libuv笔记（一）libuv中连接主机出现的10061问题


出现10061主机拒绝问题是由于我在接受连接以后没有继续accept且超过backlog数，libuv中一个uv_tcp_socket可以接受32个连接，其余的连接会以失败结束。

现在解析下源码，观察为什么会出现这种现象。

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int uv_tcp_listen(uv_tcp_t* handle, int backlog, uv_connection_cb cb) {
  unsigned int i, simultaneous_accepts;
  uv_tcp_accept_t* req;
  int err;

  assert(backlog > 0);

  if (handle->flags & UV_HANDLE_LISTENING) {
    handle->stream.serv.connection_cb = cb; //设置回调函数
  }

  if (handle->flags & UV_HANDLE_READING) {
    return WSAEISCONN;
  }

  if (handle->delayed_error) {
    return handle->delayed_error;
  }

  if (!(handle->flags & UV_HANDLE_BOUND)) { //若在调用listen之前未绑定则绑定
    err = uv_tcp_try_bind(handle,
                          (const struct sockaddr*) &uv_addr_ip4_any_,
                          sizeof(uv_addr_ip4_any_),
                          0);
    if (err)
      return err;
    if (handle->delayed_error)
      return handle->delayed_error;
  }

  if (!handle->tcp.serv.func_acceptex) { //确保AcceptEx函数可以已加载
    if (!uv_get_acceptex_function(handle->socket, &handle->tcp.serv.func_acceptex)) {
      return WSAEAFNOSUPPORT;
    }
  }

  if (!(handle->flags & UV_HANDLE_SHARED_TCP_SOCKET) &&
      listen(handle->socket, backlog) == SOCKET_ERROR) {
    return WSAGetLastError();
  }

  handle->flags |= UV_HANDLE_LISTENING;
  handle->stream.serv.connection_cb = cb;
  INCREASE_ACTIVE_COUNT(loop, handle);

  simultaneous_accepts = handle->flags & UV_HANDLE_TCP_SINGLE_ACCEPT ? 1
    : uv_simultaneous_server_accepts;

  if (handle->tcp.serv.accept_reqs == NULL) {
    handle->tcp.serv.accept_reqs =
      uv__malloc(uv_simultaneous_server_accepts * sizeof(uv_tcp_accept_t));
    if (!handle->tcp.serv.accept_reqs) {
      uv_fatal_error(ERROR_OUTOFMEMORY, "uv__malloc");
    }

    //这一步simultaneous_accepts默认32，因此可以基本上猜测出只会调用32次AcceptEx
    for (i = 0; i < simultaneous_accepts; i++) {
      req = &handle->tcp.serv.accept_reqs[i];
      UV_REQ_INIT(req, UV_ACCEPT);
      req->accept_socket = INVALID_SOCKET;
      req->data = handle;

      req->wait_handle = INVALID_HANDLE_VALUE;
      if (handle->flags & UV_HANDLE_EMULATE_IOCP) {
        req->event_handle = CreateEvent(NULL, 0, 0, NULL);
        if (req->event_handle == NULL) {
          uv_fatal_error(GetLastError(), "CreateEvent");
        }
      } else {
        req->event_handle = NULL;
      }

      uv_tcp_queue_accept(handle, req);
    }

    /* Initialize other unused requests too, because uv_tcp_endgame doesn't
     * know how many requests were initialized, so it will try to clean up
     * {uv_simultaneous_server_accepts} requests. */
    for (i = simultaneous_accepts; i < uv_simultaneous_server_accepts; i++) {
      req = &handle->tcp.serv.accept_reqs[i];
      UV_REQ_INIT(req, UV_ACCEPT);
      req->accept_socket = INVALID_SOCKET;
      req->data = handle;
      req->wait_handle = INVALID_HANDLE_VALUE;
      req->event_handle = NULL;
    }
  }

  return 0;
}

接下来跟踪最重要的uv_tcp_queue_accept函数，这一步中解释了为什么会出现10061错误的原因

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static void uv_tcp_queue_accept(uv_tcp_t* handle, uv_tcp_accept_t* req) {
  uv_loop_t* loop = handle->loop;
  BOOL success;
  DWORD bytes;
  SOCKET accept_socket;
  short family;

  assert(handle->flags & UV_HANDLE_LISTENING);
  assert(req->accept_socket == INVALID_SOCKET);

