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>

diff --git a/src/sort/sort.go b/src/sort/sort.go
index 5eb45c6d4a1cfe000e077259b9c862c57ce66d3d..ce3dc06f8868945c036317939a4538711d545e13 100644 (file)
--- 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 6c36f30e0eebba98bc41e7ed50e45064498ee33f..a5da6b2630bcc84d263933300002c569cce4a2f7 100644 (file)
--- 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++ {