safesocket: make clear which net.Conns are winio types

Follow-up to earlier #9049.

Updates #9049

Change-Id: I121fbd2468770233a23ab5ee3df42698ca1dabc2
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

safesocket/basic_test.go

@@ -25,7 +25,7 @@ func TestBasics(t *testing.T) {
 		t.Cleanup(downgradeSDDL())
 	}
 
-	l, err := Listen(sock)
+	ln, err := Listen(sock)
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -33,12 +33,12 @@ func TestBasics(t *testing.T) {
 	errs := make(chan error, 2)
 
 	go func() {
-		s, err := l.Accept()
+		s, err := ln.Accept()
 		if err != nil {
 			errs <- err
 			return
 		}
-		l.Close()
+		ln.Close()
 		s.Write([]byte("hello"))
 
 		b := make([]byte, 1024)

safesocket/pipe_windows.go

@@ -57,27 +57,26 @@ func listen(path string) (net.Listener, error) {
 // the Windows access token associated with the connection's client. The
 // embedded net.Conn must be a go-winio PipeConn.
 type WindowsClientConn struct {
-	net.Conn
+	winioPipeConn
 	token windows.Token
 }
 
-// winioPipeHandle is fulfilled by the underlying code implementing go-winio's
-// PipeConn interface.
-type winioPipeHandle interface {
+// winioPipeConn is a subset of the interface implemented by the go-winio's
+// unexported *win32pipe type, as returned by go-winio's ListenPipe
+// net.Listener's Accept method. This type is used in places where we really are
+// assuming that specific unexported type and its Fd method.
+type winioPipeConn interface {
+	net.Conn
 	// Fd returns the Windows handle associated with the connection.
 	Fd() uintptr
 }
 
-func resolvePipeHandle(c net.Conn) windows.Handle {
-	wph, ok := c.(winioPipeHandle)
-	if !ok {
-		return 0
-	}
-	return windows.Handle(wph.Fd())
+func resolvePipeHandle(pc winioPipeConn) windows.Handle {
+	return windows.Handle(pc.Fd())
 }
 
 func (conn *WindowsClientConn) handle() windows.Handle {
-	return resolvePipeHandle(conn.Conn)
+	return resolvePipeHandle(conn.winioPipeConn)
 }
 
 // ClientPID returns the pid of conn's client, or else an error.
@@ -99,11 +98,14 @@ func (conn *WindowsClientConn) Close() error {
 		conn.token.Close()
 		conn.token = 0
 	}
-	return conn.Conn.Close()
+	return conn.winioPipeConn.Close()
 }
 
+// winIOPipeListener is a net.Listener that wraps a go-winio PipeListener and
+// returns net.Conn values of type *WindowsClientConn with the associated
+// windows.Token.
 type winIOPipeListener struct {
-	net.Listener
+	net.Listener // must be from winio.ListenPipe
 }
 
 func (lw *winIOPipeListener) Accept() (net.Conn, error) {
@@ -112,22 +114,28 @@ func (lw *winIOPipeListener) Accept() (net.Conn, error) {
 		return nil, err
 	}
 
-	token, err := clientUserAccessToken(conn)
+	pipeConn, ok := conn.(winioPipeConn)
+	if !ok {
+		conn.Close()
+		return nil, fmt.Errorf("unexpected type %T from winio.ListenPipe listener (itself a %T)", conn, lw.Listener)
+	}
+
+	token, err := clientUserAccessToken(pipeConn)
 	if err != nil {
 		conn.Close()
 		return nil, err
 	}
 
 	return &WindowsClientConn{
-		Conn:  conn,
-		token: token,
+		winioPipeConn: pipeConn,
+		token:         token,
 	}, nil
 }
 
-func clientUserAccessToken(c net.Conn) (windows.Token, error) {
-	h := resolvePipeHandle(c)
+func clientUserAccessToken(pc winioPipeConn) (windows.Token, error) {
+	h := resolvePipeHandle(pc)
 	if h == 0 {
-		return 0, fmt.Errorf("not a windows handle: %T", c)
+		return 0, fmt.Errorf("clientUserAccessToken failed to get handle from pipeConn %T", pc)
 	}
 
 	// Impersonation touches thread-local state, so we need to lock until the

safesocket/pipe_windows_test.go

@@ -3,7 +3,12 @@
 
 package safesocket
 
-import "tailscale.com/util/winutil"
+import (
+	"fmt"
+	"testing"
+
+	"tailscale.com/util/winutil"
+)
 
 func init() {
 	// downgradeSDDL is a test helper that downgrades the windowsSDDL variable if
@@ -20,3 +25,84 @@ func init() {
 		return func() {}
 	}
 }
+
+// TestExpectedWindowsTypes is a copy of TestBasics specialized for Windows with
+// type assertions about the types of listeners and conns we expect.
+func TestExpectedWindowsTypes(t *testing.T) {
+	t.Cleanup(downgradeSDDL())
+	const sock = `\\.\pipe\tailscale-test`
+	ln, err := Listen(sock)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if got, want := fmt.Sprintf("%T", ln), "*safesocket.winIOPipeListener"; got != want {
+		t.Errorf("got listener type %q; want %q", got, want)
+	}
+
+	errs := make(chan error, 2)
+
+	go func() {
+		s, err := ln.Accept()
+		if err != nil {
+			errs <- err
+			return
+		}
+		ln.Close()
+
+		wcc, ok := s.(*WindowsClientConn)
+		if !ok {
+			s.Close()
+			errs <- fmt.Errorf("accepted type %T; want WindowsClientConn", s)
+			return
+		}
+		if wcc.winioPipeConn.Fd() == 0 {
+			t.Error("accepted conn had unexpected zero fd")
+		}
+		if wcc.token == 0 {
+			t.Error("accepted conn had unexpected zero token")
+		}
+
+		s.Write([]byte("hello"))
+
+		b := make([]byte, 1024)
+		n, err := s.Read(b)
+		if err != nil {
+			errs <- err
+			return
+		}
+		t.Logf("server read %d bytes.", n)
+		if string(b[:n]) != "world" {
+			errs <- fmt.Errorf("got %#v, expected %#v\n", string(b[:n]), "world")
+			return
+		}
+		s.Close()
+		errs <- nil
+	}()
+
+	go func() {
+		s := DefaultConnectionStrategy(sock)
+		c, err := Connect(s)
+		if err != nil {
+			errs <- err
+			return
+		}
+		c.Write([]byte("world"))
+		b := make([]byte, 1024)
+		n, err := c.Read(b)
+		if err != nil {
+			errs <- err
+			return
+		}
+		if string(b[:n]) != "hello" {
+			errs <- fmt.Errorf("got %#v, expected %#v\n", string(b[:n]), "hello")
+		}
+		c.Close()
+		errs <- nil
+	}()
+
+	for i := 0; i < 2; i++ {
+		if err := <-errs; err != nil {
+			t.Fatal(err)
+		}
+	}
+}