branch: make create_branch() always create a branch

With the previous commit, there are no more invocations of
create_branch() that do not create a branch because:

* BRANCH_TRACK_OVERRIDE is no longer passed
* clobber_head_ok = true and force = false is never passed

Assert these situations, delete dead code and ensure that we're handling
clobber_head_ok and force correctly by introducing tests for `git branch
--force`. As a result, create_branch() now always creates a branch.

Helped-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Glen Choo <chooglen@google.com>
Reviewed-by: Jonathan Tan <jonathantanmy@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

branch.c

@@ -429,15 +429,19 @@ void create_branch(struct repository *r,
 	char *real_ref;
 	struct strbuf ref = STRBUF_INIT;
 	int forcing = 0;
-	int dont_change_ref = 0;
+	struct ref_transaction *transaction;
+	struct strbuf err = STRBUF_INIT;
+	char *msg;
 
-	if ((track == BRANCH_TRACK_OVERRIDE || clobber_head_ok)
+	if (track == BRANCH_TRACK_OVERRIDE)
-	    ? validate_branchname(name, &ref)
+		BUG("'track' cannot be BRANCH_TRACK_OVERRIDE. Did you mean to call dwim_and_setup_tracking()?");
-	    : validate_new_branchname(name, &ref, force)) {
+	if (clobber_head_ok && !force)
-		if (!force)
+		BUG("'clobber_head_ok' can only be used with 'force'");
-			dont_change_ref = 1;
+
-		else
+	if (clobber_head_ok ?
-			forcing = 1;
+			  validate_branchname(name, &ref) :
+			  validate_new_branchname(name, &ref, force)) {
+		forcing = 1;
 	}
 
 	dwim_branch_start(r, start_name, track, &real_ref, &oid);
@@ -445,27 +449,20 @@ void create_branch(struct repository *r,
 	if (reflog)
 		log_all_ref_updates = LOG_REFS_NORMAL;
 
-	if (!dont_change_ref) {
+	if (forcing)
-		struct ref_transaction *transaction;
+		msg = xstrfmt("branch: Reset to %s", start_name);
-		struct strbuf err = STRBUF_INIT;
+	else
-		char *msg;
+		msg = xstrfmt("branch: Created from %s", start_name);
-
+	transaction = ref_transaction_begin(&err);
-		if (forcing)
+	if (!transaction ||
-			msg = xstrfmt("branch: Reset to %s", start_name);
+		ref_transaction_update(transaction, ref.buf,
-		else
+					&oid, forcing ? NULL : null_oid(),
-			msg = xstrfmt("branch: Created from %s", start_name);
+					0, msg, &err) ||
-
+		ref_transaction_commit(transaction, &err))
-		transaction = ref_transaction_begin(&err);
+		die("%s", err.buf);
-		if (!transaction ||
+	ref_transaction_free(transaction);
-		    ref_transaction_update(transaction, ref.buf,
+	strbuf_release(&err);
-					   &oid, forcing ? NULL : null_oid(),
+	free(msg);
-					   0, msg, &err) ||
-		    ref_transaction_commit(transaction, &err))
-			die("%s", err.buf);
-		ref_transaction_free(transaction);
-		strbuf_release(&err);
-		free(msg);
-	}
 
 	if (real_ref && track)
 		setup_tracking(ref.buf + 11, real_ref, track, quiet);

branch.h

@@ -52,8 +52,8 @@ void dwim_and_setup_tracking(struct repository *r, const char *new_ref,
  *
  *   - force enables overwriting an existing (non-head) branch
  *
- *   - clobber_head_ok allows the currently checked out (hence existing)
+ *   - clobber_head_ok, when enabled with 'force', allows the currently
- *     branch to be overwritten; without 'force', it has no effect.
+ *     checked out (head) branch to be overwritten
  *
  *   - reflog creates a reflog for the branch
  *

t/t3200-branch.sh

@@ -42,6 +42,23 @@ test_expect_success 'git branch abc should create a branch' '
 	git branch abc && test_path_is_file .git/refs/heads/abc
 '
 
+test_expect_success 'git branch abc should fail when abc exists' '
+	test_must_fail git branch abc
+'
+
+test_expect_success 'git branch --force abc should fail when abc is checked out' '
+	test_when_finished git switch main &&
+	git switch abc &&
+	test_must_fail git branch --force abc HEAD~1
+'
+
+test_expect_success 'git branch --force abc should succeed when abc exists' '
+	git rev-parse HEAD~1 >expect &&
+	git branch --force abc HEAD~1 &&
+	git rev-parse abc >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'git branch a/b/c should create a branch' '
 	git branch a/b/c && test_path_is_file .git/refs/heads/a/b/c
 '