  /* choose family and extension function */
  if (handle->flags & UV_HANDLE_IPV6) {
    family = AF_INET6;
  } else {
    family = AF_INET;
  }

  /* Open a socket for the accepted connection. */
  accept_socket = socket(family, SOCK_STREAM, 0); // 创建accept socket
  if (accept_socket == INVALID_SOCKET) {
    SET_REQ_ERROR(req, WSAGetLastError());
    uv_insert_pending_req(loop, (uv_req_t*)req);
    handle->reqs_pending++;
    return;
  }

  /* Make the socket non-inheritable */
  if (!SetHandleInformation((HANDLE) accept_socket, HANDLE_FLAG_INHERIT, 0)) {
    SET_REQ_ERROR(req, GetLastError());
    uv_insert_pending_req(loop, (uv_req_t*)req);
    handle->reqs_pending++;
    closesocket(accept_socket);
    return;
  }

  /* Prepare the overlapped structure. */
  memset(&(req->u.io.overlapped), 0, sizeof(req->u.io.overlapped));
  if (handle->flags & UV_HANDLE_EMULATE_IOCP) {
    assert(req->event_handle != NULL);
    req->u.io.overlapped.hEvent = (HANDLE) ((ULONG_PTR) req->event_handle | 1);
  }
  //这一步调用了AcceptEx函数，之前的源码中可以看到循环调用了32次，因此最多只有32个连接
  success = handle->tcp.serv.func_acceptex(handle->socket,
                                          accept_socket,
                                          (void*)req->accept_buffer,
                                          0,
                                          sizeof(struct sockaddr_storage),
                                          sizeof(struct sockaddr_storage),
                                          &bytes,
                                          &req->u.io.overlapped);

  if (UV_SUCCEEDED_WITHOUT_IOCP(success)) {
    /* Process the req without IOCP. */
    req->accept_socket = accept_socket;
    handle->reqs_pending++;
    uv_insert_pending_req(loop, (uv_req_t*)req);
  } else if (UV_SUCCEEDED_WITH_IOCP(success)) {
    /* The req will be processed with IOCP. */
    req->accept_socket = accept_socket;
    handle->reqs_pending++;
    if (handle->flags & UV_HANDLE_EMULATE_IOCP &&
        req->wait_handle == INVALID_HANDLE_VALUE &&
        !RegisterWaitForSingleObject(&req->wait_handle,
          req->event_handle, post_completion, (void*) req,
          INFINITE, WT_EXECUTEINWAITTHREAD)) {
      SET_REQ_ERROR(req, GetLastError());
      uv_insert_pending_req(loop, (uv_req_t*)req);
    }
  } else {
    /* Make this req pending reporting an error. */
    SET_REQ_ERROR(req, WSAGetLastError());
    uv_insert_pending_req(loop, (uv_req_t*)req);
    handle->reqs_pending++;
    /* Destroy the preallocated client socket. */
    closesocket(accept_socket);
    /* Destroy the event handle */
    if (handle->flags & UV_HANDLE_EMULATE_IOCP) {
      CloseHandle(req->event_handle);
      req->event_handle = NULL;
    }
  }
}

上诉源码中的注释详细解释了为什么会出现10061错误，根本原因是创建的uv_tcp_socket默认包含32个可连接的accept socket。因此在接受连接后，我的回调函数OnConnection中应该继续调用accept保持uv_tcp_socket可以继续接受新的连接。