tcp: Fix assert in tcp::tcb::output_one

When _snd.window == 0, it is not allowed to send a tcp segment with more
than 1 byte of data for zero window probe. However, it is legal to send
an empty segment, even if _snd.window == 0.

When a seastar tcp client connects to a tcp server:

With:

    void do_syn_sent() {
        _state = SYN_SENT;
        _snd.syn_tx_time = clock_type::now();
   +    print("In do_syn_sent: _snd.window = %ld\n", _snd.window);
        // Send <SYN> to remote
        output();
    }

I saw:

   In do_syn_sent: _snd.window = 0

So the callchain is:

do_syn_sent() (with a _snd.window == 0)-> output() -> get_packet() -> output_one -> get the assert

client: net/tcp.hh:1691: void seastar::net::tcp<InetTraits>::tcb::output_one(bool) [with InetTraits = seastar::net::ipv4_traits]: Assertion `(_snd.window > 0) || ((_snd.window == 0) && (len == 1))' failed.

$ addr2line -Cfpi -e ./client
  0x000000000041fa8a
  0x000000000041fb5c
  0x000000000041fc13
  /lib64/libpthread.so.0+0x000000000001241f
  /lib64/libc.so.6+0x00000000000369fa
  /lib64/libc.so.6+0x00000000000387ff
  /lib64/libc.so.6+0x000000000002f0d9
  /lib64/libc.so.6+0x000000000002f151
  0x00000000005d0cc7
  0x00000000005d41f9
  0x00000000005d46e8
  0x00000000005cc2f3
  0x00000000005a7ad0
  0x00000000005a7bfb
  0x000000000052c3b2
  0x000000000052d752
  0x000000000041a814
  0x000000000046b47e
  0x00000000004fb9f2
  0x00000000004fc642
  0x0000000000418e1d
  /lib64/libc.so.6+0x0000000000020889
  0x0000000000418fc9 seastar::backtrace_buffer::append_backtrace() at /home/asias/src/cloudius-systems/scylla/seastar/core/reactor.cc:378
 (inlined by) print_with_backtrace at /home/asias/src/cloudius-systems/scylla/seastar/core/reactor.cc:399
seastar::print_with_backtrace(char const*) at /home/asias/src/cloudius-systems/scylla/seastar/core/reactor.cc:406
sigabrt_action at /home/asias/src/cloudius-systems/scylla/seastar/core/reactor.cc:3914
 (inlined by) operator() at /home/asias/src/cloudius-systems/scylla/seastar/core/reactor.cc:3896
 (inlined by) _FUN at /home/asias/src/cloudius-systems/scylla/seastar/core/reactor.cc:3892
?? ??:0
?? ??:0
?? ??:0
?? ??:0
?? ??:0
seastar::net::tcp<seastar::net::ipv4_traits>::tcb::output_one(bool) at /home/asias/src/cloudius-systems/scylla/seastar/net/tcp.hh:1691 (discriminator 2)
seastar::net::tcp<seastar::net::ipv4_traits>::tcb::get_packet() at /home/asias/src/cloudius-systems/scylla/seastar/net/tcp.hh:2046
 (inlined by) seastar::net::tcp<seastar::net::ipv4_traits>::tcp(seastar::net::ipv4_l4<(seastar::net::ip_protocol_num)6>&)::{lambda()#2}::operator()() at /home/asias/src/cloudius-systems/scylla/seastar/net/tcp.hh:770
std::_Function_handler<std::experimental::fundamentals_v1::optional<seastar::net::ipv4_traits::l4packet> (), seastar::net::tcp<seastar::net::ipv4_traits>::tcp(seastar::net::ipv4_l4<(seastar::net::ip_protocol_num)6>&)::{lambda()#2}>::_M_invoke(std::_Any_data const&) at /usr/include/c++/7/bits/std_function.h:302
std::function<std::experimental::fundamentals_v1::optional<seastar::net::ipv4_traits::l4packet> ()>::operator()() const at /usr/include/c++/7/bits/std_function.h:706
