/*

 * Copyright (C) Max Dymond, <cmeister2@gmail.com>, et al.

 *

 * SPDX-License-Identifier: curl

 */

/// @file

/// @brief Implementation of the preloaded TELNET mock peer.

#include "proto_fuzzer/telnet_mock_server.h"

#include <algorithm>

#include <cstddef>

#include <memory>

#include <string>

#include "proto_fuzzer/mock_server.h"

#include "proto_fuzzer/request_data.h"

#include "proto_fuzzer/scenario_limits.h"

#include "proto_fuzzer/telnet_scenario.h"

namespace proto_fuzzer {

namespace {

// BoundTelnetResponse limits worst-case negotiation output to well below this

// floor. Rejecting a surprising platform socket configuration is preferable

// to letting curl's blocking POLLOUT path become a fuzzer timeout.

constexpr std::size_t kMinClientSendBufferBytes = 64 * 1024;

// curl currently builds each TTYPE, XDISPLOC, or NEW_ENV reply in a fixed

// 2048-byte buffer. Charging that whole buffer to every retained IAC is more

// conservative than curl's protocol grammar, while doubled upload bytes and a

// generous allowance for initial/small negotiation frames cover the remaining

// writes that can accumulate before the pre-read drain runs.

constexpr std::size_t kCurlTelnetSuboptionReplyBytes = 2048;

constexpr std::size_t kSmallNegotiationHeadroomBytes = 4 * 1024;

constexpr std::size_t kMaxBufferedClientBytes =

    scenario_limits::kMaxTelnetControlBytes * kCurlTelnetSuboptionReplyBytes +

    2 * scenario_limits::kMaxTelnetUploadBytes + kSmallNegotiationHeadroomBytes;

static_assert(kMaxBufferedClientBytes <= kMinClientSendBufferBytes,

              "TELNET mutation caps exceed the verified client send buffer");

/// Copy only the byte prefix the runtime can preload before applying the

/// control-byte budget. Compatibility inputs bypass the LPM postprocessor, so

/// copying each complete protobuf field and trimming afterwards would briefly

/// duplicate mutation-sized response strings that curl can never observe.

void CopyTelnetResponsePrefix(const curl::fuzzer::proto::Connection& source,

                              curl::fuzzer::proto::Connection* destination) {

  std::size_t bytes_left = scenario_limits::kMaxTelnetResponseBytes;

  const auto copy_fragment = [&bytes_left](const std::string& fragment, std::string* output) {

    const std::size_t retained = std::min(fragment.size(), bytes_left);

    output->assign(fragment.data(), retained);

    bytes_left -= retained;

    return retained == fragment.size();

  };

  if (!copy_fragment(source.initial_response(), destination->mutable_initial_response())) {

    return;

  }

  const std::size_t chunk_count =

      std::min<std::size_t>(scenario_limits::kMaxTelnetResponseChunks, source.on_readable_size());

  for (std::size_t index = 0; index < chunk_count && bytes_left != 0; ++index) {

    if (!copy_fragment(source.on_readable(static_cast<int>(index)), destination->add_on_readable())) {

      break;

    }

  }

}

}  // namespace

/// Start without a borrowed scenario. RunLoop installs the pointer only for

/// the synchronous multi drive in which curl may invoke HandleOpenSocket.

TelnetMockServer::TelnetMockServer() : scenario_(nullptr), socket_opened_(false) {}

/// The base owns and closes the server half after curl releases its client fd.

TelnetMockServer::~TelnetMockServer() = default;

void TelnetMockServer::ConfigureRequestData(ScenarioRequestData* request_data) {

  if (request_data != nullptr) {

    request_data->SetBeforeUploadReadCallback(&DrainBeforeUploadRead, this);

  }

}

void TelnetMockServer::DrainBeforeUploadRead(void* userdata) {

  auto* server = static_cast<TelnetMockServer*>(userdata);

  if (server != nullptr && server->connection_) {

    (void)server->connection_->DrainIncoming();

  }

}

/// Create one socketpair, preload every raw response fragment curl can observe,

/// then half-close the peer before returning the client fd. TELNET's transfer

/// function does not yield to the outer multi loop while it polls, so deferred

/// delivery would turn an ordinary mutation into a timeout. WriteAll uses a

/// non-blocking server fd; if a compatibility input exceeds the socket buffer,

/// the retained prefix is still useful and the half-close still guarantees

/// termination.

/// @param purpose Socket role requested by curl; TELNET uses one outbound

///                stream and does not need role-specific setup.

/// @param address Intended destination retained by curl; the already-connected

///                socketpair does not need to rewrite it.

/// @return The preloaded client socket, or CURL_SOCKET_BAD on setup failure.

curl_socket_t TelnetMockServer::HandleOpenSocket(curlsocktype purpose, struct curl_sockaddr* address) {

  (void)purpose;

  (void)address;

  if (scenario_ == nullptr || socket_opened_) {

    return CURL_SOCKET_BAD;

  }

  socket_opened_ = true;

  connection_ = std::make_unique<MockConnection>();

  if (!connection_->ok()) {

    connection_.reset();

    return CURL_SOCKET_BAD;

  }

  if (!connection_->EnsureClientSendBufferSize(kMinClientSendBufferBytes)) {

    connection_.reset();

    return CURL_SOCKET_BAD;

  }

  const auto write_bytes = [this](const std::string& bytes) {

    return bytes.empty() || connection_->WriteAll(reinterpret_cast<const unsigned char*>(bytes.data()), bytes.size());

  };

  // Compatibility corpus entries bypass the LPM postprocessor. Copy only the

  // raw prefix TELNET can consume, then normalize it so direct replay gets the

  // same byte/control budgets without traversing HTTP/WS-only fields.

  const auto& source = scenario_->connection();

  curl::fuzzer::proto::Connection script;

  CopyTelnetResponsePrefix(source, &script);

  BoundTelnetResponse(&script);

  bool complete = write_bytes(script.initial_response());

  const std::size_t chunk_count = static_cast<std::size_t>(script.on_readable_size());

  for (std::size_t index = 0; complete && index < chunk_count; ++index) {

    complete = write_bytes(script.on_readable(static_cast<int>(index)));

  }

  (void)complete;

  // Do not apply BackpressureConfig here. curl's TELNET driver sends from

  // inside its blocking protocol loop; tightening the socket would invalidate

  // the minimum-capacity invariant used by the pre-read drain.

  connection_->ShutdownWrite();

  return connection_->take_client_fd();

}

/// Drive a small number of non-waiting multi transitions. The first call that

/// enters telnet_do() sees a preloaded, half-closed peer and therefore returns

/// without relying on the outer loop to deliver bytes. Subsequent calls only

/// settle curl's completion bookkeeping.

/// @param multi Multi handle whose sole easy handle runs the TELNET transfer.

/// @param easy Easy handle already installed on this mock.

/// @param scenario Bounded script to preload when curl opens its socket.

void TelnetMockServer::RunLoop(CURLM* multi, CURL* easy, const curl::fuzzer::proto::Scenario& scenario) {

  (void)easy;

  scenario_ = &scenario;

  socket_opened_ = false;

  connection_.reset();

  int still_running = 1;

  for (int iteration = 0; still_running && iteration < kMaxIdleIterations; ++iteration) {

    const CURLMcode result = curl_multi_perform(multi, &still_running);

    if (result != CURLM_OK) {

      break;

    }

    if (connection_) {

      (void)connection_->DrainIncoming();

    }

  }

  scenario_ = nullptr;

}

}  // namespace proto_fuzzer