// Brings up one OpenVPN tunnel and reports what came back. Nothing above the
// ovpn module is involved -- no netstack, no SOCKS5.
//
// This exists because the design rests on one assumption that cannot be
// unit-tested: that openvpn3 is happy to treat a SOCK_DGRAM socketpair
// descriptor as its tun device, and that what arrives on our end is bare IP
// packets with no framing of its own. This tool proves or disproves that
// against a real server in about thirty seconds, and stays in the tree because
// the same question comes up again on every openvpn3 bump.
//
//   ovg_tunnel_smoke [--csv FILE] [--node HOSTNAME] [--udp] [--seconds N]
//
// With no --csv it fetches the live VPNGate list. Exit status is 0 only if the
// tunnel came up and at least one IP packet arrived.
#include <asio.hpp>

#include <algorithm>
#include <chrono>
#include <cstdio>
#include <cstring>
#include <string>
#include <vector>

#include "common/config.h"
#include "common/http_get.h"
#include "common/logging.h"
#include "ovpn/tunnel_client.h"
#include "vpngate/csv_parser.h"

using namespace ovg;

namespace {

constexpr const char *kMod = "smoke";

std::string read_file(const std::string &path) {
  std::string out;
  std::FILE *f = std::fopen(path.c_str(), "rb");
  if (!f) return out;
  char buf[65536];
  size_t n;
  while ((n = std::fread(buf, 1, sizeof(buf), f)) > 0) out.append(buf, n);
  std::fclose(f);
  return out;
}

// Enough of an IP header decode to prove the framing assumption: if these
// fields are sane, what we are being handed really is a bare IP packet.
std::string describe_ip_packet(const uint8_t *p, size_t len) {
  if (len < 1) return "empty";
  const int version = p[0] >> 4;
  if (version == 4) {
    if (len < 20) return fmt::format("truncated IPv4 ({} bytes)", len);
    const size_t ihl = (p[0] & 0x0f) * 4;
    const size_t total = (size_t(p[2]) << 8) | p[3];
    const int proto = p[9];
    const auto addr = [](const uint8_t *a) {
      return fmt::format("{}.{}.{}.{}", a[0], a[1], a[2], a[3]);
    };
    const char *pname = proto == 6    ? "TCP"
                        : proto == 17 ? "UDP"
                        : proto == 1  ? "ICMP"
                                      : "?";
    std::string s = fmt::format("IPv4 {} -> {} {} len={} (wire {})",
                                addr(p + 12), addr(p + 16), pname, total, len);
    if (total != len) s += "  <-- length mismatch!";
    if (ihl < 20 || ihl > len) s += "  <-- bad IHL";
    return s;
  }
  if (version == 6) {
    if (len < 40) return fmt::format("truncated IPv6 ({} bytes)", len);
    return fmt::format("IPv6 next-header={} len={}", p[6], len);
  }
  // The interesting failure mode: a 4-byte tun_prefix would put a small
  // integer here instead of an IP version nibble.
  return fmt::format("NOT an IP packet: first bytes {:02x} {:02x} {:02x} {:02x} "
                     "(len {}) -- framing assumption is wrong",
                     len > 0 ? p[0] : 0, len > 1 ? p[1] : 0, len > 2 ? p[2] : 0,
                     len > 3 ? p[3] : 0, len);
}

uint16_t inet_checksum(const uint8_t *p, size_t len) {
  uint32_t sum = 0;
  for (size_t i = 0; i + 1 < len; i += 2) sum += (uint32_t(p[i]) << 8) | p[i + 1];
  if (len & 1) sum += uint32_t(p[len - 1]) << 8;
  while (sum >> 16) sum = (sum & 0xffff) + (sum >> 16);
  return static_cast<uint16_t>(~sum);
}

bool parse_ipv4(const std::string &s, uint8_t out[4]) {
  unsigned a, b, c, d;
  if (std::sscanf(s.c_str(), "%u.%u.%u.%u", &a, &b, &c, &d) != 4) return false;
  if (a > 255 || b > 255 || c > 255 || d > 255) return false;
  out[0] = uint8_t(a); out[1] = uint8_t(b); out[2] = uint8_t(c); out[3] = uint8_t(d);
  return true;
}

