sort: fix for nondeterministic less function in quicksort pivot

Fixes #14377

Change-Id: I130a6e1b8bc827db44efd0a74e759b894ecc4977
Reviewed-on: https://go-review.googlesource.com/19823
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-on: https://go-review.googlesource.com/22039

2 files changed, 44 insertions, 7 deletions

diff --git a/src/sort/sort.go b/src/sort/sort.go
index ac8f4a661f..ee437d3faf 100644
--- a/src/sort/sort.go
+++ b/src/sort/sort.go
@@ -119,15 +119,15 @@ func doPivot(data Interface, lo, hi int) (midlo, midhi int) {
 	pivot := lo
 	a, c := lo+1, hi-1
 
-	for ; a != c && data.Less(a, pivot); a++ {
+	for ; a < c && data.Less(a, pivot); a++ {
 	}
 	b := a
 	for {
-		for ; b != c && !data.Less(pivot, b); b++ { // data[b] <= pivot
+		for ; b < c && !data.Less(pivot, b); b++ { // data[b] <= pivot
 		}
-		for ; b != c && data.Less(pivot, c-1); c-- { // data[c-1] > pivot
+		for ; b < c && data.Less(pivot, c-1); c-- { // data[c-1] > pivot
 		}
-		if b == c {
+		if b >= c {
 			break
 		}
 		// data[b] > pivot; data[c-1] <= pivot
@@ -167,11 +167,11 @@ func doPivot(data Interface, lo, hi int) (midlo, midhi int) {
 		//	data[a <= i < b] unexamined
 		//	data[b <= i < c] = pivot
 		for {
-			for ; a != b && !data.Less(b-1, pivot); b-- { // data[b] == pivot
+			for ; a < b && !data.Less(b-1, pivot); b-- { // data[b] == pivot
 			}
-			for ; a != b && data.Less(a, pivot); a++ { // data[a] < pivot
+			for ; a < b && data.Less(a, pivot); a++ { // data[a] < pivot
 			}
-			if a == b {
+			if a >= b {
 				break
 			}
 			// data[a] == pivot; data[b-1] < pivot
diff --git a/src/sort/sort_test.go b/src/sort/sort_test.go
index 6c36f30e0e..a5da6b2630 100644
--- a/src/sort/sort_test.go
+++ b/src/sort/sort_test.go
@@ -109,6 +109,43 @@ func TestReverseSortIntSlice(t *testing.T) {
 	}
 }
 
+type nonDeterministicTestingData struct {
+	r *rand.Rand
+}
+
+func (t *nonDeterministicTestingData) Len() int {
+	return 500
+}
+func (t *nonDeterministicTestingData) Less(i, j int) bool {
+	if i < 0 || j < 0 || i >= t.Len() || j >= t.Len() {
+		panic("nondeterministic comparison out of bounds")
+	}
+	return t.r.Float32() < 0.5
+}
+func (t *nonDeterministicTestingData) Swap(i, j int) {
+	if i < 0 || j < 0 || i >= t.Len() || j >= t.Len() {
+		panic("nondeterministic comparison out of bounds")
+	}
+}
+
+func TestNonDeterministicComparison(t *testing.T) {
+	// Ensure that sort.Sort does not panic when Less returns inconsistent results.
+	// See https://golang.org/issue/14377.
+	defer func() {
+		if r := recover(); r != nil {
+			t.Error(r)
+		}
+	}()
+
+	td := &nonDeterministicTestingData{
+		r: rand.New(rand.NewSource(0)),
+	}
+
+	for i := 0; i < 10; i++ {
+		Sort(td)
+	}
+}
+
 func BenchmarkSortString1K(b *testing.B) {
 	b.StopTimer()
 	for i := 0; i < b.N; i++ {