通过 AshX 处理程序支持可恢复的 HTTP 下载？

Question

我们通过 ASP.NET 中的 ASHX 处理程序提供应用程序设置的下载。

一位客户告诉我们，他使用一些第三方下载管理器应用程序，而我们提供文件的方式当前不支持他的下载管理器应用程序的“恢复”功能。

我的问题是：

恢复下载背后的基本思想是什么？是否有某个 HTTP GET 请求告诉我起始偏移量？

Answer 1

感谢 icktoofay 让我开始，这里有一个完整的示例，可以节省其他开发人员一些时间：

磁盘示例

/// <summary>
/// Writes the file stored in the filesystem to the response stream without buffering in memory, ideal for large files. Supports resumable downloads.
/// </summary>
/// <param name="filename">The name of the file to write to the HTTP output.</param>
/// <param name="etag">A unique identifier for the content. Required for IE9 resumable downloads, must be a strong etag which means begins and ends in a quote i.e. "\"6c132-941-ad7e3080\""</param>
public static void TransmitFile(this HttpResponse response, string filename, string etag)
{
    var request = HttpContext.Current.Request;
    var fileInfo = new FileInfo(filename);
    var responseLength = fileInfo.Exists ? fileInfo.Length : 0;
    var buffer = new byte[4096];
    var startIndex = 0;

    //if the "If-Match" exists and is different to etag (or is equal to any "*" with no resource) then return 412 precondition failed
    if (request.Headers["If-Match"] == "*" && !fileInfo.Exists ||
        request.Headers["If-Match"] != null && request.Headers["If-Match"] != "*" && request.Headers["If-Match"] != etag)
    {
        response.StatusCode = (int)HttpStatusCode.PreconditionFailed;
        response.End();
    }

    if (!fileInfo.Exists)
    {
        response.StatusCode = (int)HttpStatusCode.NotFound;
        response.End();
    }

    if (request.Headers["If-None-Match"] == etag)
    {
        response.StatusCode = (int)HttpStatusCode.NotModified;
        response.End();
    }

    if (request.Headers["Range"] != null && (request.Headers["If-Range"] == null || request.Headers["IF-Range"] == etag))
    {
        var match = Regex.Match(request.Headers["Range"], @"bytes=(\d*)-(\d*)");
        startIndex = Parse<int>(match.Groups[1].Value);
        responseLength = (Parse<int?>(match.Groups[2].Value) + 1 ?? fileInfo.Length) - startIndex;
        response.StatusCode = (int)HttpStatusCode.PartialContent;
        response.Headers["Content-Range"] = "bytes " + startIndex + "-" + (startIndex + responseLength - 1) + "/" + fileInfo.Length;
    }

    response.Headers["Accept-Ranges"] = "bytes";
    response.Headers["Content-Length"] = responseLength.ToString();
    response.Cache.SetCacheability(HttpCacheability.Public); //required for etag output
    response.Cache.SetETag(etag); //required for IE9 resumable downloads
    response.TransmitFile(filename, startIndex, responseLength);
}

public void ProcessRequest(HttpContext context)
{
    var id = Parse<int>(context.Request.QueryString["id"]);
    var version = context.Request.QueryString["v"];
    var db = new DataClassesDataContext();
    var filePath = db.Documents.Where(d => d.ID == id).Select(d => d.Fullpath).FirstOrDefault();

    if (String.IsNullOfEmpty(filePath) || !File.Exists(filePath))
    {
        context.Response.StatusCode = (int)HttpStatusCode.NotFound;
        context.Response.End();
    }

    context.Response.AddHeader("content-disposition", "filename=" + Path.GetFileName(filePath));
    context.Response.ContentType = GetMimeType(filePath);
    context.Response.TransmitFile(filePath, version);
}

