git-bisect.sh: don't accidentally override existing branch "bisect"

If a branch named "bisect" or "new-bisect" already was created in the
repo by other means than git bisect, doing a git bisect used to override
the branch without a warning.  Now if the branch "bisect" or
"new-bisect" already exists, and it was not created by git bisect itself,
git bisect start fails with an appropriate error message.  Additionally,
if checking out a new bisect state fails due to a merge problem, git
bisect cleans up the temporary branch "new-bisect".

The accidental override has been noticed by Andres Salomon, reported
through
 http://bugs.debian.org/478647

Signed-off-by: Gerrit Pape <pape@smarden.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-bisect.txt

@@ -78,7 +78,7 @@ Oh, and then after you want to reset to the original head, do a
 $ git bisect reset
 ------------------------------------------------
 
-to get back to the master branch, instead of being in one of the
+to get back to the original branch, instead of being in one of the
 bisection branches ("git bisect start" will do that for you too,
 actually: it will reset the bisection state, and before it does that
 it checks that you're not using some old bisection branch).

git-bisect.sh

@@ -65,14 +65,19 @@ bisect_start() {
 	head=$(GIT_DIR="$GIT_DIR" git symbolic-ref -q HEAD) ||
 	head=$(GIT_DIR="$GIT_DIR" git rev-parse --verify HEAD) ||
 	die "Bad HEAD - I need a HEAD"
+	#
+	# Check that we either already have BISECT_START, or that the
+	# branches bisect, new-bisect don't exist, to not override them.
+	#
+	test -s "$GIT_DIR/BISECT_START" ||
+		if git show-ref --verify -q refs/heads/bisect ||
+		    git show-ref --verify -q refs/heads/new-bisect; then
+			die 'The branches "bisect" and "new-bisect" must not exist.'
+		fi
 	start_head=''
 	case "$head" in
 	refs/heads/bisect)
-		if [ -s "$GIT_DIR/BISECT_START" ]; then
-		    branch=`cat "$GIT_DIR/BISECT_START"`
-		else
-		    branch=master
-		fi
+		branch=`cat "$GIT_DIR/BISECT_START"`
 		git checkout $branch || exit
 		;;
 	refs/heads/*|$_x40)
@@ -324,8 +329,8 @@ bisect_next() {
 	exit_if_skipped_commits "$bisect_rev"
 
 	echo "Bisecting: $bisect_nr revisions left to test after this"
-	git branch -f new-bisect "$bisect_rev"
-	git checkout -q new-bisect || exit
+	git branch -D new-bisect 2> /dev/null
+	git checkout -q -b new-bisect "$bisect_rev" || exit
 	git branch -M new-bisect bisect
 	git show-branch "$bisect_rev"
 }

t/t6030-bisect-porcelain.sh

@@ -284,6 +284,24 @@ test_expect_success 'bisect starting with a detached HEAD' '
 
 '
 
+test_expect_success 'bisect refuses to start if branch bisect exists' '
+	git bisect reset &&
+	git branch bisect &&
+	test_must_fail git bisect start &&
+	git branch -d bisect &&
+	git checkout -b bisect &&
+	test_must_fail git bisect start &&
+	git checkout master &&
+	git branch -d bisect
+'
+
+test_expect_success 'bisect refuses to start if branch new-bisect exists' '
+	git bisect reset &&
+	git branch new-bisect &&
+	test_must_fail git bisect start &&
+	git branch -d new-bisect
+'
+
 #
 #
 test_done