Re: Is "show-ref -h" a good test for an empty repository?

From: Jeff King <peff@peff.net>
Date: Sep 8, 2008, 14:42 UTC
Message-ID: <20080908144159.GA10331@coredump.intra.peff.net>
In-Reply-To: <48C52797.3030500@feds.uwaterloo.ca>

On Mon, Sep 08, 2008 at 09:24:39AM -0400, Eric Gerlach wrote:

Show 14 quoted lines

> The case I'm looking to protect against is the following:
>
> $ mkdir -p new-repo/debian
> $ cd new-repo
> $ git init
> $ vi debian/changelog (add a few lines)
> $ git add debian/changelog
> $ debcommit
>
> (which runs
> $ git diff --cached debian/changelog)
>
> If I can test before the git-diff, then I can run "diff debian/changelog  
> /dev/null" instead of the git-diff and all is well.

In that case, I would use the "rev-parse --verify HEAD" I mentioned, since it exactly covers the situation you are trying to avoid. What you are saying is something like:

  if there is a previous commit on the current branch
    diff against previous commit on the current branch
  else
    diff against /dev/null

So your "show-ref" attempt could actually say "yes, the repo is non-empty" but your diff would still fail (because the commits just weren't on the current branch).

As an aside, it's too bad "--root" isn't more widely supported. Then

  git diff --root --cached

would do what you want without the need for any conditionals. But it seems that --root only works between trees. Something like the patch below would help, but I'm sure this is covering only one of many cases.

---

diff --git a/builtin-diff.c b/builtin-diff.c
index 52470c7..43db970 100644
--- a/builtin-diff.c
+++ b/builtin-diff.c
@@ -317,8 +317,11 @@ int cmd_diff(int argc, const char **argv, const char *prefix)
 				break;
 			else if (!strcmp(arg, "--cached")) {
 				add_head_to_pending(&rev);
-				if (!rev.pending.nr)
-					die("No HEAD commit to compare with (yet)");
+				if (!rev.pending.nr) {
+					if (!rev.show_root_diff)
+						die("No HEAD commit to compare with (yet)");
+					add_empty_to_pending(&rev);
+				}
 				break;
 			}
 		}
diff --git a/revision.c b/revision.c
index 2f646de..7ec3990 100644
--- a/revision.c
+++ b/revision.c
@@ -145,16 +145,27 @@ void add_pending_object(struct rev_info *revs, struct object *obj, const char *n
 	add_pending_object_with_mode(revs, obj, name, S_IFINVALID);
 }
 
-void add_head_to_pending(struct rev_info *revs)
+static void add_to_pending_by_name(struct rev_info *revs, const char *name)
 {
 	unsigned char sha1[20];
 	struct object *obj;
-	if (get_sha1("HEAD", sha1))
+	if (get_sha1(name, sha1))
 		return;
 	obj = parse_object(sha1);
 	if (!obj)
 		return;
-	add_pending_object(revs, obj, "HEAD");
+	add_pending_object(revs, obj, name);
+}
+
+void add_head_to_pending(struct rev_info *revs)
+{
+	add_to_pending_by_name(revs, "HEAD");
+}
+
+void add_empty_to_pending(struct rev_info *revs)
+{
+	add_to_pending_by_name(revs,
+			"4b825dc642cb6eb9a060e54bf8d69288fbee4904");
 }
 
 static struct object *get_reference(struct rev_info *revs, const char *name, const unsigned char *sha1, unsigned int flags)
diff --git a/revision.h b/revision.h
index 2fdb2dd..8c990d5 100644
--- a/revision.h
+++ b/revision.h
@@ -152,6 +152,7 @@ extern void add_object(struct object *obj,
 extern void add_pending_object(struct rev_info *revs, struct object *obj, const char *name);
 
 extern void add_head_to_pending(struct rev_info *);
+extern void add_empty_to_pending(struct rev_info *);
 
 enum commit_action {
 	commit_ignore,