create pkg/reuseport (#3455)

* create pkg/reuseport

Move the core server listening functions to a new package so plugins can
use them.

Also make *all* servers use the functions here; as only the udp/tcp
listeners where using SO_REUSEPORT (if available). This is the only
actual change in this PR; in it's core it's just a move of 2 files.

This can also be used to cleanup the dance we're doing now for
re-acquiring the sockets in e.g. the metrics plugins and the ready
plugin.

Signed-off-by: Miek Gieben <miek@miek.nl>

* Also push a small doc update

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/pkg/reuseport/listen_go111.go

@@ -0,0 +1,37 @@
+// +build go1.11
+// +build aix darwin dragonfly freebsd linux netbsd openbsd
+
+package reuseport
+
+import (
+	"context"
+	"net"
+	"syscall"
+
+	"github.com/coredns/coredns/plugin/pkg/log"
+
+	"golang.org/x/sys/unix"
+)
+
+func control(network, address string, c syscall.RawConn) error {
+	c.Control(func(fd uintptr) {
+		if err := unix.SetsockoptInt(int(fd), unix.SOL_SOCKET, unix.SO_REUSEPORT, 1); err != nil {
+			log.Warningf("Failed to set SO_REUSEPORT on socket: %s", err)
+		}
+	})
+	return nil
+}
+
+// Listen announces on the local network address. See net.Listen for more information.
+// If SO_REUSEPORT is available it will be set on the socket.
+func Listen(network, addr string) (net.Listener, error) {
+	lc := net.ListenConfig{Control: control}
+	return lc.Listen(context.Background(), network, addr)
+}
+
+// ListenPacket announces on the local network address. See net.ListenPacket for more information.
+// If SO_REUSEPORT is available it will be set on the socket.
+func ListenPacket(network, addr string) (net.PacketConn, error) {
+	lc := net.ListenConfig{Control: control}
+	return lc.ListenPacket(context.Background(), network, addr)
+}

plugin/pkg/reuseport/listen_go_not111.go

@@ -0,0 +1,13 @@
+// +build !go1.11 !aix,!darwin,!dragonfly,!freebsd,!linux,!netbsd,!openbsd
+
+package reuseport
+
+import "net"
+
+// Listen is a wrapper around net.Listen.
+func Listen(network, addr string) (net.Listener, error) { return net.Listen(network, addr) }
+
+// ListenPacket is a wrapper around net.ListenPacket.
+func ListenPacket(network, addr string) (net.PacketConn, error) {
+	return net.ListenPacket(network, addr)
+}