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  2. How can I recover HEAD^'S tree?

How can I recover HEAD^'S tree?

由 拈花ヽ惹草 提交于 2019-12-04 07:47:21
Andrew C

Try git fetch-pack to recover missing objects available from another repository. Instructions below.

For recovery of unpushed commits, specifically HEAD^1 I would start with 

git diff-tree -r HEAD~2^{tree} HEAD^{tree}

You'll get a list of all trees/blobs that have changed and their SHAs (which would include the changes from both HEAD and HEAD^1). Depending on how much information is available you may be able to recreate some of all of the missing tree. Missing blobs are more problematic though.

Use of git fetch-pack

Intentionally corrupt repository

me@myvm:/scratch/corrupt/.git  (GIT_DIR!)$ cd objects/
me@myvm:/scratch/corrupt/.git/objects  (GIT_DIR!)$ ll
total 20
drwxrwxr-x 2 andrewc warp 4096 Oct  7 06:03 20
drwxrwxr-x 2 andrewc warp 4096 Oct  7 06:03 22
drwxrwxr-x 2 andrewc warp 4096 Oct  7 06:03 25
drwxrwxr-x 2 andrewc warp 4096 Oct  7 06:03 info
drwxrwxr-x 2 andrewc warp 4096 Oct  7 06:03 pack
me@myvm:/scratch/corrupt/.git/objects  (GIT_DIR!)$ rm -rf 22

Verify head in bad state

me@myvm:/scratch/corrupt/.git/objects  (GIT_DIR!)$ cd ../../
me@myvm:/scratch/corrupt  (master)$ git status
fatal: bad object HEAD

recover missing objects 

me@myvm:/scratch/corrupt  (master)$ git fetch-pack --all $(git config --get remote.origin.url)
error: refs/heads/master does not point to a valid object!
error: refs/remotes/origin/HEAD does not point to a valid object!
error: refs/remotes/origin/master does not point to a valid object!
error: refs/heads/master does not point to a valid object!
error: refs/remotes/origin/HEAD does not point to a valid object!
error: refs/remotes/origin/master does not point to a valid object!
remote: Counting objects: 3, done.
remote: Total 3 (delta 0), reused 0 (delta 0)
Unpacking objects: 100% (3/3), done.
22ecde746be79c65b27a5cf1dc421764d8ff6e17 HEAD
22ecde746be79c65b27a5cf1dc421764d8ff6e17 refs/heads/master
me@myvm:/scratch/corrupt  (master)$ git status
On branch master
Your branch is up-to-date with 'origin/master'.
nothing to commit, working directory clean

missing objects restored 

me@myvm:/scratch/corrupt  (master)$ ll .git/objects/
total 20
drwxrwxr-x 2 andrewc warp 4096 Oct  7 06:03 20
drwxrwxr-x 2 andrewc warp 4096 Oct  7 06:05 22
drwxrwxr-x 2 andrewc warp 4096 Oct  7 06:03 25
drwxrwxr-x 2 andrewc warp 4096 Oct  7 06:03 info
drwxrwxr-x 2 andrewc warp 4096 Oct  7 06:03 pack
me@myvm:/scratch/corrupt  (master)$ 


me@myvm:/scratch/corrupt  (master)$ git status
On branch master
Your branch is up-to-date with 'origin/master'.
nothing to commit, working directory clean

If you end up in a state where you can find a broken tree object and a broken blob object you can manually recover those. You can git cat-file -p BLOB_SHA for any blob, this will dump the contents. If you can figure out by looking at the contents what the file is that can help you recover the file. Likewise git cat-file -p TREE_SHA will dump the tree, which tells you file names and blob SHAs. At this point you would be attempting to manually construct tree and commit objects from presumably partial data. If your HEAD commit is OK then you are only missing history and should at least have the most recent state covered.

So my assumption is that I'm merely missing some information that can be recovered from GitHub.

Generally true, but it helps if you can identify from where exactly that broken link comes from.

That is what will propose Git 2.10 (Q3 2016) with:

git fsck --name-objects

See commit 90cf590, commit 1cd772c, commit 7b35efd, commit 993a21b (17 Jul 2016) by Johannes Schindelin (dscho).
(Merged by Junio C Hamano -- gitster -- in commit 9db3979, 25 Jul 2016)

fsck: optionally show more helpful info for broken links

When "git fsck" reports a broken link (e.g. a tree object contains a blob that does not exist), both containing object and the object that is referred to were reported with their 40-hex object names.
The command learned the "--name-objects" option to show the path to the containing object from existing refs (e.g. "HEAD~24^2:file.txt").

标签
git
data-recovery
git-fsck
git-plumbing
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