Merge branch 'ci/forbid-unwanted-current-branch-update'

* ci/forbid-unwanted-current-branch-update:
  Show interpreted branch name in error messages
  Prevent force-updating of the current branch

branch.c

@@ -135,6 +135,26 @@ static int setup_tracking(const char *new_ref, const char *orig_ref,
 	return 0;
 }
 
+int validate_new_branchname(const char *name, struct strbuf *ref, int force)
+{
+	const char *head;
+	unsigned char sha1[20];
+
+	if (strbuf_check_branch_ref(ref, name))
+		die("'%s' is not a valid branch name.", name);
+
+	if (!ref_exists(ref->buf))
+		return 0;
+	else if (!force)
+		die("A branch named '%s' already exists.", ref->buf + strlen("refs/heads/"));
+
+	head = resolve_ref("HEAD", sha1, 0, NULL);
+	if (!is_bare_repository() && head && !strcmp(head, ref->buf))
+		die("Cannot force update the current branch.");
+
+	return 1;
+}
+
 void create_branch(const char *head,
 		   const char *name, const char *start_name,
 		   int force, int reflog, enum branch_track track)
@@ -151,17 +171,11 @@ void create_branch(const char *head,
 	if (track == BRANCH_TRACK_EXPLICIT || track == BRANCH_TRACK_OVERRIDE)
 		explicit_tracking = 1;
 
-	if (strbuf_check_branch_ref(&ref, name))
-		die("'%s' is not a valid branch name.", name);
-
-	if (resolve_ref(ref.buf, sha1, 1, NULL)) {
-		if (!force && track == BRANCH_TRACK_OVERRIDE)
+	if (validate_new_branchname(name, &ref, force || track == BRANCH_TRACK_OVERRIDE)) {
+		if (!force)
 			dont_change_ref = 1;
-		else if (!force)
-			die("A branch named '%s' already exists.", name);
-		else if (!is_bare_repository() && head && !strcmp(head, name))
-			die("Cannot force update the current branch.");
-		forcing = 1;
+		else
+			forcing = 1;
 	}
 
 	real_ref = NULL;

branch.h

@@ -15,6 +15,14 @@
 void create_branch(const char *head, const char *name, const char *start_name,
 		   int force, int reflog, enum branch_track track);
 
+/*
+ * Validates that the requested branch may be created, returning the
+ * interpreted ref in ref, force indicates whether (non-head) branches
+ * may be overwritten. A non-zero return value indicates that the force
+ * parameter was non-zero and the branch already exists.
+ */
+int validate_new_branchname(const char *name, struct strbuf *ref, int force);
+
 /*
  * Remove information about the state of working on the current
  * branch. (E.g., MERGE_HEAD)

builtin/branch.c

@@ -566,11 +566,7 @@ static void rename_branch(const char *oldname, const char *newname, int force)
 			die(_("Invalid branch name: '%s'"), oldname);
 	}
 
-	if (strbuf_check_branch_ref(&newref, newname))
-		die(_("Invalid branch name: '%s'"), newname);
-
-	if (resolve_ref(newref.buf, sha1, 1, NULL) && !force)
-		die(_("A branch named '%s' already exists."), newref.buf + 11);
+	validate_new_branchname(newname, &newref, force);
 
 	strbuf_addf(&logmsg, "Branch: renamed %s to %s",
 		 oldref.buf, newref.buf);

builtin/checkout.c

@@ -1072,15 +1072,9 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 
 	if (opts.new_branch) {
 		struct strbuf buf = STRBUF_INIT;
-		if (strbuf_check_branch_ref(&buf, opts.new_branch))
-			die(_("git checkout: we do not like '%s' as a branch name."),
-			    opts.new_branch);
-		if (ref_exists(buf.buf)) {
-			opts.branch_exists = 1;
-			if (!opts.new_branch_force)
-				die(_("git checkout: branch %s already exists"),
-				    opts.new_branch);
-		}
+
+		opts.branch_exists = validate_new_branchname(opts.new_branch, &buf, !!opts.new_branch_force);
+
 		strbuf_release(&buf);
 	}
 

t/t2018-checkout-branch.sh

@@ -118,6 +118,15 @@ test_expect_success 'checkout -b to an existing branch fails' '
 	test_must_fail do_checkout branch2 $HEAD2
 '
 
+test_expect_success 'checkout -b to @{-1} fails with the right branch name' '
+	git reset --hard HEAD &&
+	git checkout branch1 &&
+	git checkout branch2 &&
+	echo  >expect "fatal: A branch named '\''branch1'\'' already exists." &&
+	test_must_fail git checkout -b @{-1} 2>actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'checkout -B to an existing branch resets branch to HEAD' '
 	git checkout branch1 &&
 
@@ -180,4 +189,12 @@ test_expect_success 'checkout -b <describe>' '
 	test_cmp expect actual
 '
 
+test_expect_success 'checkout -B to the current branch fails before merging' '
+	git checkout branch1 &&
+	setup_dirty_mergeable &&
+	git commit -mfooble &&
+	test_must_fail git checkout -B branch1 initial &&
+	test_must_fail test_dirty_mergeable
+'
+
 test_done

t/t3200-branch.sh

@@ -98,6 +98,18 @@ test_expect_success 'git branch -m q r/q should fail when r exists' '
 	test_must_fail git branch -m q r/q
 '
 
+test_expect_success 'git branch -M foo bar should fail when bar is checked out' '
+	git branch bar &&
+	git checkout -b foo &&
+	test_must_fail git branch -M bar foo
+'
+
+test_expect_success 'git branch -M baz bam should succeed when baz is checked out' '
+	git checkout -b baz &&
+	git branch bam &&
+	git branch -M baz bam
+'
+
 mv .git/config .git/config-saved
 
 test_expect_success 'git branch -m q q2 without config should succeed' '