Re: git fsck: unreachable vs. dangling

[Junio C Hamano <gitster@pobox.com>]

On Tue, Apr 14, 2015 at 12:16 AM, Sebastian Schuberth
<sschuberth@gmail.com> wrote:
> Hi,
>
> reading through the fsck docs [1] I'm having a hard time understanding
> what the difference between "unreachable" and "dangling" objects are.
>
> By example, suppose I have a commit A that is the tip of exactly one
> branch (and no tag or other ref points to A). If I delete that branch,
> is A now dangling, or unreachable, or both?

Suppose that branch consists of two commits, A and A^.
When you lose that branch (git branch -D that-branch),
both A and A^ become unreachable. So are trees and
blobs that appear only in A and A^ and nowhere else;
they are also unreachable.

A dangling object is an unreachable object that cannot be
made reachable by any way other than pointing at it
directly with a ref. A^ is not dangling, because you can
make it reachable by pointing A (the tip of the original
branch you just lost) with a ref. A on the other hand is
dangling (if you had a tag object that points at A that
you lost, then A is merely unreachable but not dangling,
because you can point at that tag with a ref and make A
reachable).