从互联网下载文件时，连接不会超时 [英] The connection does not timeout while downloading file from internet

问题描述

与我的职位相关（如何检索文件从互联网通过HTTP？）关于如何轻松，强大地从互联网上下载文件，我找到了一个可能的解决方案 - 但是没有工作，因为它应该是工作。

Related to a post of mine ( How to retrieve a file from Internet via HTTP? ) about how to easily and robustly download a file from Internet, I have found a possible solution - however is not working as it was supposed to work.

根据MS文档，下面的代码应该是在我断开互联网之后的500ms超时。但是，它完全忽略了INTERNET_OPTION_RECEIVE_TIMEOUT设置。应用程序在下载过程中冻结。这个功能需要大约20-30，才能意识到互联网连接正在关闭，并让控制回到GUI。

According to MS documentation, the code below is supposed to time-out at 500ms after I disconnect myself from internet. However, it looks like it totally ignores the 'INTERNET_OPTION_RECEIVE_TIMEOUT' setting. The application freezes during download. It takes about 20-30 to this function to realize that there the Internet connection is down and to give the control back to the GUI.

有人知道为什么吗？

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;

以下是文档：

{

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：
我通过拔下电缆断开互联网用于无线）从应用程序启动下载后（我选择下载大文件）。它模拟网站脱机。

I disconnect the Internet by unplugging the cable or (for wireless) from software AFTER the application starts the download (I chose to download large file). It simulates the web site going offline.

推荐答案

MS IE代码中有一个记录的错误。只能通过使用线程中的代码并重新实现超时机制来解决。

There is a documented bug in MS IE code. Can only be solved by using the code in a thread and re-implementing the time out mechanism.

详细信息：

此动作显示InternetSetOption API错误的解决方法，通过创建一个第二个线程
InternetSetOption不设置超时值

"This acticle shows a workaround to the InternetSetOption API bug on setting timeout values by creating a second thread. InternetSetOption Does Not Set Timeout Values"

http://support.microsoft.com/default.aspx?scid=kb;en-us;Q224318

（链接被报告为破坏，责备MS不是我）

http://support.microsoft.com/default.aspx?scid=kb;en-us;Q224318
(Link was reported broken. Blame MS not me)

也许有人可以帮助在Delphi中实现这个错误修复。我个人没有C的经验。即使是伪帕斯卡的骨干也会很好。

Maybe somebody can help with implementing this bug fix also in Delphi. I personally don't have experience with C. Even the backbone in pseudo-Pascal will be nice.

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