Fix a potential indefinite blocking in packetimpact testbench

1. setsockopt(SO_RCVTIMEO, 0) == never timeout
2. float64(time.Microsecond/time.Second) == 0
3. packetimpact tests use a lot of 1s timeouts

This becomes a more significant problem because of a recent change that binds
the sniffer only on the specific testNet interface so now the traffic on the
ctrlNet cannot wake up the blocking call anymore.

PiperOrigin-RevId: 344123465

test/packetimpact/testbench/rawsockets.go

@@ -74,7 +74,8 @@ func (n *DUTTestNet) NewSniffer(t *testing.T) (Sniffer, error) {
 // packet too large for the buffer arrives, the test will get a fatal error.
 const maxReadSize int = 65536
 
-// Recv tries to read one frame until the timeout is up.
+// Recv tries to read one frame until the timeout is up. If the timeout given
+// is 0, then no read attempt will be made.
 func (s *Sniffer) Recv(t *testing.T, timeout time.Duration) []byte {
 	t.Helper()
 
@@ -87,9 +88,13 @@ func (s *Sniffer) Recv(t *testing.T, timeout time.Duration) []byte {
 		whole, frac := math.Modf(timeout.Seconds())
 		tv := unix.Timeval{
 			Sec:  int64(whole),
-			Usec: int64(frac * float64(time.Microsecond/time.Second)),
+			Usec: int64(frac * float64(time.Second/time.Microsecond)),
+		}
+		// The following should never happen, but having this guard here is better
+		// than blocking indefinitely in the future.
+		if tv.Sec == 0 && tv.Usec == 0 {
+			t.Fatal("setting SO_RCVTIMEO to 0 means blocking indefinitely")
 		}
-
 		if err := unix.SetsockoptTimeval(s.fd, unix.SOL_SOCKET, unix.SO_RCVTIMEO, &tv); err != nil {
 			t.Fatalf("can't setsockopt SO_RCVTIMEO: %s", err)
 		}