ptrguard: assert that //go:uintptrescapes actually works

This adds a test that assures that the special //go:uintptrescapes comment
before the storeUntilRelease() function works as intended, that is the
garbage collector doesn't touch the object referenced by the uintptr until
the function returns after Release() is called.  The test will fail if the
//go:uintptrescapes comment is disabled (removed) or stops working in future
versions of go.

Signed-off-by: Sven Anderson <sven@redhat.com>

internal/cutil/ptrguard_test.go

@@ -2,6 +2,7 @@ package cutil
 
 import (
 	"math/rand"
+	"runtime"
 	"testing"
 	"unsafe"
 
@@ -34,6 +35,32 @@ func TestPtrGuard(t *testing.T) {
 		assert.Zero(t, *(*unsafe.Pointer)(cPtr))
 	})
 
+	t.Run("uintptrescapesTest", func(t *testing.T) {
+		// This test assures that the special //go:uintptrescapes comment before
+		// the storeUntilRelease() function works as intended, that is the
+		// garbage collector doesn't touch the object referenced by the uintptr
+		// until the function returns after Release() is called. The test will
+		// fail if the //go:uintptrescapes comment is disabled (removed) or
+		// stops working in future versions of go.
+		var pg_done, u_done bool
+		var goPtr = func(b *bool) unsafe.Pointer {
+			s := "ok"
+			runtime.SetFinalizer(&s, func(p *string) { *b = true })
+			return unsafe.Pointer(&s)
+		}
+		cPtr := Malloc(PtrSize)
+		defer Free(cPtr)
+		pg := NewPtrGuard(cPtr, goPtr(&pg_done))
+		u := uintptr(goPtr(&u_done))
+		runtime.GC()
+		assert.True(t, u_done)
+		assert.False(t, pg_done)
+		pg.Release()
+		runtime.GC()
+		assert.True(t, pg_done)
+		assert.NotZero(t, u) // avoid "unused" error
+	})
+
 	t.Run("stressTest", func(t *testing.T) {
 		// Because the default thread limit of the Go runtime is 10000, creating
 		// 20000 parallel PtrGuards asserts, that Go routines of PtrGuards don't