runtime: make gostringnocopy update maxstring

Fixes #8706

LGTM=josharian
R=josharian
CC=golang-codereviews
https://golang.org/cl/143880043

src/runtime/export_test.go

@@ -160,3 +160,6 @@ func GostringW(w []uint16) (s string) {
 	})
 	return
 }
+
+var Gostringnocopy = gostringnocopy
+var Maxstring = &maxstring

src/runtime/string.c

@@ -42,10 +42,15 @@ String
 runtime·gostringnocopy(byte *str)
 {
 	String s;
+	uintptr ms;
 	
 	s.str = str;
 	s.len = runtime·findnull(str);
+	while(true) {
+		ms = runtime·maxstring;
+		if(s.len <= ms || runtime·casp((void**)&runtime·maxstring, (void*)ms, (void*)s.len))
 			return s;
+	}
 }
 
 // TODO: move this elsewhere

src/runtime/string_test.go

@@ -145,3 +145,16 @@ func main() {
 	panic(s)
 }
 `
+
+func TestGostringnocopy(t *testing.T) {
+	max := *runtime.Maxstring
+	b := make([]byte, max+10)
+	for i := uintptr(0); i < max+9; i++ {
+		b[i] = 'a'
+	}
+	_ = runtime.Gostringnocopy(&b[0])
+	newmax := *runtime.Maxstring
+	if newmax != max+9 {
+		t.Errorf("want %d, got %d", max+9, newmax)
+	}
+}