从Internet下载文件时连接不会超时

Question

与我的帖子(如何通过HTTP从Internet检索文件？)有关如何从Internet轻松,健壮地下载文件相关,我找到了一个可能的解决方案 - 但是不能正常工作.

根据MS文档,我断开自己的互联网后,下面的代码应该在500ms超时.但是,它看起来完全忽略了'INTERNET_OPTION_RECEIVE_TIMEOUT'设置.应用程序在下载期间冻结.此功能需要大约20-30才能实现Internet连接断开并将控制权交还给GUI.

谁知道为什么？

function GetBinFileHTTP (const aUrl: string; const pStream: TStream; wTimeOut: Word= 500; wSleep: Word= 500; wAttempts: Word= 10): Integer;
CONST
  BufferSize = 1024;
VAR
  hSession, hService: HINTERNET;
  Buffer     : array[0..BufferSize-1] of Char;
  dwBytesRead, dwBytesAvail: DWORD;
  lSucc        : LongBool;
  lRetries, dwTimeOut: Integer;   
begin
 Result:= 0;
 if NOT IsConnectedToInternet then
  begin
   Result:= -1;
   EXIT;
  end;

 hSession := InternetOpen(PChar(ExtractFileName(Application.ExeName)), INTERNET_OPEN_TYPE_PRECONFIG, nil, nil, 0);  { The INTERNET_OPEN_TYPE_PRECONFIG flag specifies that if the user has configured Internet Explorer to use a proxy server, WinInet will use it as well. }
 if NOT Assigned(hSession) then
  begin
   Result:= -4;
   EXIT;
  end;

 TRY
   hService := InternetOpenUrl(hSession, PChar(aUrl), nil, 0, INTERNET_FLAG_RELOAD, 0);
   if NOT Assigned(hService) then Exit;
   TRY
     FillChar(Buffer, SizeOf(Buffer), 0);

     { Set time out }
     dwTimeOut:= wTimeOut;
     InternetSetOption(hService, INTERNET_OPTION_RECEIVE_TIMEOUT, @dwTimeOut, SizeOf(dwTimeOut));   { use INTERNET_FLAG_RELOAD instead of NIL to redownload the file instead of using the cache }


     InternetSetOption(hService, INTERNET_OPTION_CONNECT_TIMEOUT, @dwTimeOut, SizeOf(dwTimeOut));

     REPEAT
       lRetries := 0;

       REPEAT
         lSucc:= InternetQueryDataAvailable( hService, dwBytesAvail, 0, 0);
         if NOT lSucc
         then Sleep( wSleep );
         if lRetries > wAttempts
         then Result:= -2;
       UNTIL lSucc OR (Result= -2);

       if NOT InternetReadFile(hService, @Buffer, BufferSize, dwBytesRead) then
        begin
          Result:= -3;                                                          { Error: File not found/File cannot be downloaded }
          EXIT;
        end;

       if dwBytesRead = 0
       then Break;

       pStream.WriteBuffer(Buffer[0], dwBytesRead);

     UNTIL False;
   FINALLY
     InternetCloseHandle(hService);
   end;
 FINALLY
   InternetCloseHandle(hSession);
 end;

 Result:= 1;
end;

这是文档:

{

INTERNET_OPTION_CONNECT_TIMEOUT         Sets or retrieves an unsigned long integer value that contains the time-out value to use for Internet connection requests. If a connection request takes longer than this time-out value, the request is canceled. When attempting to connect to multiple IP addresses for a single host (a multihome host), the timeout limit is cumulative for all of the IP addresses. This option can be used on any HINTERNET handle, including a NULL handle. It is used by InternetQueryOption  and InternetSetOption.
INTERNET_OPTION_RECEIVE_TIMEOUT         Sets or retrieves an unsigned long integer value that contains the time-out value to receive a response to a request.      If the response takes longer than this time-out value, the request is canceled. This option can be used on any HINTERNET handle, including a NULL handle. It is used by InternetQueryOption and InternetSetOption. For using WinInet synchronously, only the default value for this flag can be changed by calling InternetSetOption and passing NULL in the hInternet parameter.
                  INTERNET_OPTION_CONTROL_RECEIVE_TIMEOUT - Identical to INTERNET_OPTION_RECEIVE_TIMEOUT. This is used by InternetQueryOption and InternetSetOption.
}

编辑:我在应用程序开始下载后从软件拔下电缆或(用于无线)断开Internet连接(我选择下载大文件).它模拟网站脱机.

Answer 1

连接超时显然不适用于您的测试,因为在您开始测试时(即拔插头),连接已经建立.实际上,在您开始设置超时选项之前,已经建立了连接.

接收超时的有效性也是可疑的,因为您已经开始接收响应.

看起来最有希望的超时是断开连接超时,但MSDN表示尚未实现.

在我看来,要走的路是使用异步操作.使用InternetReadFileEx和使用irf_Async和irf_No_Wait标志.如果在没有接收任何数据的情况下经过太多时间,请关闭连接.另一种选择是坚持使用同步调用,但InternetCloseHandle如果下载时间过长,则从另一个线程调用.