Re: Bug: "git checkout -b" should be allowed in empty repo

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

Jeff King <peff@peff.net> writes:

> Is it really worth warning? After all, by definition you are not leaving
> any commits or useful work behind.

I actually do not know if this change itself is worth doing, but if we
were to do this, then I think the user benefits from the warning.

The patch is made on maint-1.7.6 track for no good reason, so it may have
some merge conflicts around "resolve_ref()" vs "resolve_refdup()" if we
were to apply it on a more modern codebase, but the resolution should be
trivial.

---
Subject: [PATCH] git checkout -b: allow switching out of an unborn branch

Running "git checkout -b another" immediately after "git init" when you do
not even have a commit on 'master' is forbidden, with a readable message:

    $ git checkout -b another
    fatal: You are on a branch yet to be born

It is readable but not easily understandable unless the user knows what
"yet to be born" really means.

So let's try allowing it and see what happens. I strongly suspect that
this may just shift the confusion one step further without adding much
value to the resulting system, because the next question that would come
to somebody who does not understand what "yet to be born" is is "why don't
I see 'master' in the output from 'git branch' command?", and the new
warning may not be descriptive enough to explain what the user is doing.

The early part of switch_branches() that computes old is probably be
better moved to the caller cmd_checkout() and used in the new code that
detects the "unborn" case, and passed as to switch_branches() as the third
parameter.  Such improvements and tests are left as an exercise for the
interested and motivated, as usual ;-)

Signed-off-by: Junio C Hamano <gitster@pobox.com>
---
 builtin/checkout.c |   24 ++++++++++++++++++++++++
 1 files changed, 24 insertions(+), 0 deletions(-)

diff --git a/builtin/checkout.c b/builtin/checkout.c
index 4c20dae..5894f40 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -916,6 +916,19 @@ static int parse_branchname_arg(int argc, const char **argv,
 	return argcount;
 }
 
+static int switch_unborn_to_new_branch(struct checkout_opts *opts, const char *old_ref)
+{
+	int status;
+	struct strbuf branch_ref = STRBUF_INIT;
+
+	strbuf_addf(&branch_ref, "refs/heads/%s", opts->new_branch);
+	warning(_("Leaving the unborn branch '%s' behind..."),
+		skip_prefix(old_ref, "refs/heads/"));
+	status = create_symref("HEAD", branch_ref.buf, "checkout -b");
+	strbuf_release(&branch_ref);
+	return status;
+}
+
 int cmd_checkout(int argc, const char **argv, const char *prefix)
 {
 	struct checkout_opts opts;
@@ -1089,5 +1102,16 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 	if (opts.writeout_stage)
 		die(_("--ours/--theirs is incompatible with switching branches."));
 
+	if (!new.commit) {
+		unsigned char rev[20];
+		int flag, status;
+		const char *old_ref = resolve_ref("HEAD", rev, 0, &flag);
+
+		if ((flag & REF_ISSYMREF) && is_null_sha1(rev)) {
+			status = switch_unborn_to_new_branch(&opts, old_ref);
+			free((char *)old_ref);
+			return status;
+		}
+	}
 	return switch_branches(&opts, &new);
 }
-- 
1.7.9.172.ge26ae