net: add multi listener impl for net.Listener

This adds an implementation of net.Listener which listens
on and accepts connections from multiple addresses.

Signed-off-by: Daman Arora <aroradaman@gmail.com>

net/multi_listen.go

@@ -0,0 +1,151 @@
+/*
+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"
+	"sync"
+	"sync/atomic"
+)
+
+// connAndErrPair packs results of accept call on actual listeners together along with the
+// index of the listener. It is used for communication between main and listener goroutines.
+type connAndErrPair struct {
+	index int
+	conn  net.Conn
+	err   error
+}
+
+// multiListener implements net.Listener
+type multiListener struct {
+	latestAcceptedIndex atomic.Int32
+	addrs               []net.Addr
+	listeners           []net.Listener
+
+	wg           sync.WaitGroup
+	connAndErrCh chan connAndErrPair
+	stopCh       chan struct{}
+}
+
+// compile time check to ensure *multiListener implements net.Listener.
+var _ net.Listener = &multiListener{}
+
+// MultiListen returns net.Listener which can listen for and accept
+// TCP connections on multiple addresses.
+func MultiListen(addresses []string) (net.Listener, error) {
+	return multiListen(addresses, net.ResolveTCPAddr, net.Listen)
+}
+
+func multiListen(
+	addresses []string,
+	resolveTCPAddrFunc func(network, address string) (*net.TCPAddr, error),
+	listenFunc func(network, address string) (net.Listener, error),
+) (net.Listener, error) {
+	ml := &multiListener{
+		connAndErrCh: make(chan connAndErrPair),
+		stopCh:       make(chan struct{}),
+	}
+
+	for _, address := range addresses {
+		addr, err := resolveTCPAddrFunc("tcp", address)
+		if err != nil {
+			// close all listeners
+			_ = ml.Close()
+			return nil, err
+		}
+
+		var network string
+		host, _, err := net.SplitHostPort(addr.String())
+		if err != nil {
+			// close all listeners
+			_ = ml.Close()
+			return nil, err
+		}
+		switch IPFamilyOf(ParseIPSloppy(host)) {
+		case IPv4:
+			network = "tcp4"
+		case IPv6:
+			network = "tcp6"
+		default:
+			// close all listeners
+			_ = ml.Close()
+			return nil, fmt.Errorf("failed to identify ip family of address '%s", addr.String())
+		}
+
+		l, err := listenFunc(network, addr.String())
+		if err != nil {
+			// close all listeners
+			_ = ml.Close()
+			return nil, err
+		}
+
+		ml.addrs = append(ml.addrs, addr)
+		ml.listeners = append(ml.listeners, l)
+	}
+
+	for i := range ml.listeners {
+		index := i
+		ml.wg.Add(1)
+		// spawn a go routine for every listener to wait for incoming connection requests
+		go func() {
+			defer ml.wg.Done()
+			for {
+				conn, err := ml.listeners[index].Accept()
+				if err != nil {
+					select {
+					// Accept() will throw "use of closed network connection" when listener is closed,
+					// we can ignore that error and break out of this goroutine.
+					case <-ml.stopCh:
+						return
+					default:
+						ml.connAndErrCh <- connAndErrPair{conn: conn, err: err, index: index}
+					}
+				}
+				ml.connAndErrCh <- connAndErrPair{conn: conn, err: err, index: index}
+			}
+		}()
+	}
+	return ml, nil
+}
+
+// Accept is part of net.Listener interface.
+func (ml *multiListener) Accept() (net.Conn, error) {
+	connAndErr, ok := <-ml.connAndErrCh
+	if !ok {
+		return nil, fmt.Errorf("use of closed network connection")
+	}
+	// update latestAcceptedIndex with index of the listener which accepted the connection
+	ml.latestAcceptedIndex.Store(int32(connAndErr.index))
+	return connAndErr.conn, connAndErr.err
+}
+
+// Close is part of net.Listener interface.
+func (ml *multiListener) Close() error {
+	close(ml.stopCh)
+	close(ml.connAndErrCh)
+	for i := range ml.listeners {
+		_ = ml.listeners[i].Close()
+	}
+	ml.wg.Wait()
+	return nil
+}
+
+// Addr is part of net.Listener interface.
+func (ml *multiListener) Addr() net.Addr {
+	return ml.addrs[ml.latestAcceptedIndex.Load()]
+}