safesocket: document

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

safesocket/pipe_windows.go

@@ -15,16 +15,8 @@ func path(vendor, name string, port uint16) string {
 	return fmt.Sprintf("127.0.0.1:%v", port)
 }
 
-func ConnCloseRead(c net.Conn) error {
-	return c.(*net.TCPConn).CloseRead()
-}
-
-func ConnCloseWrite(c net.Conn) error {
-	return c.(*net.TCPConn).CloseWrite()
-}
-
 // TODO(apenwarr): handle magic cookie auth
-func Connect(path string, port uint16) (net.Conn, error) {
+func connect(path string, port uint16) (net.Conn, error) {
 	pipe, err := net.Dial("tcp", fmt.Sprintf("127.0.0.1:%d", port))
 	if err != nil {
 		return nil, err
@@ -45,7 +37,7 @@ func setFlags(network, address string, c syscall.RawConn) error {
 //   just always using a TCP session on a fixed port on localhost. As a
 //   result, on Windows we ignore the vendor and name strings.
 // TODO(apenwarr): handle magic cookie auth
-func Listen(path string, port uint16) (net.Listener, uint16, error) {
+func listen(path string, port uint16) (_ net.Listener, gotPort uint16, _ error) {
 	lc := net.ListenConfig{
 		Control: setFlags,
 	}

safesocket/safesocket.go

@@ -0,0 +1,42 @@
+// Copyright (c) 2020 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.
+
+// +build !windows
+
+// Package safesocket creates either a Unix socket, if possible, or
+// otherwise a localhost TCP connection.
+package safesocket
+
+import (
+	"net"
+)
+
+type closeable interface {
+	CloseRead() error
+	CloseWrite() error
+}
+
+// ConnCloseRead calls c's CloseRead method. c is expected to be
+// either a UnixConn or TCPConn as returned from this package.
+func ConnCloseRead(c net.Conn) error {
+	return c.(closeable).CloseRead()
+}
+
+// ConnCloseWrite calls c's CloseWrite method. c is expected to be
+// either a UnixConn or TCPConn as returned from this package.
+func ConnCloseWrite(c net.Conn) error {
+	return c.(closeable).CloseWrite()
+}
+
+// Connect connects to either path (on Unix) or the provided localhost port (on Windows).
+func Connect(path string, port uint16) (net.Conn, error) {
+	return connect(path, port)
+}
+
+// Listen returns a listener either on Unix socket path (on Unix), or
+// the localhost port (on Windows).
+// If port is 0, the returned gotPort says which port was selected on Windows.
+func Listen(path string, port uint16) (_ net.Listener, gotPort uint16, _ error) {
+	return listen(path, port)
+}

safesocket/unixsocket.go

@@ -12,16 +12,8 @@ import (
 	"os"
 )
 
-func ConnCloseRead(c net.Conn) error {
-	return c.(*net.UnixConn).CloseRead()
-}
-
-func ConnCloseWrite(c net.Conn) error {
-	return c.(*net.UnixConn).CloseWrite()
-}
-
 // TODO(apenwarr): handle magic cookie auth
-func Connect(path string, port uint16) (net.Conn, error) {
+func connect(path string, port uint16) (net.Conn, error) {
 	pipe, err := net.Dial("unix", path)
 	if err != nil {
 		return nil, err
@@ -30,7 +22,7 @@ func Connect(path string, port uint16) (net.Conn, error) {
 }
 
 // TODO(apenwarr): handle magic cookie auth
-func Listen(path string, port uint16) (net.Listener, uint16, error) {
+func listen(path string, port uint16) (ln net.Listener, _ uint16, err error) {
 	// Unix sockets hang around in the filesystem even after nobody
 	// is listening on them. (Which is really unfortunate but long-
 	// entrenched semantics.) Try connecting first; if it works, then