Hi,

Until now, my interest in corrupted repositories has been very
limited.  Just now, the power went out for a second and my UPS failed
me.  As a result, my ~/src/git is completely borked.  For your
amusement, here's a quick session showing me bumbling around:

  $ ~/src/git
  error: object file
.git/objects/8e/6a6dda24b017915449897fcc1353a9b848fd2f is empty
  error: object file
.git/objects/8e/6a6dda24b017915449897fcc1353a9b848fd2f is empty
  fatal: loose object 8e6a6dda24b017915449897fcc1353a9b848fd2f (stored
in .git/objects/8e/6a6dda24b017915449897fcc1353a9b848fd2f) is corrupt
  artagnon|remote-cruft*+:~/src/git$ rm
.git/objects/8e/6a6dda24b017915449897fcc1353a9b848fd2f
  artagnon|remote-cruft*+:~/src/git$ git prune
  artagnon|remote-cruft*+:~/src/git$ git status
  fatal: bad object HEAD
  fatal: bad object HEAD
  artagnon|remote-cruft*+:~/src/git$ git symbolic-ref HEAD refs/heads/master
  artagnon|master*+=:~/src/git$ git status
  ## master
  MM Documentation/git-ls-remote.txt
  MM remote.c
  MM t/t5505-remote.sh
  MM t/t5510-fetch.sh
  MM t/t5515-fetch-merge-logic.sh
  MM t/t5516-fetch-push.sh
  ?? lib/
  ?? outgoing/

That status is completely bogus, by the way.

  artagnon|master*+=:~/src/git$ git reset --hard
  artagnon|master*+=:~/src/git$ git checkout remote-cruft
  fatal: reference is not a tree: remote-cruft
  artagnon|master=:~/src/git$ git reflog
  21ff915 HEAD@{10 minutes ago}: rebase -i (finish): returning to
refs/heads/remote-cruft

What happened to the rest of my reflog?!  Okay, I give up.  Let's go
back to what's present on Github.  I push often, so it's not a
problem.

  artagnon|master=:~/src/git$ git branch -D remote-cruft
  error: Couldn't look up commit object for 'refs/heads/remote-cruft'
  artagnon|master=:~/src/git$ rm .git/refs/heads/remote-cruft
  artagnon|master=:~/src/git$ git checkout -b remote-cruft
  Switched to a new branch 'remote-cruft'

Huh?  What happened to my upstream?

  artagnon|remote-cruft:~/src/git$ git branch -u ram/remote-cruft
  warning: ignoring broken ref refs/remotes/ram/remote-cruft.

Fine, let's fetch.

  artagnon|remote-cruft:~/src/git$ git fetch ram
  remote: Counting objects: 101, done.
  remote: Compressing objects: 100% (24/24), done.
  remote: Total 92 (delta 78), reused 82 (delta 68)
  error: object file
.git/objects/08/2b069c11e8d4f372b963b038cbf5b71a676ef6 is empty
  fatal: loose object 082b069c11e8d4f372b963b038cbf5b71a676ef6 (stored
in .git/objects/08/2b069c11e8d4f372b963b038cbf5b71a676ef6) is corrupt
  fatal: unpack-objects failed

Fine, let's run an fsck and get rid of all the corrupted objects.

  $ git fsck
  error: object file
.git/objects/08/2b069c11e8d4f372b963b038cbf5b71a676ef6 is empty
  error: object file
.git/objects/08/2b069c11e8d4f372b963b038cbf5b71a676ef6 is empty
  fatal: loose object 082b069c11e8d4f372b963b038cbf5b71a676ef6 (stored
in .git/objects/08/2b069c11e8d4f372b963b038cbf5b71a676ef6) is corrupt
  artagnon|remote-cruft:~/src/git$ rm
.git/objects/08/2b069c11e8d4f372b963b038cbf5b71a676ef6
  artagnon|remote-cruft:~/src/git$ git repack
  artagnon|remote-cruft:~/src/git$ git fetch ram
  remote: Counting objects: 101, done.
  remote: Compressing objects: 100% (24/24), done.
  remote: Total 92 (delta 78), reused 82 (delta 68)
  Unpacking objects: 100% (92/92), done.
  error: object file
.git/objects/64/fa33d706658278b871a6e2ca66694efcadacca is empty
  fatal: loose object 64fa33d706658278b871a6e2ca66694efcadacca (stored
in .git/objects/64/fa33d706658278b871a6e2ca66694efcadacca) is corrupt
  error: github.com:artagnon/git did not send all necessary objects

Fine, my packfiles are corrupt.  Let's unpack-objects by hand.

  artagnon|remote-cruft:~/src/git$ mv .git/objects/pack .git/objects/pack.old
  artagnon|remote-cruft+:~/src/git$ for i in
.git/objects/pack.old/*.pack; do git unpack-objects -r <$i; done
  artagnon|remote-cruft:~/src/git$ git fetch ram
  remote: Counting objects: 101, done.
  remote: Compressing objects: 100% (24/24), done.
  remote: Total 92 (delta 78), reused 82 (delta 68)
  Unpacking objects: 100% (92/92), done.
  error: object file
.git/objects/64/fa33d706658278b871a6e2ca66694efcadacca is empty
  fatal: loose object 64fa33d706658278b871a6e2ca66694efcadacca (stored
in .git/objects/64/fa33d706658278b871a6e2ca66694efcadacca) is corrupt
  error: github.com:artagnon/git did not send all necessary objects

  Auto packing the repository for optimum performance. You may also
  run "git gc" manually. See "git help gc" for more information.
  error: bad ref for refs/remotes/ram/remote-cruft
  error: bad ref for refs/remotes/ram/remote-cruft
  Counting objects: 161917, done.
  Delta compression using up to 8 threads.
  Compressing objects: 100% (159963/159963), done.
  Writing objects: 100% (161917/161917), done.
  Total 161917 (delta 117725), reused 0 (delta 0)
  Removing duplicate objects: 100% (256/256), done.
  error: bad ref for refs/remotes/ram/remote-cruft
  Checking connectivity: 161917, done.
  warning: There are too many unreachable loose objects; run 'git
prune' to remove them.

I'm assuming it just went back and fetched everything the second time.
 Why didn't it do that in the first place?

  artagnon|remote-cruft:~/src/git$ git log ram/remote-cruft
  warning: ignoring broken ref refs/remotes/ram/remote-cruft.

Now what?  Why didn't the fetch update this ref?

  artagnon|remote-cruft:~/src/git$ rm .git/refs/remotes/ram/remote-cruft
  artagnon|remote-cruft:~/src/git$ git fetach ram
  remote: Counting objects: 101, done.
  remote: Compressing objects: 100% (24/24), done.
  remote: Total 92 (delta 78), reused 82 (delta 68)
  Unpacking objects: 100% (92/92), done.
  From github.com:artagnon/git
   * [new branch]      remote-cruft -> ram/remote-cruft
   * [new branch]      upstream-fix -> ram/upstream-fix

Yes!  Everything finally works.

Was I being stupid, or is fixing corrupted repositories really this
non-trivial?  Comments appreciated.

Thanks.
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