Re: [funny] "git checkout -t origin/xyzzy" seems to misbehave

On Sun, 21 Sep 2008, Junio C Hamano wrote:

When you

 * are on a branch "foo" that is not "bar",
 * have "origin/bar",
 * and already have a local branch "bar",

"git checkout -t origin/bar" seems to misbehave.

It's always been the case if the thing that fails is changing the ref for 
HEAD, you're left with the index and working tree changed but the ref 
unchanged. OTOH, the C conversion lost the early checks that a new branch 
name is plausible in advance of trying anything.

commit c36a025b20ac752f8960bc36dcbab98ca1824657
Author: Daniel Barkalow [off-list ref]
Date:   Sun Sep 21 14:25:31 2008 -0400

    Check early that a new branch is new and valid
    
    If you fail to update refs to change branches in checkout, your index
    and working tree are left already updated. We don't have an easy way
    to undo this, but at least we can check things that would make the
    creation of a new branch fail. These checks were in the shell version,
    and were lost in the C conversion.
    
    The messages are from the shell version, and should probably be made nicer.
    
    Signed-off-by: Daniel Barkalow [off-list ref]

diff --git a/builtin-checkout.c b/builtin-checkout.c
index 9377a1c..4497b70 100644
--- a/builtin-checkout.c
+++ b/builtin-checkout.c

@@ -580,6 +580,18 @@ no_reference:
 		return checkout_paths(source_tree, pathspec);
 	}
 
+	if (opts.new_branch) {
+		struct strbuf buf;
+		strbuf_init(&buf, 0);
+		strbuf_addstr(&buf, "refs/heads/");
+		strbuf_addstr(&buf, opts.new_branch);
+		if (!get_sha1(buf.buf, rev))
+			die("git checkout: branch %s already exists", opts.new_branch);
+		if (check_ref_format(buf.buf))
+			die("git checkout: we do not like '%s' as a branch name.", opts.new_branch);
+		strbuf_release(&buf);
+	}
+
 	if (new.name && !new.commit) {
 		die("Cannot switch branch to a non-commit.");
 	}