RFC: reverse bisect

[Michal Vyskocil <mvyskocil@suse.cz>]

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Hi all,

Following proposed patch try to implement reverse mode for git bisect.

The git bisect command is written in the regression-finding in mind. IOW
it expects the good commit is older than the later one, which caused a
regression.

However common usage for (at least) package maintainer is not to find a
regression and fix it. The main task it to identify a bugfix!. In this
case git bisect is still helpfull as it reduces a time a lot, but user
needs to exchange the good<->bad in his mind, which is confusing and in
case there are delays in the work, it's trivial to forgot that I have to
type git bisect good, when I'm in the bad revision.

This simple patch try to address the problem of poor package
maintainer's brain and introduces --reverse argument for the git bisect
start command.

In this mode, bisect internally exchange the behavior of good/bad
itself, so there's no need to do it manually. I did some basic testing
and

git bisect start --reverse HEAD~999 HEAD
git bisect good/bad/skip/run

really works well, allowing user to identify a first good commit instead
of the first bad one. I did not test other commands like visualize or
replay.

What do you think about it? Do you see other problems I'm not aware of?
---
 bisect.c      |    2 +-
 git-bisect.sh |   49 ++++++++++++++++++++++++++++++++++++++++++++-----
 2 files changed, 45 insertions(+), 6 deletions(-)

diff --git a/bisect.c b/bisect.c
index c7b7d79..33aaeaa 100644
--- a/bisect.c
+++ b/bisect.c
@@ -768,7 +768,7 @@ static void handle_bad_merge_base(void)
 
 	fprintf(stderr, "Some good revs are not ancestor of the bad rev.\n"
 		"git bisect cannot work properly in this case.\n"
-		"Maybe you mistake good and bad revs?\n");
+		"Try --reverse to switch the bisect logic.\n");
 	exit(1);
 }
 
diff --git a/git-bisect.sh b/git-bisect.sh
index 2524060..5c95f25 100755
--- a/git-bisect.sh
+++ b/git-bisect.sh
@@ -33,6 +33,25 @@ OPTIONS_SPEC=
 _x40='[0-9a-f][0-9a-f][0-9a-f][0-9a-f][0-9a-f]'
 _x40="$_x40$_x40$_x40$_x40$_x40$_x40$_x40$_x40"
 
+bisect_reverse_mode() {
+	test -f "$GIT_DIR/BISECT_REVERSE"
+}
+
+bisect_reverse_state() {
+
+	if bisect_reverse_mode; then
+		if test "$1" = "good"; then
+			echo "bad"
+			return 0
+		elif test "$1" = "bad"; then
+			echo "good"
+			return 0
+		fi
+	fi
+
+	echo $1
+}
+
 bisect_head()
 {
 	if test -f "$GIT_DIR/BISECT_HEAD"
@@ -69,6 +88,11 @@ bisect_start() {
 	# Check for one bad and then some good revisions.
 	#
 	has_double_dash=0
+	#
+	# Exchange the internal meainng of good/bad allowing bisect to find
+	# a commit fixing a bug, not "only" the one causes a regression
+	#
+	reverse_mode=1
 	for arg; do
 		case "$arg" in --) has_double_dash=1; break ;; esac
 	done
@@ -91,6 +115,9 @@ bisect_start() {
 		--no-checkout)
 			mode=--no-checkout
 			shift ;;
+		--reverse)
+			reverse_mode=0
+			shift ;;
 		--*)
 			die "$(eval_gettext "unrecognised option: '\$arg'")" ;;
 		*)
@@ -99,10 +126,17 @@ bisect_start() {
 				die "$(eval_gettext "'\$arg' does not appear to be a valid revision")"
 				break
 			}
-			case $bad_seen in
-			0) state='bad' ; bad_seen=1 ;;
-			*) state='good' ;;
-			esac
+			if test $reverse_mode -ne 0; then
+				case $bad_seen in
+				0) state='bad' ; bad_seen=1 ;;
+				*) state='good' ;;
+				esac
+			else
+				case $bad_seen in
+				0) state='good' ; bad_seen=1 ;;
+				*) state='bad' ;;
+				esac
+			fi
 			eval="$eval bisect_write '$state' '$rev' 'nolog' &&"
 			shift
 			;;
@@ -170,6 +204,9 @@ bisect_start() {
 	git rev-parse --sq-quote "$@" >"$GIT_DIR/BISECT_NAMES" &&
 	eval "$eval true" &&
 	echo "git bisect start$orig_args" >>"$GIT_DIR/BISECT_LOG" || exit
+	if test $reverse_mode -eq 0; then
+		/bin/touch "$GIT_DIR/BISECT_REVERSE" || exit
+	fi
 	#
 	# Check if we can proceed to the next bisect state.
 	#
@@ -225,7 +262,7 @@ bisect_skip() {
 
 bisect_state() {
 	bisect_autostart
-	state=$1
+	state=$(bisect_reverse_state $1)
 	case "$#,$state" in
 	0,*)
 		die "$(gettext "Please call 'bisect_state' with at least one argument.")" ;;
@@ -377,6 +414,7 @@ bisect_clean_state() {
 	rm -f "$GIT_DIR/BISECT_LOG" &&
 	rm -f "$GIT_DIR/BISECT_NAMES" &&
 	rm -f "$GIT_DIR/BISECT_RUN" &&
+	rm -f "$GIT_DIR/BISECT_REVERSE" &&
 	# Cleanup head-name if it got left by an old version of git-bisect
 	rm -f "$GIT_DIR/head-name" &&
 	git update-ref -d --no-deref BISECT_HEAD &&
@@ -402,6 +440,7 @@ bisect_replay () {
 			cmd="bisect_start $rev"
 			eval "$cmd" ;;
 		good|bad|skip)
+			command=$(bisect_reverse_state $1)
 			bisect_write "$command" "$rev" ;;
 		*)
 			die "$(gettext "?? what are you talking about?")" ;;
-- 
1.7.6.3


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