unix: fix MremapPtr test failing on NetBSD

NetBSD apparently doesn't allow remapping into used address space.
This means that the test that uses mremap to move a mmapped page
into a new address should first mmap (to reserve)
then munmap (to free) the destination.

Fixes golang/go#68180

Change-Id: If66b67e7166ca4dc4331a8cfc3e3a285416e9849
GitHub-Last-Rev: 92058c2
GitHub-Pull-Request: #198
Cq-Include-Trybots: luci.golang.try:x_sys-gotip-netbsd-amd64
Reviewed-on: https://go-review.googlesource.com/c/sys/+/594756
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Joedian Reid <joedian@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

unix/mremap_test.go

@@ -47,30 +47,31 @@ func TestMremap(t *testing.T) {
 }
 
 func TestMremapPtr(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)
+	p1, err := unix.MmapPtr(-1, 0, nil, uintptr(2*unix.Getpagesize()),
+		unix.PROT_READ|unix.PROT_WRITE, unix.MAP_ANON|unix.MAP_PRIVATE)
 	if err != nil {
-		t.Fatalf("Mmap: %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 {
 		t.Fatalf("MmapPtr: %v", err)
 	}
 
-	b[0] = 42
+	p2 := unsafe.Add(p1, unix.Getpagesize())
+	if err := unix.MunmapPtr(p2, uintptr(unix.Getpagesize())); err != nil {
+		t.Fatalf("MunmapPtr: %v", err)
+	}
+
+	*(*byte)(p1) = 42
 
 	if _, err := unix.MremapPtr(
-		unsafe.Pointer(&b[0]), uintptr(unix.Getpagesize()),
-		unsafe.Pointer(&b[unix.Getpagesize()]), uintptr(unix.Getpagesize()),
+		p1, uintptr(unix.Getpagesize()),
+		p2, uintptr(unix.Getpagesize()),
 		unix.MremapFixed|unix.MremapMaymove); err != nil {
 		t.Fatalf("MremapPtr: %v", err)
 	}
-	if got := b[unix.Getpagesize()]; got != 42 {
+
+	if got := *(*byte)(p2); got != 42 {
 		t.Errorf("got %d, want 42", got)
 	}
 
-	if err := unix.Munmap(b); err != nil {
-		t.Fatalf("Munmap: %v", err)
+	if err := unix.MunmapPtr(p2, uintptr(unix.Getpagesize())); err != nil {
+		t.Fatalf("MunmapPtr: %v", err)
 	}
 }