X pipenet: Don't use 0 port in exchanges -- like with IP it is used only to...

X pipenet: Don't use 0 port in exchanges -- like with IP it is used only to signal autobind to listen

go/xcommon/xnet/pipenet/pipenet.go

@@ -83,7 +83,7 @@ type Host struct {
 	name    string
 
 	// NOTE protected by Network.mu
-	socketv []*socket // port -> listener | conn
+	socketv []*socket // port -> listener | conn  ; [0] is always nil
 }
 
 var _ xnet.Networker = (*Host)(nil)
@@ -176,14 +176,14 @@ func (h *Host) resolveAddr(addr string) (host *Host, port int, err error) {
 		return nil, 0, &net.AddrError{Err: "no such host", Addr: addr}
 	}
 
-	return host, port, nil
+	return host, a.Port, nil
 }
 
 // XXX temp
 //trace:event traceListen(laddr string)
 
 // Listen starts new listener
-// It either allocates free port if laddr is "", or binds to laddr.
+// It either allocates free port if laddr is "" or with 0 port, or binds to laddr.
 // Once listener is started, Dials could connect to listening address.
 // Connection requests created by Dials could be accepted via Accept.
 func (h *Host) Listen(laddr string) (net.Listener, error) {
@@ -193,12 +193,10 @@ func (h *Host) Listen(laddr string) (net.Listener, error) {
 
 	var sk *socket
 
-	// find first free port if autobind requested
 	if laddr == "" {
-		sk = h.allocFreeSocket()
+		laddr = ":0"
+	}
 
-	// else we resolve/verify address, check whether it is already used, and if not allocate socket in-place
-	} else {
 	var netladdr net.Addr
 	lerr := func(err error) error {
 		return &net.OpError{Op: "listen", Net: h.Network(), Addr: netladdr, Err: err}
@@ -215,6 +213,12 @@ func (h *Host) Listen(laddr string) (net.Listener, error) {
 		return nil, lerr(errAddrNoListen)
 	}
 
+	// find first free port if autobind requested
+	if port == 0 {
+		sk = h.allocFreeSocket()
+
+	// else allocate socket in-place
+	} else {
 		// grow if needed
 		for port >= len(h.socketv) {
 			h.socketv = append(h.socketv, nil)
@@ -378,17 +382,17 @@ func (c *conn) RemoteAddr() net.Addr {
 
 // ----------------------------------------
 
-// allocFreeSocket finds first free port and allocates socket entry for it
+// allocFreeSocket finds first free port and allocates socket entry for it.
 // must be called with Network.mu held
 func (h *Host) allocFreeSocket() *socket {
 	// find first free port
-	port := 0
+	port := 1 // never allocate port 0 - it is used for autobind on listen only
 	for ; port < len(h.socketv); port++ {
 		if h.socketv[port] == nil {
 			break
 		}
 	}
-	// if all busy it exits with port == len(h.socketv)
+	// if all busy it exits with port >= len(h.socketv)
 
 	// grow if needed
 	for port >= len(h.socketv) {

go/xcommon/xnet/pipenet/pipenet_test.go

@@ -103,41 +103,45 @@ func TestPipeNet(t *testing.T) {
 	assertEq(t, err, &net.OpError{Op: "dial", Net: "pipet", Addr: xaddr("α:0"), Err: errConnRefused})
 
 	l1 := xlisten(hα, "")
-	assertEq(t, l1.Addr(), xaddr("α:0"))
+	assertEq(t, l1.Addr(), xaddr("α:1"))
+
+	// zero port always stays unused even after autobind
+	_, err = hα.Dial(context.Background(), ":0")
+	assertEq(t, err, &net.OpError{Op: "dial", Net: "pipet", Addr: xaddr("α:0"), Err: errConnRefused})
 
 	wg := &xsync.WorkGroup{}
 	wg.Gox(func() {
 		c1s := xaccept(l1)
-		assertEq(t, c1s.LocalAddr(), xaddr("α:1"))
-		assertEq(t, c1s.RemoteAddr(), xaddr("β:0"))
+		assertEq(t, c1s.LocalAddr(), xaddr("α:2"))
+		assertEq(t, c1s.RemoteAddr(), xaddr("β:1"))
 
 		assertEq(t, xread(c1s), "ping")
 		xwrite(c1s, "pong")
 
 		c2s := xaccept(l1)
-		assertEq(t, c2s.LocalAddr(), xaddr("α:2"))
-		assertEq(t, c2s.RemoteAddr(), xaddr("β:1"))
+		assertEq(t, c2s.LocalAddr(), xaddr("α:3"))
+		assertEq(t, c2s.RemoteAddr(), xaddr("β:2"))
 
 		assertEq(t, xread(c2s), "hello")
 		xwrite(c2s, "world")
 	})
 
-	c1c := xdial(hβ, "α:0")
-	assertEq(t, c1c.LocalAddr(), xaddr("β:0"))
-	assertEq(t, c1c.RemoteAddr(), xaddr("α:1"))
+	c1c := xdial(hβ, "α:1")
+	assertEq(t, c1c.LocalAddr(), xaddr("β:1"))
+	assertEq(t, c1c.RemoteAddr(), xaddr("α:2"))
 
 	xwrite(c1c, "ping")
 	assertEq(t, xread(c1c), "pong")
 
-	c2c := xdial(hβ, "α:0")
-	assertEq(t, c2c.LocalAddr(), xaddr("β:1"))
-	assertEq(t, c2c.RemoteAddr(), xaddr("α:2"))
+	c2c := xdial(hβ, "α:1")
+	assertEq(t, c2c.LocalAddr(), xaddr("β:2"))
+	assertEq(t, c2c.RemoteAddr(), xaddr("α:3"))
 
 	xwrite(c2c, "hello")
 	assertEq(t, xread(c2c), "world")
 
 	xwait(wg)
 
-	l2 := xlisten(hα, "")
-	assertEq(t, l2.Addr(), xaddr("α:3"))
+	l2 := xlisten(hα, ":0") // autobind again
+	assertEq(t, l2.Addr(), xaddr("α:4"))
 }