bisect--helper: reimplement `bisect_autostart` shell function in C

Reimplement the `bisect_autostart()` shell function in C and add the
C implementation from `bisect_next()` which was previously left
uncovered.

Add `--bisect-autostart` subcommand to be called from git-bisect.sh.
Using `--bisect-autostart` subcommand is a temporary measure to port
the shell function to C so as to use the existing test suite. As more
functions are ported, this subcommand will be retired and
bisect_autostart() will be called directly by `bisect_state()`.

Change behavior of shell script that returned success when user aborted
the bisection.

Mentored-by: Lars Schneider <larsxschneider@gmail.com>
Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Mentored-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Pranit Bauva <pranit.bauva@gmail.com>
Signed-off-by: Tanushree Tumane <tanushreetumane@gmail.com>
Signed-off-by: Miriam Rubio <mirucam@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/bisect--helper.c

@@ -29,6 +29,7 @@ static const char * const git_bisect_helper_usage[] = {
 	N_("git bisect--helper --bisect-terms [--term-good | --term-old | --term-bad | --term-new]"),
 	N_("git bisect--helper --bisect-start [--term-{old,good}=<term> --term-{new,bad}=<term>]"
 					    " [--no-checkout] [--first-parent] [<bad> [<good>...]] [--] [<paths>...]"),
+	N_("git bisect--helper --bisect-autostart"),
 	NULL
 };
 
@@ -653,6 +654,38 @@ finish:
 	return res;
 }
 
+static inline int file_is_not_empty(const char *path)
+{
+	return !is_empty_or_missing_file(path);
+}
+
+static int bisect_autostart(struct bisect_terms *terms)
+{
+	int res;
+	const char *yesno;
+
+	if (file_is_not_empty(git_path_bisect_start()))
+		return 0;
+
+	fprintf_ln(stderr, _("You need to start by \"git bisect "
+			  "start\"\n"));
+
+	if (!isatty(STDIN_FILENO))
+		return -1;
+
+	/*
+	 * TRANSLATORS: Make sure to include [Y] and [n] in your
+	 * translation. The program will only accept English input
+	 * at this point.
+	 */
+	yesno = git_prompt(_("Do you want me to do it for you "
+			     "[Y/n]? "), PROMPT_ECHO);
+	res = tolower(*yesno) == 'n' ?
+		-1 : bisect_start(terms, empty_strvec, 0);
+
+	return res;
+}
+
 int cmd_bisect__helper(int argc, const char **argv, const char *prefix)
 {
 	enum {
@@ -665,7 +698,8 @@ int cmd_bisect__helper(int argc, const char **argv, const char *prefix)
 		CHECK_AND_SET_TERMS,
 		BISECT_NEXT_CHECK,
 		BISECT_TERMS,
-		BISECT_START
+		BISECT_START,
+		BISECT_AUTOSTART,
 	} cmdmode = 0;
 	int res = 0, nolog = 0;
 	struct option options[] = {
@@ -689,6 +723,8 @@ int cmd_bisect__helper(int argc, const char **argv, const char *prefix)
 			 N_("print out the bisect terms"), BISECT_TERMS),
 		OPT_CMDMODE(0, "bisect-start", &cmdmode,
 			 N_("start the bisect session"), BISECT_START),
+		OPT_CMDMODE(0, "bisect-autostart", &cmdmode,
+			 N_("start the bisection if it has not yet been started"), BISECT_AUTOSTART),
 		OPT_BOOL(0, "no-log", &nolog,
 			 N_("no log for BISECT_WRITE")),
 		OPT_END()
@@ -748,6 +784,12 @@ int cmd_bisect__helper(int argc, const char **argv, const char *prefix)
 		set_terms(&terms, "bad", "good");
 		res = bisect_start(&terms, argv, argc);
 		break;
+	case BISECT_AUTOSTART:
+		if (argc)
+			return error(_("--bisect-autostart does not accept arguments"));
+		set_terms(&terms, "bad", "good");
+		res = bisect_autostart(&terms);
+		break;
 	default:
 		BUG("unknown subcommand %d", cmdmode);
 	}

git-bisect.sh

@@ -49,27 +49,6 @@ bisect_head()
 	fi
 }
 
-bisect_autostart() {
-	test -s "$GIT_DIR/BISECT_START" || {
-		gettextln "You need to start by \"git bisect start\"" >&2
-		if test -t 0
-		then
-			# TRANSLATORS: Make sure to include [Y] and [n] in your
-			# translation. The program will only accept English input
-			# at this point.
-			gettext "Do you want me to do it for you [Y/n]? " >&2
-			read yesno
-			case "$yesno" in
-			[Nn]*)
-				exit ;;
-			esac
-			bisect_start
-		else
-			exit 1
-		fi
-	}
-}
-
 bisect_start() {
 	git bisect--helper --bisect-start $@ || exit
 
@@ -108,7 +87,7 @@ bisect_skip() {
 }
 
 bisect_state() {
-	bisect_autostart
+	git bisect--helper --bisect-autostart || exit
 	state=$1
 	git bisect--helper --check-and-set-terms $state $TERM_GOOD $TERM_BAD || exit
 	get_terms
@@ -149,7 +128,7 @@ bisect_auto_next() {
 
 bisect_next() {
 	case "$#" in 0) ;; *) usage ;; esac
-	bisect_autostart
+	git bisect--helper --bisect-autostart || exit
 	git bisect--helper --bisect-next-check $TERM_GOOD $TERM_BAD $TERM_GOOD|| exit
 
 	# Perform all bisection computation, display and checkout