reset: allow reset on unborn branch

Some users seem to think, knowingly or not, that being on an unborn
branch is like having a commit with an empty tree checked out, but
when run on an unborn branch, "git reset" currently fails with:

  fatal: Failed to resolve 'HEAD' as a valid ref.

Instead of making users figure out that they should run

 git rm --cached -r .

, let's teach "git reset" without a revision argument, when on an
unborn branch, to behave as if the user asked to reset to an empty
tree. Don't take the analogy with an empty commit too far, though, but
still disallow explictly referring to HEAD in "git reset HEAD".

Signed-off-by: Martin von Zweigbergk <martinvonz@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/reset.c

@@ -240,7 +240,7 @@ static int update_refs(const char *rev, const unsigned char *sha1)
 int cmd_reset(int argc, const char **argv, const char *prefix)
 {
 	int reset_type = NONE, update_ref_status = 0, quiet = 0;
-	int patch_mode = 0;
+	int patch_mode = 0, unborn;
 	const char *rev;
 	unsigned char sha1[20];
 	const char **pathspec = NULL;
@@ -265,7 +265,11 @@ int cmd_reset(int argc, const char **argv, const char *prefix)
 						PARSE_OPT_KEEP_DASHDASH);
 	pathspec = parse_args(argv, prefix, &rev);
 
-	if (!pathspec) {
+	unborn = !strcmp(rev, "HEAD") && get_sha1("HEAD", sha1);
+	if (unborn) {
+		/* reset on unborn branch: treat as reset to empty tree */
+		hashcpy(sha1, EMPTY_TREE_SHA1_BIN);
+	} else if (!pathspec) {
 		struct commit *commit;
 		if (get_sha1_committish(rev, sha1))
 			die(_("Failed to resolve '%s' as a valid revision."), rev);
@@ -286,7 +290,7 @@ int cmd_reset(int argc, const char **argv, const char *prefix)
 	if (patch_mode) {
 		if (reset_type != NONE)
 			die(_("--patch is incompatible with --{hard,mixed,soft}"));
-		return run_add_interactive(rev, "--patch=reset", pathspec);
+		return run_add_interactive(sha1_to_hex(sha1), "--patch=reset", pathspec);
 	}
 
 	/* git reset tree [--] paths... can be used to
@@ -340,16 +344,16 @@ int cmd_reset(int argc, const char **argv, const char *prefix)
 			die(_("Could not write new index file."));
 	}
 
-	if (!pathspec) {
+	if (!pathspec && !unborn) {
 		/* Any resets without paths update HEAD to the head being
 		 * switched to, saving the previous head in ORIG_HEAD before. */
 		update_ref_status = update_refs(rev, sha1);
 
 		if (reset_type == HARD && !update_ref_status && !quiet)
 			print_new_head_line(lookup_commit_reference(sha1));
-
-		remove_branch_state();
 	}
+	if (!pathspec)
+		remove_branch_state();
 
 	return update_ref_status;
 }

t/t7106-reset-unborn-branch.sh

@@ -0,0 +1,52 @@
+#!/bin/sh
+
+test_description='git reset should work on unborn branch'
+. ./test-lib.sh
+
+test_expect_success 'setup' '
+	echo a >a &&
+	echo b >b
+'
+
+test_expect_success 'reset' '
+	git add a b &&
+	git reset &&
+	test "$(git ls-files)" = ""
+'
+
+test_expect_success 'reset HEAD' '
+	rm .git/index &&
+	git add a b &&
+	test_must_fail git reset HEAD
+'
+
+test_expect_success 'reset $file' '
+	rm .git/index &&
+	git add a b &&
+	git reset a &&
+	test "$(git ls-files)" = "b"
+'
+
+test_expect_success 'reset -p' '
+	rm .git/index &&
+	git add a &&
+	echo y | git reset -p &&
+	test "$(git ls-files)" = ""
+'
+
+test_expect_success 'reset --soft is a no-op' '
+	rm .git/index &&
+	git add a &&
+	git reset --soft
+	test "$(git ls-files)" = "a"
+'
+
+test_expect_success 'reset --hard' '
+	rm .git/index &&
+	git add a &&
+	git reset --hard &&
+	test "$(git ls-files)" = "" &&
+	test_path_is_missing a
+'
+
+test_done