unix: fix MmapPtr test failing on OpenBSD

OpenBSD apparently doesn't allow unmapping address space
if part of the region is already unmapped.
This tweaks the test so that munmapping twice no longer happens.

Cq-Include-Trybots: luci.golang.try:gotip-openbsd-amd64

Author: ncruces

unix/mmap_unix_test.go

@@ -9,7 +9,6 @@ package unix_test
 import (
 	"runtime"
 	"testing"
-	"unsafe"
 
 	"golang.org/x/sys/unix"
 )
@@ -52,23 +51,20 @@ func TestMmap(t *testing.T) {
 }
 
 func TestMmapPtr(t *testing.T) {
-	mmapProt := unix.PROT_NONE
-	mmapPtrProt := unix.PROT_READ | unix.PROT_WRITE
-	b, err := unix.Mmap(-1, 0, 2*unix.Getpagesize(), mmapProt, unix.MAP_ANON|unix.MAP_PRIVATE)
+	p, err := unix.MmapPtr(-1, 0, nil, uintptr(2*unix.Getpagesize()),
+		unix.PROT_NONE, unix.MAP_ANON|unix.MAP_PRIVATE)
 	if err != nil {
-		t.Fatalf("Mmap: %v", err)
+		t.Fatalf("MmapPtr: %v", err)
 	}
-	if _, err := unix.MmapPtr(-1, 0, unsafe.Pointer(&b[0]), uintptr(unix.Getpagesize()),
-		mmapPtrProt, unix.MAP_ANON|unix.MAP_PRIVATE|unix.MAP_FIXED); err != nil {
+
+	if _, err := unix.MmapPtr(-1, 0, p, uintptr(unix.Getpagesize()),
+		unix.PROT_READ|unix.PROT_WRITE, unix.MAP_ANON|unix.MAP_PRIVATE|unix.MAP_FIXED); err != nil {
 		t.Fatalf("MmapPtr: %v", err)
 	}
 
-	b[0] = 42
+	*(*byte)(p) = 42
 
-	if err := unix.MunmapPtr(unsafe.Pointer(&b[0]), uintptr(unix.Getpagesize())); err != nil {
+	if err := unix.MunmapPtr(p, uintptr(2*unix.Getpagesize())); err != nil {
 		t.Fatalf("MunmapPtr: %v", err)
 	}
-	if err := unix.Munmap(b); err != nil {
-		t.Fatalf("Munmap: %v", err)
-	}
 }