Re: Bug: git branch --unset-upstream command can nuke config when disk is full.

On Wed, Sep 13, 2017 at 03:38:52PM +0200, demerphq wrote:

I just double checked the terminal history and this is all i saw:

$ git status
On branch yves/xxx
Your branch is based on 'origin/yves/xxx', but the upstream is gone.
  (use "git branch --unset-upstream" to fixup)

nothing to commit, working tree clean
$ git branch --unset-upstream
fatal: could not unset 'branch.yves/xxx.merge'
$ git status
On branch yves/xxx
nothing to commit, working tree clean
$ git fetch
fatal: No remote repository specified.  Please, specify either a URL or a
remote name from which new revisions should be fetched.

As a side note, I'm surprised that commands work at all when your
.git/config is empty. I'd expect check_respository_format() to complain
that you are not in a repository.  Looks like it is due to this block:

  452		 /*
  453		  * For historical use of check_repository_format() in git-init,
  454		  * we treat a missing config as a silent "ok", even when nongit_ok
  455		  * is unset.
  456		  */
  457		if (candidate.version < 0)
  458			return 0;

quoted

No, it writes the new content to "config.lock" and then renames it into
place.
All of the write() calls to the temporary file are checked.

I was going to say that perhaps the write was not checked... But if
you are confident they are then...

You're welcome to read over the function to double-check, but I just
looked it over and couldn't find any unchecked writes.

quoted

Given that your output is consistent with it failing to find the key,
and that the result is an empty file, it sounds like somehow the mmap'd
input appeared empty (but neither open nor fstat nor mmap returned an
error). You're not on any kind of exotic filesystem, are you?

I don't think so, but I don't know. Is there a command I can run to check?

BTW, with a bit of faffing I can probably recreate this problem.
Should I try? Is there something I could do during recreation that
would help?

If you think you can reproduce, the output of "strace" on a failing
invocation would be very interesting.

-Peff