From: Ephrim Khong Date: Fri, 11 Jul 2014 09:37:25 GMT Subject: [Bug] data loss with cyclic alternates Message-ID: <53BFB055.206@gmail.com> Hi, git seems to have issues with alternates when cycles are present (repo A has B/objects as alternates, B has A/objects as alternates). In such cases, gc and repack might delete objects that are present in only one of the alternates, leading to data loss. I understand that this is no big use case, and requires manual editing of objects/info/alternates. But for the sake of preventing unneccesary data loss, and since I found no warning regarding alternate cycles in the documentation, it might make sense to handle such cases properly (maybe it's a simple "after finding all alternates directories, remove duplicates"?). Here is a small test case that adds the object storage of a repository as alternate to itsself. After git repack -adl, all objects are deleted. --- rm -rf test_repo && mkdir test_repo && cd test_repo && git init && touch a && git add a && git commit -m "c1" && git repack -adl && echo $PWD/.git/objects > .git/objects/info/alternates && echo ">> re-packing..." && git repack -adl && echo ">> git fsck..." && git fsck --- Output: --- Initialized empty Git repository in /somewhere/test_repo/.git/ [master (root-commit) ab9e123] c1 1 file changed, 0 insertions(+), 0 deletions(-) create mode 100644 a Counting objects: 3, done. Writing objects: 100% (3/3), done. Total 3 (delta 0), reused 0 (delta 0) >> re-packing... Nothing new to pack. error: refs/heads/master does not point to a valid object! >> git fsck... Checking object directories: 100% (256/256), done. Checking object directories: 100% (256/256), done. error: HEAD: invalid sha1 pointer 1494ec24356cbbbd66e19f22cef762dd83de7f54 error: refs/heads/master does not point to a valid object! notice: No default references missing blob e69de29bb2d1d6434b8b29ae775ad8c2e48c5391 --- Thanks - Eph