unix: add Major, Minor and Mkdev functions on OpenBSD

Add Major, Minor and Mkdev functions for converting devices numbers to
their major/minor components and vice versa.

The functions follow the behavior of the macros defined in OpenBSD's
sys/types.h header. However, the parameter and return types are changed
to match the existing implementations of these functions.

Test the conversion macros with some well-known device numbers.

Updates golang/go#8106

Change-Id: Ia50b7ccab18ff7c7e9dd77ddc7e4aa6cf5c79963
Reviewed-on: https://go-review.googlesource.com/63070Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

unix/dev_openbsd.go

+// Copyright 2017 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.
+
+// Functions to access/create device major and minor numbers matching the
+// encoding used in OpenBSD's sys/types.h header.
+
+package unix
+
+// Major returns the major component of an OpenBSD device number.
+func Major(dev uint64) uint32 {
+	return uint32((dev & 0x0000ff00) >> 8)
+}
+
+// Minor returns the minor component of an OpenBSD device number.
+func Minor(dev uint64) uint32 {
+	minor := uint32((dev & 0x000000ff) >> 0)
+	minor |= uint32((dev & 0xffff0000) >> 8)
+	return minor
+}
+
+// Mkdev returns an OpenBSD device number generated from the given major and minor
+// components.
+func Mkdev(major, minor uint32) uint64 {
+	dev := (uint64(major) << 8) & 0x0000ff00
+	dev |= (uint64(minor) << 8) & 0xffff0000
+	dev |= (uint64(minor) << 0) & 0x000000ff
+	return dev
+}

unix/dev_openbsd_test.go

+// Copyright 2017 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 unix_test
+
+import (
+	"fmt"
+	"testing"
+
+	"golang.org/x/sys/unix"
+)
+
+func TestDevices(t *testing.T) {
+	testCases := []struct {
+		path  string
+		major uint32
+		minor uint32
+	}{
+		// well known major/minor numbers according to /dev/MAKEDEV on
+		// OpenBSD 6.0
+		{"/dev/null", 2, 2},
+		{"/dev/zero", 2, 12},
+		{"/dev/ttyp0", 5, 0},
+		{"/dev/ttyp1", 5, 1},
+		{"/dev/random", 45, 0},
+		{"/dev/srandom", 45, 1},
+		{"/dev/urandom", 45, 2},
+		{"/dev/arandom", 45, 3},
+	}
+	for _, tc := range testCases {
+		t.Run(fmt.Sprintf("%s %v:%v", tc.path, tc.major, tc.minor), func(t *testing.T) {
+			var stat unix.Stat_t
+			err := unix.Stat(tc.path, &stat)
+			if err != nil {
+				t.Errorf("failed to stat device: %v", err)
+				return
+			}
+
+			dev := uint64(stat.Rdev)
+			if unix.Major(dev) != tc.major {
+				t.Errorf("for %s Major(%#x) == %d, want %d", tc.path, dev, unix.Major(dev), tc.major)
+			}
+			if unix.Minor(dev) != tc.minor {
+				t.Errorf("for %s Minor(%#x) == %d, want %d", tc.path, dev, unix.Minor(dev), tc.minor)
+			}
+			if unix.Mkdev(tc.major, tc.minor) != dev {
+				t.Errorf("for %s Mkdev(%d, %d) == %#x, want %#x", tc.path, tc.major, tc.minor, unix.Mkdev(tc.major, tc.minor), dev)
+			}
+		})
+	}
+}