net/netutil: unify two oneConnListeners into a new package

I was about to add a third copy, so unify them now instead.

Change-Id: I3b93896aa1249b1250a6b1df4829d57717f2311a
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

cmd/tailscaled/depaware.txt

@@ -203,6 +203,7 @@ tailscale.com/cmd/tailscaled dependencies: (generated by github.com/tailscale/de
         tailscale.com/net/netknob                                    from tailscale.com/logpolicy+
         tailscale.com/net/netns                                      from tailscale.com/cmd/tailscaled+
      💣 tailscale.com/net/netstat                                    from tailscale.com/ipn/ipnserver
+        tailscale.com/net/netutil                                    from tailscale.com/ipn/ipnlocal+
         tailscale.com/net/packet                                     from tailscale.com/net/tstun+
         tailscale.com/net/portmapper                                 from tailscale.com/cmd/tailscaled+
         tailscale.com/net/proxymux                                   from tailscale.com/cmd/tailscaled

ipn/ipnlocal/peerapi.go

@@ -38,6 +38,7 @@
 	"tailscale.com/logtail/backoff"
 	"tailscale.com/net/dns/resolver"
 	"tailscale.com/net/interfaces"
+	"tailscale.com/net/netutil"
 	"tailscale.com/syncs"
 	"tailscale.com/tailcfg"
 	"tailscale.com/util/clientmetric"
@@ -506,27 +507,9 @@ func (pln *peerAPIListener) ServeConn(src netaddr.IPPort, c net.Conn) {
 	if addH2C != nil {
 		addH2C(httpServer)
 	}
-	go httpServer.Serve(&oneConnListener{Listener: pln.ln, conn: c})
+	go httpServer.Serve(netutil.NewOneConnListenerFrom(c, pln.ln))
 }
 
-type oneConnListener struct {
-	net.Listener
-	conn net.Conn
-}
-
-func (l *oneConnListener) Accept() (c net.Conn, err error) {
-	c = l.conn
-	if c == nil {
-		err = io.EOF
-		return
-	}
-	err = nil
-	l.conn = nil
-	return
-}
-
-func (l *oneConnListener) Close() error { return nil }
-
 // peerAPIHandler serves the Peer API for a source specific client.
 type peerAPIHandler struct {
 	ps         *peerAPIServer

ipn/ipnserver/server.go

@@ -39,6 +39,7 @@
 	"tailscale.com/ipn/store/aws"
 	"tailscale.com/logtail/backoff"
 	"tailscale.com/net/netstat"
+	"tailscale.com/net/netutil"
 	"tailscale.com/net/tsdial"
 	"tailscale.com/paths"
 	"tailscale.com/safesocket"
@@ -308,7 +309,7 @@ func (s *Server) serveConn(ctx context.Context, c net.Conn, logf logger.Logf) {
 			ErrorLog:    logger.StdLogger(logf),
 			Handler:     s.localhostHandler(ci),
 		}
-		httpServer.Serve(&oneConnListener{&protoSwitchConn{s: s, br: br, Conn: c}})
+		httpServer.Serve(netutil.NewOneConnListener(&protoSwitchConn{s: s, br: br, Conn: c}))
 		return
 	}
 
@@ -1061,29 +1062,6 @@ func getEngineUntilItWorksWrapper(getEngine func() (wgengine.Engine, error)) fun
 	}
 }
 
-type dummyAddr string
-type oneConnListener struct {
-	conn net.Conn
-}
-
-func (l *oneConnListener) Accept() (c net.Conn, err error) {
-	c = l.conn
-	if c == nil {
-		err = io.EOF
-		return
-	}
-	err = nil
-	l.conn = nil
-	return
-}
-
-func (l *oneConnListener) Close() error { return nil }
-
-func (l *oneConnListener) Addr() net.Addr { return dummyAddr("unused-address") }
-
-func (a dummyAddr) Network() string { return string(a) }
-func (a dummyAddr) String() string  { return string(a) }
-
 // protoSwitchConn is a net.Conn that's we want to speak HTTP to but
 // it's already had a few bytes read from it to determine that it's
 // HTTP. So we Read from its bufio.Reader. On Close, we we tell the

net/netutil/netutil.go

@@ -0,0 +1,50 @@
+// Copyright (c) 2022 Tailscale Inc & 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 netutil contains misc shared networking code & types.
+package netutil
+
+import (
+	"io"
+	"net"
+)
+
+// NewOneConnListener returns a net.Listener that returns c on its first
+// Accept and EOF thereafter. The Listener's Addr is a dummy address.
+func NewOneConnListener(c net.Conn) net.Listener {
+	return NewOneConnListenerFrom(c, dummyListener{})
+}
+
+// NewOneConnListenerFrom returns a net.Listener wrapping ln where
+// its Accept returns c on the first call and io.EOF thereafter.
+func NewOneConnListenerFrom(c net.Conn, ln net.Listener) net.Listener {
+	return &oneConnListener{c, ln}
+}
+
+type oneConnListener struct {
+	conn net.Conn
+	net.Listener
+}
+
+func (l *oneConnListener) Accept() (c net.Conn, err error) {
+	c = l.conn
+	if c == nil {
+		err = io.EOF
+		return
+	}
+	err = nil
+	l.conn = nil
+	return
+}
+
+type dummyListener struct{}
+
+func (dummyListener) Close() error                    { return nil }
+func (dummyListener) Addr() net.Addr                  { return dummyAddr("unused-address") }
+func (dummyListener) Accept() (c net.Conn, err error) { return nil, io.EOF }
+
+type dummyAddr string
+
+func (a dummyAddr) Network() string { return string(a) }
+func (a dummyAddr) String() string  { return string(a) }