// A complete IPv4 + ICMP echo request. Built by hand because the whole point
// is to put a real IP packet on the pipe without a netstack in the way: if the
// reply comes back, both directions of the framing assumption hold.
std::vector<uint8_t> build_icmp_echo(const std::string &src,
                                     const std::string &dst, uint16_t id,
                                     uint16_t seq) {
  std::vector<uint8_t> pkt(20 + 8 + 16, 0);
  uint8_t *ip = pkt.data();
  ip[0] = 0x45;                              // IPv4, IHL 5
  ip[2] = uint8_t(pkt.size() >> 8);          // total length
  ip[3] = uint8_t(pkt.size() & 0xff);
  ip[4] = uint8_t(id >> 8);                  // identification
  ip[5] = uint8_t(id & 0xff);
  ip[6] = 0x40;                              // don't fragment
  ip[8] = 64;                                // TTL
  ip[9] = 1;                                 // ICMP
  if (!parse_ipv4(src, ip + 12) || !parse_ipv4(dst, ip + 16)) return {};
  const uint16_t ipsum = inet_checksum(ip, 20);
  ip[10] = uint8_t(ipsum >> 8);
  ip[11] = uint8_t(ipsum & 0xff);

  uint8_t *icmp = pkt.data() + 20;
  icmp[0] = 8;                               // echo request
  icmp[4] = uint8_t(id >> 8);
  icmp[5] = uint8_t(id & 0xff);
  icmp[6] = uint8_t(seq >> 8);
  icmp[7] = uint8_t(seq & 0xff);
  for (size_t i = 0; i < 16; ++i) icmp[8 + i] = uint8_t('a' + i);
  const uint16_t icsum = inet_checksum(icmp, 8 + 16);
  icmp[2] = uint8_t(icsum >> 8);
  icmp[3] = uint8_t(icsum & 0xff);
  return pkt;
}

struct Options {
  std::string csv;
  std::string node;
  std::string ping = "8.8.8.8";
  bool udp = false;
  int seconds = 40;
};

bool parse_args(int argc, char **argv, Options *o) {
  for (int i = 1; i < argc; ++i) {
    const std::string a = argv[i];
    const auto next = [&](std::string *dst) {
      if (i + 1 >= argc) return false;
      *dst = argv[++i];
      return true;
    };
    if (a == "--csv") {
      if (!next(&o->csv)) return false;
    } else if (a == "--node") {
      if (!next(&o->node)) return false;
    } else if (a == "--ping") {
      if (!next(&o->ping)) return false;
    } else if (a == "--udp") {
      o->udp = true;
    } else if (a == "--seconds") {
      std::string s;
      if (!next(&s)) return false;
      o->seconds = std::atoi(s.c_str());
    } else {
      std::fprintf(stderr,
                   "usage: %s [--csv FILE] [--node HOSTNAME] [--udp] "
                   "[--seconds N]\n",
                   argv[0]);
      return false;
    }
  }
  return true;
}

}  // namespace

int main(int argc, char **argv) {
  Options opt;
  if (!parse_args(argc, argv, &opt)) return 2;

  log::set_level(log::Level::Debug);
  asio::io_context io;

