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.

Fixes golang/go#68181

Change-Id: I588255f5e10ec015dbb7188eac79cee6be570680
GitHub-Last-Rev: 2535abd
GitHub-Pull-Request: #199
Cq-Include-Trybots: luci.golang.try:go1.22-openbsd-amd64
Reviewed-on: https://go-review.googlesource.com/c/sys/+/595095
TryBot-Bypass: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>

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)
-	}
 }