Re: Multiple pushurls, different 'objects\info\packs'

From: brian m. carlson <sandals@crustytoothpaste.net>
Date: Apr 22, 2020, 03:14 UTC
Message-ID: <20200422031455.GG6465@camp.crustytoothpaste.net>
In-Reply-To: <CAAihzVJTN5V3o_aFeUuJH_8+nqZrKHJww3vFQ9K5W0DtbeM14w@mail.gmail.com>

On 2020-04-21 at 18:07:42, Mikhail Strelnikov wrote:

Show 40 quoted lines

> Hi,
> 
> I have a repo with two pushurls configured like this:
> 
> 
> C:\folder\1>git init --bare
> Initialized empty Git repository in C:/folder/1/
> 
> C:\folder\2>git init --bare
> Initialized empty Git repository in C:/folder/2/
> 
> C:\folder\w>git init
> Initialized empty Git repository in C:/folder/w/.git/
> 
> C:\folder\w>git add work.txt
> 
> C:\folder\w>git commit -m "Initial commit"
> [master (root-commit) 1b314f3] Initial commit
> 1 file changed, 1 insertion(+)
> create mode 100644 work.txt
> 
> C:\folder\w>git remote add origin C:\folder\1
> 
> C:\folder\w>git remote set-url origin --push --add C:\folder\1
> 
> C:\folder\w>git remote set-url origin --push --add C:\folder\2
> 
> C:\folder\w>git push --set-upstream origin master
> 
> 
> I would expect those two folders (C:\folder\1 and C:\folder\2) to
> contain exactly the same bytes. And they did for quite some time. But
> now there is a difference in 'objects\info\packs' (and some of
> objects\pack\pack-*.idx/pack are also different).
> 
> (all the commits are the same in both and all my data is also the same
> and 'fast-export --all' yields the same result)
> 
> I'd like to know what might have caused this nondeterminism and if
> there is something to do to prevent that.

You can get nondeterminism because the push to each repository happens independently and delta compression is multithreaded. You can therefore compute different packs on push and get different packs in the result.

You could try to avoid it by disabling threading for pushes, but that has to be done on each client that pushes to them. In general, this is not worth worrying about as long as the data is intact (that is, it passes git fsck) and the refs are identical. It is also not especially easy to avoid, since determinism of pack files is not considered a goal of Git.

Maybe if you tell us a little more about your reason for wanting bit-for-bit identical replicas we can provide some assistance in helping you achieve your goals.

-- 
brian m. carlson: Houston, Texas, US
OpenPGP: https://keybase.io/bk2204