Recover from nil pointers when logging

Signed-off-by: Davanum Srinivas <davanum@gmail.com>

klog.go

@@ -847,7 +847,7 @@ func kvListFormat(b *bytes.Buffer, keysAndValues ...interface{}) {
 		// (https://github.com/kubernetes/kubernetes/pull/106594#issuecomment-975526235).
 		switch v := v.(type) {
 		case fmt.Stringer:
-			writeStringValue(b, true, v.String())
+			writeStringValue(b, true, stringertoString(v))
 		case string:
 			writeStringValue(b, true, v)
 		case error:
@@ -872,6 +872,16 @@ func kvListFormat(b *bytes.Buffer, keysAndValues ...interface{}) {
 	}
 }
 
+func stringertoString(s fmt.Stringer) (ret string) {
+	defer func() {
+		if err := recover(); err != nil {
+			ret = "nil"
+		}
+	}()
+	ret = s.String()
+	return
+}
+
 func writeStringValue(b *bytes.Buffer, quote bool, v string) {
 	data := []byte(v)
 	index := bytes.IndexByte(data, '\n')

klog_test.go

@@ -1063,8 +1063,20 @@ func TestErrorS(t *testing.T) {
 	}
 }
 
+// point conforms to fmt.Stringer interface as it implements the String() method
+type point struct {
+	x int
+	y int
+}
+
+// we now have a value receiver
+func (p point) String() string {
+	return fmt.Sprintf("x=%d, y=%d", p.x, p.y)
+}
+
 // Test that kvListFormat works as advertised.
 func TestKvListFormat(t *testing.T) {
+	var emptyPoint *point
 	var testKVList = []struct {
 		keysValues []interface{}
 		want       string
@@ -1159,6 +1171,10 @@ No whitespace.`,
 			})},
 			want: " pods=[kube-system/kube-dns mi-conf]",
 		},
+		{
+			keysValues: []interface{}{"point-1", point{100, 200}, "point-2", emptyPoint},
+			want:       " point-1=\"x=100, y=200\" point-2=\"nil\"",
+		},
 	}
 
 	for _, d := range testKVList {