Re: git-clone causes out of memory

On Fri, Oct 13, 2017 at 12:26:46PM +0200, Christian Couder wrote:

On Fri, Oct 13, 2017 at 12:06 PM, Mike Hommey [off-list ref] wrote:

quoted

On Fri, Oct 13, 2017 at 12:51:58PM +0300, Constantine wrote:

quoted

There's a gitbomb on github. It is undoubtedly creative and funny, but since
this is a bug in git, I thought it'd be nice to report. The command:

      $ git clone https://github.com/x0rz/ShadowBrokersFiles

What fills memory is actually the checkout part of the command. git
clone -n doesn't fail.

Credit should go where it's due: https://kate.io/blog/git-bomb/
(with the bonus that it comes with explanations)

Yeah, there is a thread on Hacker News about this too:

https://news.ycombinator.com/item?id=15457076

The original repo on GitHub is:

https://github.com/Katee/git-bomb.git

After cloning it with -n, there is the following "funny" situation:

$ time git rev-list HEAD
7af99c9e7d4768fa681f4fe4ff61259794cf719b
18ed56cbc5012117e24a603e7c072cf65d36d469
45546f17e5801791d4bc5968b91253a2f4b0db72

real    0m0.004s
user    0m0.000s
sys     0m0.004s
$ time git rev-list HEAD -- d0/d0/d0/d0/d0/d0/d0/d0/d0/d0/f0

real    0m0.004s
user    0m0.000s
sys     0m0.000s
$ time git rev-list HEAD -- d0/d0/d0/d0/d0/d0/d0/d0/d0/d0

real    0m0.004s
user    0m0.000s
sys     0m0.000s
$ time git rev-list HEAD -- d0/d0/d0/d0/d0/d0/d0/d0/
45546f17e5801791d4bc5968b91253a2f4b0db72

real    0m0.005s
user    0m0.008s
sys     0m0.000s
$ time git rev-list HEAD -- d0/d0/d0/d0/d0/
45546f17e5801791d4bc5968b91253a2f4b0db72

real    0m0.203s
user    0m0.112s
sys     0m0.088s
$ time git rev-list HEAD -- d0/d0/d0/d0/
45546f17e5801791d4bc5968b91253a2f4b0db72

real    0m1.305s
user    0m0.720s
sys     0m0.580s
$ time git rev-list HEAD -- d0/d0/d0/
45546f17e5801791d4bc5968b91253a2f4b0db72

real    0m12.135s
user    0m6.700s
sys     0m5.412s

So `git rev-list` becomes exponentially more expensive when you run it
on a shorter directory path, though it is fast if you run it without a
path.

That's because there are 10^7 files under d0/d0/d0, 10^6 under
d0/d0/d0/d0/, 10^5 under d0/d0/d0/d0/d0/ etc.

So really, this is all about things being slower when there's a crazy
number of files. Picture me surprised.

What makes it kind of special is that the repository contains a lot of
paths/files, but very few objects, because it's duplicating everything.

All the 10^10 blobs have the same content, all the 10^9 trees that point
to them have the same content, all the 10^8 trees that point to those
trees have the same content, etc.

If git wasn't effectively deduplicating identical content, the repository
would be multiple gigabytes large.

Mike