  // ---- node list ----------------------------------------------------------
  std::string body;
  if (!opt.csv.empty()) {
    body = read_file(opt.csv);
    if (body.empty()) {
      LOG_ERROR(kMod, "cannot read {}", opt.csv);
      return 2;
    }
  } else {
    http::Options ho;
    ho.timeout = std::chrono::seconds(30);
    std::error_code fetch_ec;
    http::async_get(io, "http://www.vpngate.net/api/iphone/", ho,
                    [&](std::error_code ec, http::Response resp) {
                      fetch_ec = ec;
                      body = std::move(resp.body);
                    });
    io.run();
    io.restart();
    if (fetch_ec) {
      LOG_ERROR(kMod, "fetching the node list failed: {}", fetch_ec.message());
      return 2;
    }
  }

  vpngate::ParseResult pr;
  std::string err;
  if (!vpngate::parse_node_list(body, &pr, &err)) {
    LOG_ERROR(kMod, "node list is not parseable: {}", err);
    return 2;
  }
  LOG_INFO(kMod, "{} nodes parsed -- {}", pr.nodes.size(), pr.stats.summary());

  // ---- pick one -----------------------------------------------------------
  // Deliberately not the selector: this tool is about the tunnel, and mixing
  // in the scoring logic would make a failure ambiguous.
  const vpngate::Node *chosen = nullptr;
  const vpngate::Remote *remote = nullptr;
  if (!opt.node.empty()) {
    for (const auto &n : pr.nodes) {
      if (n.host_name == opt.node || n.ip == opt.node) {
        chosen = &n;
        break;
      }
    }
    if (!chosen) {
      LOG_ERROR(kMod, "no node matching '{}'", opt.node);
      return 2;
    }
    remote = chosen->pick_remote(opt.udp);
  } else {
    // Highest VPNGate score that offers the protocol we want.
    int64_t best = -1;
    for (const auto &n : pr.nodes) {
      const vpngate::Remote *r = n.pick_remote(opt.udp);
      if (!r) continue;
      if (opt.udp && r->proto != vpngate::Proto::Udp) continue;
      if (n.api.score > best) {
        best = n.api.score;
        chosen = &n;
        remote = r;
      }
    }
  }
  if (!chosen || !remote) {
    LOG_ERROR(kMod, "no usable node found");
    return 2;
  }
  LOG_INFO(kMod, "trying {} ({}, score {}) via {}:{}/{}", chosen->id(),
           chosen->country_short, chosen->api.score, remote->host, remote->port,
           vpngate::proto_name(remote->proto));

  // ---- connect ------------------------------------------------------------
  OvpnConfig cfg;
  cfg.connect_timeout_s = 25;
  cfg.tunnel_up_timeout_s = 35;

  auto tc = ovpn::TunnelClient::create(io, cfg);
  if (!ovpn::TunnelClient::supported()) {
    LOG_ERROR(kMod, "this build has no openvpn3 (-DOVG_WITH_TUNNEL=ON)");
    return 2;
  }

  bool came_up = false;
  bool finished = false;
  size_t packets = 0;
  size_t echo_replies = 0;
  std::vector<uint8_t> buf(ovpn::kMaxPacketSize);

  // Reads whatever the tunnel delivers and decodes just enough of each packet
  // to show that the framing is what we assumed.
  std::function<void()> read_one = [&] {
    tc->pipe().socket().async_receive(
        asio::buffer(buf), [&](std::error_code ec, size_t n) {
          if (ec) {
            if (ec != asio::error::operation_aborted)
              LOG_INFO(kMod, "tun read ended: {}", ec.message());
            return;
          }
          tc->pipe().note_received(n);
          // ICMP echo reply: type 0 at the start of the payload.
          if (n >= 28 && (buf[0] >> 4) == 4 && buf[9] == 1 &&
              buf[(buf[0] & 0x0f) * 4] == 0)
            ++echo_replies;
          if (++packets <= 12)
            LOG_INFO(kMod, "rx #{}: {}", packets,
                     describe_ip_packet(buf.data(), n));
          else if (packets % 200 == 0)
            LOG_INFO(kMod, "rx {} packets", packets);
          read_one();
        });
  };

