Re: [RFC/PATCH] Fix git-diff --cached to not error out if HEAD points to a nonexistant branch

From: Junio C Hamano <junkio@cox.net>
Date: Feb 24, 2007, 21:03 UTC
Message-ID: <7vvehrw9mz.fsf@assigned-by-dhcp.cox.net>
In-Reply-To: <20070224172037.GA31963@xp.machine.xx>

Peter Baumann <waste.manager@gmx.de> writes:

Show 8 quoted lines

> The documentation mentions "git-diff --cached" to see what is staged for
> the next commit. But this failes if you haven't done any commits yet.
> So lets fix it.
> ...
> ...  I am
> not sure if this is the right fix and/or if git-diff-index
> should also be fixed. I decided against it and let the core
> cmd git-diff-index stay as it is now.

I think you decision here is a correct one. The plumbing level command git-diff-index should error out if you do not give a tree to compare against.

My preference for 'git-diff --cached' issue is to fix the explanation. Clearly document that --cached is to review the difference between any commit (we could even be more precise to say any tree, but I think we should say commit here, as the description is at the end-user level) and what is staged for the commit that will be created with your next 'git-commit'. For convenience it defaults to 'HEAD', the latest commit on your current branch, because that is what people would do most often.

Until you have a commit at HEAD, there really is nothing to diff against. I think "foo is a new entry, no comparison available." is one of the very few things that CVS got right.

The bug in the current code is that we do not check if that HEAD is sensible when we add it as the default commit to compare with. The error message coming out of the low-level diff-index code might be sensible if that 'HEAD' were what the user actually gave us, but clearly not the right error message in this case.

---

 builtin-diff.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)

diff --git a/builtin-diff.c b/builtin-diff.c
index c387ebb..67f4932 100644
--- a/builtin-diff.c
+++ b/builtin-diff.c
@@ -261,6 +261,8 @@ int cmd_diff(int argc, const char **argv, const char *prefix)
 				break;
 			else if (!strcmp(arg, "--cached")) {
 				add_head(&rev);
+				if (!rev.pending.nr)
+					die("No HEAD commit to compare with (yet)");
 				break;
 			}
 		}