bisect: test merge base if good rev is not an ancestor of bad rev

Before this patch, "git bisect", when it was given some good revs that
are not ancestor of the bad rev, didn't check if the merge bases were
good. "git bisect" just supposed that the user knew what he was doing,
and that, when he said the revs were good, he knew that it meant that
all the revs in the history leading to the good revs were also
considered good.

But in pratice, the user may not know that a good rev is not an
ancestor of the bad rev, or he may not know/remember that all revs
leading to the good rev will be considered good. So he may give a good
rev that is a sibling, instead of an ancestor, of the bad rev, when in
fact there can be one rev becoming good in the branch of the good rev
(because the bug was already fixed there, for example) instead of one
rev becoming bad in the branch of the bad rev.

For example, if there is the following history:

    A--B--C--D
	\
	 E--F

and we launch "git bisect start D F" then only C and D would have been
considered as possible first bad commit before this patch. This could
invite user errors; F could be the commit that fixes the bug that exists
everywhere else.

The purpose of this patch is to detect when "git bisect" is passed
some good revs that are not ancestors of the bad rev, and then to first
ask the user to test the merge bases between the good and bad revs.

If the merge bases are good then all is fine, we can continue
bisecting. Otherwise, if one merge base is bad, it means that the
assumption that all revs leading to the good one are good too is
wrong and we error out. In the case where one merge base is skipped we
issue a warning and then continue bisecting anyway.

These checks will also catch the case where good and bad have been
mistaken. This means that we can remove the check that was done latter
on the output of "git rev-list --bisect-vars".

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 102 insertions, 25 deletions

diff --git a/git-bisect.sh b/git-bisect.sh
index 97ac600..b314d47 100755
--- a/git-bisect.sh
+++ b/git-bisect.sh
@@ -243,33 +243,18 @@ bisect_auto_next() {
 	bisect_next_check && bisect_next || :
 }
 
-eval_rev_list() {
-	_eval="$1"
-
-	eval $_eval
-	res=$?
-
-	if [ $res -ne 0 ]; then
-		echo >&2 "'git rev-list --bisect-vars' failed:"
-		echo >&2 "maybe you mistake good and bad revs?"
-		exit $res
-	fi
-
-	return $res
-}
-
 filter_skipped() {
 	_eval="$1"
 	_skip="$2"
 
 	if [ -z "$_skip" ]; then
-		eval_rev_list "$_eval"
+		eval "$_eval"
 		return
 	fi
 
 	# Let's parse the output of:
 	# "git rev-list --bisect-vars --bisect-all ..."
-	eval_rev_list "$_eval" | while read hash line
+	eval "$_eval" | while read hash line
 	do
 		case "$VARS,$FOUND,$TRIED,$hash" in
 			# We display some vars.
@@ -332,20 +317,113 @@ exit_if_skipped_commits () {
 	fi
 }
 