  asio::steady_timer pinger(io);
  uint16_t seq = 0;
  std::string tun_ip;
  std::function<void()> ping_once = [&] {
    const auto pkt = build_icmp_echo(tun_ip, opt.ping, 0x4f56, ++seq);
    if (pkt.empty()) {
      LOG_ERROR(kMod, "cannot build an echo request for {} -> {}", tun_ip,
                opt.ping);
      return;
    }
    const auto st = tc->pipe().send_packet(pkt.data(), pkt.size());
    LOG_INFO(kMod, "tx echo request #{} {} -> {} ({} bytes): {}", seq, tun_ip,
             opt.ping, pkt.size(),
             st == ovpn::PacketPipe::SendStatus::Ok      ? "queued"
             : st == ovpn::PacketPipe::SendStatus::Dropped ? "DROPPED"
                                                           : "PIPE CLOSED");
    pinger.expires_after(std::chrono::seconds(2));
    pinger.async_wait([&](std::error_code ec) {
      if (!ec) ping_once();
    });
  };

  asio::steady_timer deadline(io);

  // Everything that keeps the io_context alive has to be taken down together,
  // the pending tun read included: leaving it armed means the final drain
  // never returns.
  auto shutdown = [&] {
    deadline.cancel();
    pinger.cancel();
    if (tc->pipe().is_open()) {
      std::error_code ignored;
      tc->pipe().socket().cancel(ignored);
    }
    tc->stop([&] { finished = true; });
  };

  deadline.expires_after(std::chrono::seconds(opt.seconds));
  deadline.async_wait([&](std::error_code ec) {
    if (ec) return;
    LOG_INFO(kMod, "{}s elapsed, shutting down", opt.seconds);
    shutdown();
  });

  if (!tc->start(*chosen, *remote, [&](ovpn::TunnelState st,
                                       const ovpn::TunnelInfo &info,
                                       const std::string &detail) {
        LOG_INFO(kMod, "state -> {}{}{}", ovpn::tunnel_state_name(st),
                 detail.empty() ? "" : ": ", detail);
        if (st == ovpn::TunnelState::Up) {
          came_up = true;
          LOG_INFO(kMod,
                   "pushed: ip={}/{} gw={} mtu={} dns=[{}] redirect_gw={} "
                   "routes={} server={}",
                   info.ipv4, info.prefix4, info.gateway4, info.mtu,
                   fmt::join(info.dns, ","), info.redirect_gateway,
                   info.routes.size(), info.server_ip);
          // A tunnel that comes up but carries nothing is the failure this
          // tool is really looking for. Nothing else generates traffic here,
          // so send something that has to be answered.
          tun_ip = info.ipv4;
          ping_once();
        } else if (st == ovpn::TunnelState::Down) {
          shutdown();
        }
      },
                 &err)) {
    LOG_ERROR(kMod, "start failed: {}", err);
    return 2;
  }

  // Only now: pipe() is guaranteed valid from a successful start(), not before.
  read_one();

  while (!finished && !io.stopped()) {
    if (io.run_one() == 0) break;
  }
  io.run();  // drain the stop callback

  const auto ctr = tc->pipe().counters();
  LOG_INFO(kMod,
           "result: up={} tx={} pkts/{} B (dropped {}) rx={} pkts/{} B, "
           "{} echo replies",
           came_up, ctr.tx_packets, ctr.tx_bytes, ctr.tx_dropped,
           ctr.rx_packets, ctr.rx_bytes, echo_replies);

  if (!came_up) {
    LOG_ERROR(kMod, "tunnel never came up");
    return 1;
  }
  if (ctr.rx_packets == 0) {
    LOG_ERROR(kMod,
              "tunnel came up but no IP packet ever arrived on the pipe -- "
              "the socketpair-as-tun assumption needs re-checking");
    return 1;
  }
  if (echo_replies == 0) {
    LOG_ERROR(kMod,
              "packets arrive but none of them answered our echo requests; "
              "the node may be filtering ICMP, so this is inconclusive rather "
              "than a verdict on the tun plumbing");
    return 1;
  }
  LOG_INFO(kMod,
           "OK: openvpn3 took the socketpair as its tun, our hand-built IP "
           "packet reached {} and the reply came back unframed",
           opt.ping);
  return 0;
}