syscall: allow nacl's fake network code to Listen twice on the same address

Noticed from nacl trybot failures on new tests in
https://golang.org/cl/16630

Related earlier fix of mine to nacl's listen code:

  syscall: fix nacl listener to not accept connections once closed
  https://go-review.googlesource.com/15940

Perhaps a better fix (in the future?) would be to remove the listener
from the map at close, but that didn't seem entirely straightforward
last time I looked into it. It's not my code, but it seems that the
map entry continues to have a purpose even after Listener close. (?)

But given that this code is only really used for running tests and the
playground, this seems fine.

Change-Id: I43bfedc57c07f215f4d79c18f588d3650687a48f
Reviewed-on: https://go-review.googlesource.com/16650
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/net_test.go

@@ -261,3 +261,26 @@ func TestPacketConnClose(t *testing.T) {
 		}
 	}
 }
+
+// nacl was previous failing to reuse an address.
+func TestListenCloseListen(t *testing.T) {
+	const maxTries = 10
+	for tries := 0; tries < maxTries; tries++ {
+		ln, err := newLocalListener("tcp")
+		if err != nil {
+			t.Fatal(err)
+		}
+		addr := ln.Addr().String()
+		if err := ln.Close(); err != nil {
+			t.Fatal(err)
+		}
+		ln, err = Listen("tcp", addr)
+		if err == nil {
+			// Success. nacl couldn't do this before.
+			ln.Close()
+			return
+		}
+		t.Errorf("failed on try %d/%d: %v", tries+1, maxTries, err)
+	}
+	t.Fatal("failed to listen/close/listen on same address after %d tries", maxTries)
+}

src/syscall/net_nacl.go

@@ -526,8 +526,8 @@ func (f *netFile) listen(backlog int) error {
 	if f.listener != nil {
 		return EINVAL
 	}
-	_, ok := net.listener[netAddr{f.proto, f.sotype, f.addr.key()}]
-	if ok {
+	old, ok := net.listener[netAddr{f.proto, f.sotype, f.addr.key()}]
+	if ok && !old.listenerClosed() {
 		return EADDRINUSE
 	}
 	net.listener[netAddr{f.proto, f.sotype, f.addr.key()}] = f