[PATCH 2/2] rebase: refuse to switch to branch already checked out elsewhere

[Eric Sunshine <sunshine@sunshineco.com>]

The invocation "git rebase <upstream> <branch>" switches to <branch>
before performing the rebase operation. However, unlike git-switch,
git-checkout, and git-worktree which all refuse to switch to a branch
that is already checked out in some other worktree, git-rebase switches
to <branch> unconditionally. Curb this careless behavior by making
git-rebase also refuse to switch to a branch checked out elsewhere.

Reported-by: Mike Hommey <mh@glandium.org>
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
---
 builtin/rebase.c  |  5 +++--
 t/t3400-rebase.sh | 18 ++++++++++++++++++
 2 files changed, 21 insertions(+), 2 deletions(-)

diff --git a/builtin/rebase.c b/builtin/rebase.c
index 6154ad8fa5..41b9372c0b 100644
--- a/builtin/rebase.c
+++ b/builtin/rebase.c
@@ -1945,10 +1945,11 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 		/* Is it a local branch? */
 		strbuf_reset(&buf);
 		strbuf_addf(&buf, "refs/heads/%s", branch_name);
-		if (!read_ref(buf.buf, &options.orig_head))
+		if (!read_ref(buf.buf, &options.orig_head)) {
+			die_if_checked_out(buf.buf, 1);
 			options.head_name = xstrdup(buf.buf);
 		/* If not is it a valid ref (branch or commit)? */
-		else if (!get_oid(branch_name, &options.orig_head))
+		} else if (!get_oid(branch_name, &options.orig_head))
 			options.head_name = NULL;
 		else
 			die(_("fatal: no such branch/commit '%s'"),
diff --git a/t/t3400-rebase.sh b/t/t3400-rebase.sh
index 6e746dca00..9aa5268a06 100755
--- a/t/t3400-rebase.sh
+++ b/t/t3400-rebase.sh
@@ -377,4 +377,22 @@ test_expect_success 'rebase -c rebase.useBuiltin=false warning' '
 	test_must_be_empty err
 '
 
+test_expect_success 'switch to branch checked out here' '
+	git checkout master &&
+	git rebase master master
+'
+
+test_expect_success 'switch to branch not checked out' '
+	git checkout master &&
+	git branch other &&
+	git rebase master other
+'
+
+test_expect_success 'refuse to switch to branch checked out elsewhere' '
+	git checkout master &&
+	git worktree add wt &&
+	test_must_fail git -C wt rebase master master 2>err &&
+	test_i18ngrep "already checked out" err
+'
+
 test_done
-- 
2.25.1.526.gf05a752211