net: dialChannel should not treat an expired deadline as noDeadline.

Now, only a zero deadline is interpreted as noDeadline.  Any other time
in the past yields an immediate timeout.

TestConnectDeadlineInThePast already covers this case.  We just need to
un-skip it for plan9, where dialChannel is used.

Change-Id: I995fd1a632c31f8004dac772c3d7c43a2a5853b0
Reviewed-on: https://go-review.googlesource.com/8435Reviewed-by: Mikio Hara <mikioh.mikioh@gmail.com>

src/net/dial_gen.go

@@ -12,12 +12,12 @@ import "time"
 // used on operating systems where the deadline hasn't been pushed
 // down into the pollserver. (Plan 9 and some old versions of Windows)
 func dialChannel(net string, ra Addr, dialer func(time.Time) (Conn, error), deadline time.Time) (Conn, error) {
-	var timeout time.Duration
-	if !deadline.IsZero() {
-		timeout = deadline.Sub(time.Now())
+	if deadline.IsZero() {
+		return dialer(noDeadline)
 	}
+	timeout := deadline.Sub(time.Now())
 	if timeout <= 0 {
-		return dialer(noDeadline)
+		return nil, &OpError{Op: "dial", Net: net, Addr: ra, Err: errTimeout}
 	}
 	t := time.NewTimer(timeout)
 	defer t.Stop()

src/net/timeout_test.go

@@ -682,11 +682,6 @@ func TestAcceptDeadlineConnectionAvailable(t *testing.T) {
 // TestConnectDeadlineInThePast tests that connect deadlines work, even
 // if the connection can be established w/o blocking.
 func TestConnectDeadlineInThePast(t *testing.T) {
-	switch runtime.GOOS {
-	case "plan9":
-		t.Skipf("skipping test on %q", runtime.GOOS)
-	}
-
 	ln := newLocalListener(t).(*TCPListener)
 	defer ln.Close()