 (inlined by) seastar::net::ipv4_l4<(seastar::net::ip_protocol_num)6>::register_packet_provider(std::function<std::experimental::fundamentals_v1::optional<seastar::net::ipv4_traits::l4packet> ()>)::{lambda()#1}::operator()() const at /home/asias/src/cloudius-systems/scylla/seastar/net/ip.hh:424
 (inlined by) std::_Function_handler<std::experimental::fundamentals_v1::optional<seastar::net::ipv4_traits::l4packet> (), seastar::net::ipv4_l4<(seastar::net::ip_protocol_num)6>::register_packet_provider(std::function<std::experimental::fundamentals_v1::optional<seastar::net::ipv4_traits::l4packet> ()>)::{lambda()#1}>::_M_invoke(std::_Any_data const&) at /usr/include/c++/7/bits/std_function.h:302
std::function<std::experimental::fundamentals_v1::optional<seastar::net::ipv4_traits::l4packet> ()>::operator()() const at /usr/include/c++/7/bits/std_function.h:706
 (inlined by) seastar::net::ipv4::get_packet() at /home/asias/src/cloudius-systems/scylla/seastar/net/ip.cc:302
operator() at /home/asias/src/cloudius-systems/scylla/seastar/net/ip.cc:54
 (inlined by) _M_invoke at /usr/include/c++/7/bits/std_function.h:302
std::function<std::experimental::fundamentals_v1::optional<seastar::net::l3_protocol::l3packet> ()>::operator()() const at /usr/include/c++/7/bits/std_function.h:706
 (inlined by) operator() at /home/asias/src/cloudius-systems/scylla/seastar/net/net.cc:246
 (inlined by) _M_invoke at /usr/include/c++/7/bits/std_function.h:302
std::function<std::experimental::fundamentals_v1::optional<seastar::net::packet> ()>::operator()() const at /usr/include/c++/7/bits/std_function.h:706
 (inlined by) seastar::net::qp::poll_tx() at /home/asias/src/cloudius-systems/scylla/seastar/net/net.cc:68
 (inlined by) operator() at /home/asias/src/cloudius-systems/scylla/seastar/net/net.cc:89
 (inlined by) poll at /home/asias/src/cloudius-systems/scylla/seastar/core/reactor.hh:1147
seastar::reactor::poll_once() at /home/asias/src/cloudius-systems/scylla/seastar/core/reactor.cc:3254
 (inlined by) operator() at /home/asias/src/cloudius-systems/scylla/seastar/core/reactor.cc:3150
 (inlined by) _M_invoke at /usr/include/c++/7/bits/std_function.h:302
std::function<bool ()>::operator()() const at /usr/include/c++/7/bits/std_function.h:706
 (inlined by) seastar::reactor::run() at /home/asias/src/cloudius-systems/scylla/seastar/core/reactor.cc:3175
seastar::app_template::run_deprecated(int, char**, std::function<void ()>&&) at /home/asias/src/cloudius-systems/scylla/seastar/core/app-template.cc:185
seastar::app_template::run(int, char**, std::function<seastar::future<int> ()>&&) at /home/asias/src/cloudius-systems/scylla/seastar/core/app-template.cc:113
main at /home/asias/src/cloudius-systems/scylla/seastar/seastar_example_from_shanshanpt/client.cc:167

Message-Id: <5b6fddc098a1c936dd7c6833e33059d0264bb3b7.1526611968.git.asias@scylladb.com>

net/tcp.hh

@@ -1676,7 +1676,7 @@ void tcp<InetTraits>::tcb::output_one(bool data_retransmit) {
     // if advertised TCP receive window is 0 we may only transmit zero window probing segment.
     // Payload size of this segment is 1. Queueing anything bigger when _snd.window == 0 is bug
     // and violation of RFC
-    assert((_snd.window > 0) || ((_snd.window == 0) && (len == 1)));
+    assert((_snd.window > 0) || ((_snd.window == 0) && (len <= 1)));
     queue_packet(std::move(p));
 }