unix: add methods to manipulate *FdSet

Add Set, Clear, IsSet and Zero methods to manipulate an *FdSet. These
implement the same functionality as the FD_SET, FD_CLR, FD_ISSET and
FD_ZERO macros in <sys/select.h>.

Change-Id: I6b7bccb98e58ee5e719096ed5743f6edcd232840
Reviewed-on: https://go-review.googlesource.com/c/sys/+/205397
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

unix/fdset_test.go

@@ -0,0 +1,53 @@
+// Copyright 2019 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.
+
+// +build aix darwin dragonfly freebsd linux netbsd openbsd solaris
+
+package unix_test
+
+import (
+	"testing"
+
+	"golang.org/x/sys/unix"
+)
+
+func TestFdSet(t *testing.T) {
+	var fdSet unix.FdSet
+	fdSet.Zero()
+	for fd := 0; fd < unix.FD_SETSIZE; fd++ {
+		if fdSet.IsSet(fd) {
+			t.Fatalf("Zero did not clear fd %d", fd)
+		}
+		fdSet.Set(fd)
+	}
+
+	for fd := 0; fd < unix.FD_SETSIZE; fd++ {
+		if !fdSet.IsSet(fd) {
+			t.Fatalf("IsSet(%d): expected true, got false", fd)
+		}
+	}
+
+	fdSet.Zero()
+	for fd := 0; fd < unix.FD_SETSIZE; fd++ {
+		if fdSet.IsSet(fd) {
+			t.Fatalf("Zero did not clear fd %d", fd)
+		}
+	}
+
+	for fd := 1; fd < unix.FD_SETSIZE; fd += 2 {
+		fdSet.Set(fd)
+	}
+
+	for fd := 0; fd < unix.FD_SETSIZE; fd++ {
+		if fd&0x1 == 0x1 {
+			if !fdSet.IsSet(fd) {
+				t.Fatalf("IsSet(%d): expected true, got false", fd)
+			}
+		} else {
+			if fdSet.IsSet(fd) {
+				t.Fatalf("IsSet(%d): expected false, got true", fd)
+			}
+		}
+	}
+}