled) on my Debian
testing system.

Cc'ing Derrick and Junio.

Thanks for the report!

-- 
An old man doll... just what I always wanted! - Clara