[PATCH] checkout: do not switch branch during a rebase unless -f is given

["Nguyễn Thái Ngọc Duy" <pclouds@gmail.com>]

It does not make much sense to switch branch when you are in a middle
of a rebase. Sometimes you might want to switch away for a moment then
back with "git checkout - ". But I find myself so many times switching
away then forget that I was rebasing something.

Avoid doing that by default. Users can use -f if they really want to.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
---
 Sorry, the previous patch had wrong subject line

 I know there are other commands like rebase ("git am" comes to mind)
 but I don't use those. Feel free to put some more on top if somebody
 finds it a good thing to do.

 builtin/checkout.c         |    7 +++++++
 t/t2018-checkout-branch.sh |   22 ++++++++++++++++++++++
 2 files changed, 29 insertions(+), 0 deletions(-)

diff --git a/builtin/checkout.c b/builtin/checkout.c
index cc622c0..7d8ba04 100644
--- a/builtin/checkout.c
+++ b/builtin/checkout.c
@@ -571,6 +571,13 @@ static int switch_branches(struct checkout_opts *opts, struct branch_info *new)
 	struct branch_info old;
 	unsigned char rev[20];
 	int flag;
+	struct stat st;
+
+	if (!opts->force &&
+	    ((!stat(git_path("rebase-merge"), &st) && S_ISDIR(st.st_mode)) ||
+	     (!stat(git_path("rebase-apply"), &st) && S_ISDIR(st.st_mode))))
+		die("You should not switch branch during a rebase. Use '-f' if you really want to.");
+
 	memset(&old, 0, sizeof(old));
 	old.path = resolve_ref("HEAD", rev, 0, &flag);
 	old.commit = lookup_commit_reference_gently(rev, 1);
diff --git a/t/t2018-checkout-branch.sh b/t/t2018-checkout-branch.sh
index fa69016..95a1da8 100755
--- a/t/t2018-checkout-branch.sh
+++ b/t/t2018-checkout-branch.sh
@@ -169,4 +169,26 @@ test_expect_success 'checkout -f -B to an existing branch with mergeable changes
 	test_must_fail test_dirty_mergeable
 '
 
+test_expect_success 'checkout fails during rebase' '
+	git reset --hard &&
+	git checkout branch1 &&
+	mkdir .git/rebase-merge &&
+	test_must_fail git checkout branch2 &&
+	git checkout -f branch2
+'
+
+test_expect_success 'checkout fails during rebase (2)' '
+	rmdir .git/rebase-merge &&
+	git reset --hard &&
+	git checkout branch1 &&
+	mkdir .git/rebase-apply &&
+	test_must_fail git checkout branch2 &&
+	git checkout -f branch2
+'
+
+# this is to be incorporated to the next test
+test_expect_success 'cleanup' '
+	rmdir .git/rebase-apply
+'
+
 test_done
-- 
1.7.1.rc1.70.g788ca