+bisect_checkout() {
+	_rev="$1"
+	_msg="$2"
+	echo "Bisecting: $_msg"
+	git checkout -q "$_rev" || exit
+	git show-branch "$_rev"
+}
+
+is_among() {
+	_rev="$1"
+	_list="$2"
+	case "$_list" in *$_rev*) return 0 ;; esac
+	return 1
+}
+
+is_testing_merge_base() {
+	grep "^testing $1$" "$GIT_DIR/BISECT_MERGE_BASES" >/dev/null 2>&1
+}
+
+mark_testing_merge_base() {
+	echo "testing $1" >> "$GIT_DIR/BISECT_MERGE_BASES"
+}
+
+handle_bad_merge_base() {
+	_badmb="$1"
+	_good="$2"
+	if is_testing_merge_base "$_badmb"; then
+		cat >&2 <<EOF
+The merge base $_badmb is bad.
+This means the bug has been fixed between $_badmb and [$_good].
+EOF
+		exit 3
+	else
+		cat >&2 <<EOF
+Some good revs are not ancestor of the bad rev.
+git bisect cannot work properly in this case.
+Maybe you mistake good and bad revs?
+EOF
+		exit 1
+	fi
+}
+
+handle_skipped_merge_base() {
+	_mb="$1"
+	_bad="$2"
+	_good="$3"
+	cat >&2 <<EOF
+Warning: the merge base between $_bad and [$_good] must be skipped.
+So we cannot be sure the first bad commit is between $_mb and $_bad.
+We continue anyway.
+EOF
+}
+
+check_merge_bases() {
+	_bad="$1"
+	_good="$2"
+	_skip="$3"
+	for _mb in $(git merge-base --all $_bad $_good)
+	do
+		if is_among "$_mb" "$_good"; then
+			continue
+		elif test "$_mb" = "$_bad"; then
+			handle_bad_merge_base "$_bad" "$_good"
+		elif is_among "$_mb" "$_skip"; then
+			handle_skipped_merge_base "$_mb" "$_bad" "$_good"
+		else
+			mark_testing_merge_base "$_mb"
+			bisect_checkout "$_mb" "a merge base must be tested"
+			checkout_done=1
+			return
+		fi
+	done
+}
+
+check_good_are_ancestors_of_bad() {
+	_bad="$1"
+	_good=$(echo $2 | sed -e 's/\^//g')
+	_skip="$3"
+
+	# Bisecting with no good rev is ok
+	test -z "$_good" && return
+
+	_side=$(git rev-list $_good ^$_bad)
+	if test -n "$_side"; then
+		check_merge_bases "$_bad" "$_good" "$_skip"
+	fi
+}
+
 bisect_next() {
 	case "$#" in 0) ;; *) usage ;; esac
 	bisect_autostart
 	bisect_next_check good
 
+	# Get bad, good and skipped revs
+	bad=$(git rev-parse --verify refs/bisect/bad) &&
+	good=$(git for-each-ref --format='^%(objectname)' \
+		"refs/bisect/good-*" | tr '\012' ' ') &&
 	skip=$(git for-each-ref --format='%(objectname)' \
-		"refs/bisect/skip-*" | tr '\012' ' ') || exit
+		"refs/bisect/skip-*" | tr '\012' ' ') &&
 
+	# Maybe some merge bases must be tested first
+	check_good_are_ancestors_of_bad "$bad" "$good" "$skip" || exit
+	test "$checkout_done" -eq "1" && checkout_done='' && return
+
+	# Get bisection information
 	BISECT_OPT=''
 	test -n "$skip" && BISECT_OPT='--bisect-all'
-
-	bad=$(git rev-parse --verify refs/bisect/bad) &&
-	good=$(git for-each-ref --format='^%(objectname)' \
-		"refs/bisect/good-*" | tr '\012' ' ') &&
 	eval="git rev-list --bisect-vars $BISECT_OPT $good $bad --" &&
 	eval="$eval $(cat "$GIT_DIR/BISECT_NAMES")" &&
 	eval=$(filter_skipped "$eval" "$skip") &&
@@ -366,9 +444,7 @@ bisect_next() {
 	# commit is also a "skip" commit (see above).
 	exit_if_skipped_commits "$bisect_rev"
 
-	echo "Bisecting: $bisect_nr revisions left to test after this"
-	git checkout -q "$bisect_rev" || exit
-	git show-branch "$bisect_rev"
+	bisect_checkout "$bisect_rev" "$bisect_nr revisions left to test after this"
 }
 
 bisect_visualize() {
@@ -415,6 +491,7 @@ bisect_clean_state() {
 	do
 		git update-ref -d $ref $hash || exit
 	done
+	rm -f "$GIT_DIR/BISECT_MERGE_BASES" &&
 	rm -f "$GIT_DIR/BISECT_LOG" &&
 	rm -f "$GIT_DIR/BISECT_NAMES" &&
 	rm -f "$GIT_DIR/BISECT_RUN" &&