net: add multi listener impl for net.Listener

This adds an implementation of net.Listener which uses synchronous
multiplexing to listen on and accept connections from multiple addrs.

Signed-off-by: Daman Arora <adaman@vmware.com>

net/multi_listen.go

@@ -0,0 +1,189 @@
+/*
+Copyright 2024 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package net
+
+import (
+	"fmt"
+	"net"
+	"os"
+	"syscall"
+)
+
+// multiListener implements net.Listener and uses synchronous multiplexing to listen to and accept
+// TCP connections from multiple addresses.
+type multiListener struct {
+	latestAcceptedFDIndex int
+	fds                   []int
+	addrs                 []net.Addr
+}
+
+// compile time check to ensure *multiListener implements net.Listener.
+var _ net.Listener = &multiListener{}
+
+// NewMultiListener returns *multiListener as net.Listener allowing consumers to synchronously
+// listen for TCP connections on multiple addresses.
+func NewMultiListener(addresses []string) (net.Listener, error) {
+	ml := &multiListener{}
+	for _, address := range addresses {
+		fd, addr, err := createBindAndListen(address)
+		if err != nil {
+			return nil, err
+		}
+		ml.fds = append(ml.fds, fd)
+		ml.addrs = append(ml.addrs, addr)
+	}
+	return ml, nil
+}
+
+// createBindAndListen creates a TCP socket, binds it to the specified address, and starts listening on it.
+func createBindAndListen(address string) (int, net.Addr, error) {
+	host, _, err := net.SplitHostPort(address)
+	if err != nil {
+		return -1, nil, err
+	}
+
+	ipFamily := IPFamilyOf(ParseIPSloppy(host))
+	var network string
+	var domain int
+	switch ipFamily {
+	case IPv4:
+		network = "tcp4"
+		domain = syscall.AF_INET
+	case IPv6:
+		network = "tcp6"
+		domain = syscall.AF_INET6
+	default:
+		return -1, nil, fmt.Errorf("failed to idenfity ip family of host '%s'", host)
+
+	}
+
+	// resolve tcp addr
+	addr, err := net.ResolveTCPAddr(network, address)
+	if err != nil {
+		return -1, nil, err
+	}
+
+	// create socket
+	fd, err := syscall.Socket(domain, syscall.SOCK_STREAM, 0)
+	if err != nil {
+		return -1, nil, err
+	}
+
+	// define socket address for bind
+	var sockAddr syscall.Sockaddr
+	if ipFamily == IPv4 {
+		var ipBytes [4]byte
+		copy(ipBytes[:], addr.IP.To4())
+		sockAddr = &syscall.SockaddrInet4{
+			Addr: ipBytes,
+			Port: addr.Port,
+		}
+	} else {
+		var ipBytes [16]byte
+		copy(ipBytes[:], addr.IP.To16())
+		sockAddr = &syscall.SockaddrInet6{
+			Addr: ipBytes,
+			Port: addr.Port,
+		}
+	}
+
+	// bind socket to specified addr
+	if err = syscall.Bind(fd, sockAddr); err != nil {
+		_ = syscall.Close(fd)
+		return -1, nil, err
+	}
+
+	// start listening on socket
+	if err = syscall.Listen(fd, syscall.SOMAXCONN); err != nil {
+		_ = syscall.Close(fd)
+		return -1, nil, err
+	}
+
+	return fd, addr, nil
+}
+
+// Accept is part of net.Listener interface.
+func (ml *multiListener) Accept() (net.Conn, error) {
+	for {
+		readFds := &syscall.FdSet{}
+		maxfd := 0
+
+		for _, fd := range ml.fds {
+			if fd > maxfd {
+				maxfd = fd
+			}
+			addFDToFDSet(fd, readFds)
+		}
+
+		// wait for any of the sockets to be ready for accepting new connection
+		err := syscall.Select(maxfd+1, readFds, nil, nil, nil)
+		if err != nil {
+			return nil, err
+		}
+
+		for i, fd := range ml.fds {
+			if isFDInFDSet(fd, readFds) {
+				conn, err := acceptConnection(fd)
+				if err != nil {
+					return nil, err
+				}
+				ml.latestAcceptedFDIndex = i
+				return conn, nil
+			}
+		}
+	}
+}
+
+// addFDToFDSet adds fd to the given fd set
+func addFDToFDSet(fd int, p *syscall.FdSet) {
+	mask := 1 << (uint(fd) % syscall.FD_SETSIZE)
+	p.Bits[fd/syscall.FD_SETSIZE] |= int32(mask)
+}
+
+// isFDInFDSet returns true if fd is in fd set, false otherwise
+func isFDInFDSet(fd int, p *syscall.FdSet) bool {
+	mask := 1 << (uint(fd) % syscall.FD_SETSIZE)
+	return p.Bits[fd/syscall.FD_SETSIZE]&int32(mask) != 0
+}
+
+// acceptConnection accepts connection and returns remote connection object
+func acceptConnection(fd int) (net.Conn, error) {
+	connFD, _, err := syscall.Accept(fd)
+	if err != nil {
+		return nil, err
+	}
+
+	conn, err := net.FileConn(os.NewFile(uintptr(connFD), fmt.Sprintf("fd %d", connFD)))
+	if err != nil {
+		_ = syscall.Close(connFD)
+		return nil, err
+	}
+	return conn, nil
+}
+
+// Close is part of net.Listener interface.
+func (ml *multiListener) Close() error {
+	for _, fd := range ml.fds {
+		_ = syscall.Close(fd)
+	}
+	return nil
+}
+
+// Addr is part of net.Listener interface.
+func (ml *multiListener) Addr() net.Addr {
+	return ml.addrs[ml.latestAcceptedFDIndex]
+}