Prevent force-updating of the current branch

"git branch -M <foo> <current-branch>" allows updating the current branch
which HEAD points, without the necessary house-keeping that git reset
normally does to make this operation sensible. It also leaves the reflog
in a confusing state (you would be warned when trying to read it).

"git checkout -B <current branch> <foo>" is also partly vulnerable to this
bug; due to inconsistent pre-flight checks it would perform half of its
task and then abort just before rewriting the branch. Again this
manifested itself as the index file getting out-of-sync with HEAD.

"git branch -f" already guarded against this problem, and aborts with
a fatal error.

Update "git branch -M", "git checkout -B" and "git branch -f" to share the
same check before allowing a branch to be created. These prevent you from
updating the current branch.

We considered suggesting the use of "git reset" in the failure message
but concluded that it was not possible to discern what the user was
actually trying to do.

Signed-off-by: Conrad Irwin <conrad.irwin@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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.", name);
+
+	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

@@ -1071,15 +1071,9 @@ 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

@@ -180,4 +180,12 @@ 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' '