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

On 13 September 2017 at 14:34, Jeff King [off-list ref] wrote:

On Wed, Sep 13, 2017 at 01:59:17PM +0200, demerphq wrote:

quoted

After being away for a while I saw the following message in one of my git repos:

$ 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/simple_projection.merge'

Hrm. I wonder what caused this failure. The error would be in
git_config_set_multivar_in_file_gently(). Most errors there produce
another error message before hitting the die(). In fact, the only case I
see where it would not produce another message is if it found nothing to
unset (but in that case, "branch" would never have called the function
in the first place).

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.

quoted

At this point my .git/config file was empty, and all of my config was lost.

I assume that things that rewrite .git/config do not check for a
successful write before deleting the old version of the file.

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...

The
old data is copied over after having been ready by mmap (which is also
error-checked).

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?

cheers,
Yves

-- 
perl -Mre=debug -e "/just|another|perl|hacker/"