code review 7913043: runtime: prevent garbage collection during hashmap insertion

src/pkg/runtime/gc_test.go

 // Copyright 2011 The Go Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
 package runtime_test
 
 import (
         "os"
         "runtime"
+        "runtime/debug"
         "testing"
 )
 
 func TestGcSys(t *testing.T) {
         if os.Getenv("GOGC") == "off" {
                 t.Fatalf("GOGC=off in environment; test cannot pass")
         }
         data := struct{ Short bool }{testing.Short()}
         got := executeTest(t, testGCSysSource, &data)
         want := "OK\n"
@@ skipping 55 matching lines @@
         // This makes sure new(T) is allocated on heap, not on the stack.
         t.Logf("%p", a)
 
         a[0][0][0][0][0][0][0][0][0][0] = new(int)
         *a[0][0][0][0][0][0][0][0][0][0] = 13
         runtime.GC()
         if *a[0][0][0][0][0][0][0][0][0][0] != 13 {
                 t.Fail()
         }
 }
+
+func TestGcHashmapIndirection(t *testing.T) {
+        defer debug.SetGCPercent(debug.SetGCPercent(1))
+        runtime.GC()
+        type T struct {
+                a [256]int
+        }
+        m := make(map[T]T)
+        for i := 0; i < 2000; i++ {
+                var a T
+                a.a[0] = i
+                m[a] = T{}
+        }
+}