safesocket, paths: add Plan 9 support

Updates #5794

Change-Id: I69150ec18d101f55baabb38613512cde858447cb
Co-authored-by: Brad Fitzpatrick <bradfitz@tailscale.com>
Signed-off-by: Skip Tavakkolian <skip.tavakkolian@gmail.com>

paths/paths.go

@@ -27,6 +27,9 @@ func DefaultTailscaledSocket() string {
 	if runtime.GOOS == "darwin" {
 		return "/var/run/tailscaled.socket"
 	}
+	if runtime.GOOS == "plan9" {
+		return "/srv/tailscaled.sock"
+	}
 	switch distro.Get() {
 	case distro.Synology:
 		if distro.DSMVersion() == 6 {

safesocket/safesocket_plan9.go

@@ -0,0 +1,124 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build plan9
+
+package safesocket
+
+import (
+	"fmt"
+	"net"
+	"os"
+	"syscall"
+	"time"
+
+	"golang.org/x/sys/plan9"
+)
+
+// Plan 9's devsrv srv(3) is a server registry  and
+// it is conventionally bound to "/srv" in the default
+// namespace. It is "a one level directory for holding
+// already open channels to services". Post one end of
+// a pipe to "/srv/tailscale.sock" and use the other
+// end for communication with a requestor. Plan 9 pipes
+// are bidirectional.
+
+type plan9SrvAddr string
+
+func (sl plan9SrvAddr) Network() string {
+	return "/srv"
+}
+
+func (sl plan9SrvAddr) String() string {
+	return string(sl)
+}
+
+// There is no net.FileListener for Plan 9 at this time
+type plan9SrvListener struct {
+	name string
+	srvf *os.File
+	file *os.File
+}
+
+func (sl *plan9SrvListener) Accept() (net.Conn, error) {
+	// sl.file is the server end of the pipe that's
+	// connected to /srv/tailscale.sock
+	return plan9FileConn{name: sl.name, file: sl.file}, nil
+}
+
+func (sl *plan9SrvListener) Close() error {
+	sl.file.Close()
+	return sl.srvf.Close()
+}
+
+func (sl *plan9SrvListener) Addr() net.Addr {
+	return plan9SrvAddr(sl.name)
+}
+
+type plan9FileConn struct {
+	name string
+	file *os.File
+}
+
+func (fc plan9FileConn) Read(b []byte) (n int, err error) {
+	return fc.file.Read(b)
+}
+func (fc plan9FileConn) Write(b []byte) (n int, err error) {
+	return fc.file.Write(b)
+}
+func (fc plan9FileConn) Close() error {
+	return fc.file.Close()
+}
+func (fc plan9FileConn) LocalAddr() net.Addr {
+	return plan9SrvAddr(fc.name)
+}
+func (fc plan9FileConn) RemoteAddr() net.Addr {
+	return plan9SrvAddr(fc.name)
+}
+func (fc plan9FileConn) SetDeadline(t time.Time) error {
+	return syscall.EPLAN9
+}
+func (fc plan9FileConn) SetReadDeadline(t time.Time) error {
+	return syscall.EPLAN9
+}
+func (fc plan9FileConn) SetWriteDeadline(t time.Time) error {
+	return syscall.EPLAN9
+}
+
+func connect(s *ConnectionStrategy) (net.Conn, error) {
+	f, err := os.OpenFile(s.path, os.O_RDWR, 0666)
+	if err != nil {
+		return nil, err
+	}
+
+	return plan9FileConn{name: s.path, file: f}, nil
+}
+
+// Create an entry in /srv, open a pipe, write the
+// client end to the entry and return the server
+// end of the pipe to the caller. When the server
+// end of the pipe is closed, /srv name associated
+// with it will be removed (controlled by ORCLOSE flag)
+func listen(path string) (net.Listener, error) {
+	const O_RCLOSE = 64 // remove on close; should be in plan9 package
+	var pip [2]int
+
+	err := plan9.Pipe(pip[:])
+	if err != nil {
+		return nil, err
+	}
+	defer plan9.Close(pip[1])
+
+	srvfd, err := plan9.Create(path, plan9.O_WRONLY|plan9.O_CLOEXEC|O_RCLOSE, 0600)
+	if err != nil {
+		return nil, err
+	}
+	srv := os.NewFile(uintptr(srvfd), path)
+
+	_, err = fmt.Fprintf(srv, "%d", pip[1])
+	if err != nil {
+		return nil, err
+	}
+
+	return &plan9SrvListener{name: path, srvf: srv, file: os.NewFile(uintptr(pip[0]), path)}, nil
+}

safesocket/unixsocket.go

@@ -1,7 +1,7 @@
 // Copyright (c) Tailscale Inc & AUTHORS
 // SPDX-License-Identifier: BSD-3-Clause
 
-//go:build !windows && !js
+//go:build !windows && !js && !plan9
 
 package safesocket