From: Avery Pennarun <apenwarr@gmail.com>
Date: Thu, 17 Sep 2009 21:40:12 GMT
Subject: Re: [PATCH JGIT] Circular references shouldn't be created
Message-ID: <32541b130909171440w1a6d2394t4acc6a2f791c143@mail.gmail.com>
In-Reply-To: <C89280B882467443A695734861B942B28759DB95@DEWDFECCR09.wdf.sap.corp>

On Thu, Sep 17, 2009 at 3:23 PM, Sohn, Matthias <matthias.sohn@sap.com> wrote:
>        void link(final String name, final String target) throws IOException {
> +               if (name.equals(target))
> +                       throw new IllegalArgumentException(
> +                                       "illegal circular reference : symref " + name
> +                                                       + " cannot refer to " + target);

This isn't a very thorough fix.  It doesn't catch longer loops, like

    HEAD -> chicken -> HEAD

or

   a -> b -> c -> d -> a

Experimenting with original git.git's implementation, I see that this
is allowed:

   git symbolic-ref refs/heads/boink refs/heads/boink

It succeeds and creates a file that looks like this:

   ref: refs/heads/boink

And "git show-ref refs/heads/boink" says: nothing (but returns an error code).

And "git log refs/heads/boink" says:

   warning: ignoring dangling symref refs/heads/boink.
   fatal: ambiguous argument 'refs/heads/boink': unknown revision or
path not in the working tree.
   Use '--' to separate paths from revisions

Clearly, in git.git, symref loops are caught at ref read time, not
write time.  This makes sense, since someone might foolishly twiddle
the repository by hand and you don't want to get into an infinite loop
in that case.  Also, it's potentially useful to allow people to set
invalid symrefs *temporarily*, as part of a multi step process.

Have fun,

Avery