数据库示例

/// <summary>
/// Writes the file stored in the database to the response stream without buffering in memory, ideal for large files. Supports resumable downloads.
/// </summary>
/// <param name="retrieveBinarySql">The sql to retrieve the binary data of the file from the database to be transmitted to the client. Parameters can be reffered to by {0} the index in the supplied parameter array.</param>
/// <param name="retrieveBinarySqlParameters">The parameters used in the sql query. Specify null if no parameters are required.</param>
/// <param name="connectionString">The connectring string for the sql database.</param>
/// <param name="contentLength">The length of the content in bytes.</param>
/// <param name="etag">A unique identifier for the content. Required for IE9 resumable downloads, must be a strong etag which means begins and ends in a quote i.e. "\"6c132-941-ad7e3080\""</param>
/// <param name="useFilestream">If the binary data is stored using Sql's Filestream feature set this to true to stream the file directly.</param>
public static void TransmitFile(this HttpResponse response, string retrieveBinarySql, object[] retrieveBinarySqlParameters, string connectionString, int contentLength, string etag, bool useFilestream)
{
    var request = HttpContext.Current.Request;
    var responseLength = contentLength;
    var buffer = new byte[4096];
    var startIndex = 0;

    //if the "If-Match" exists and is different to etag (or is equal to any "*" with no resource) then return 412 precondition failed
    if (request.Headers["If-Match"] == "*" && contentLength == 0 ||
        request.Headers["If-Match"] != null && request.Headers["If-Match"] != "*" && request.Headers["If-Match"] != etag)
    {
        response.StatusCode = (int)HttpStatusCode.PreconditionFailed;
        response.End();
    }

    if (contentLength == 0)
    {
        response.StatusCode = (int)HttpStatusCode.NotFound;
        response.End();
    }

    if (request.Headers["If-None-Match"] == etag)
    {
        response.StatusCode = (int)HttpStatusCode.NotModified;
        response.End();
    }

    if (request.Headers["Range"] != null && (request.Headers["If-Range"] == null || request.Headers["IF-Range"] == etag))
    {
        var match = Regex.Match(request.Headers["Range"], @"bytes=(\d*)-(\d*)");
        startIndex = Parse<int>(match.Groups[1].Value);
        responseLength = (Parse<int?>(match.Groups[2].Value) + 1 ?? contentLength) - startIndex;
        response.StatusCode = (int)HttpStatusCode.PartialContent;
        response.Headers["Content-Range"] = "bytes " + startIndex + "-" + (startIndex + responseLength - 1) + "/" + contentLength;
    }

    response.Headers["Accept-Ranges"] = "bytes";
    response.Headers["Content-Length"] = responseLength.ToString();
    response.Cache.SetCacheability(HttpCacheability.Public); //required for etag output
    response.Cache.SetETag(etag); //required for IE9 resumable downloads
    response.BufferOutput = false; //don't load entire data into memory (buffer) before sending

    if (!useFilestream)
    {
        using (var connection = new SqlConnection(connectionString))
        {
            connection.Open();
            var command = new SqlCommand(retrieveBinarySql, connection);

            for (var i = 0; retrieveBinarySqlParameters != null && i < retrieveBinarySqlParameters.Length; i++)
            {
                command.Parameters.AddWithValue("p" + i, retrieveBinarySqlParameters[i]);
                command.CommandText = command.CommandText.Replace("{" + i + "}", "@p" + i);
            }

            var reader = command.ExecuteReader(CommandBehavior.SequentialAccess);
            if (!reader.Read())
            {
                response.StatusCode = (int)HttpStatusCode.NotFound;
                response.End();
            }

            for (var i = startIndex; i < contentLength; i += buffer.Length)
            {
                var bytesRead = (int)reader.GetBytes(0, i, buffer, 0, buffer.Length);
                response.OutputStream.Write(buffer, 0, bytesRead);
            }
        }
    }
    else
    {
        using (var connection = new SqlConnection(connectionString))
        {
            connection.Open();
            var tran = connection.BeginTransaction(IsolationLevel.ReadCommitted);
            var command = new SqlCommand(Regex.Replace(retrieveBinarySql, @"select \w+ ", v => v.Value.TrimEnd() + ".PathName(), GET_FILESTREAM_TRANSACTION_CONTEXT() "), connection);
            command.Transaction = tran;

            for (var i = 0; retrieveBinarySqlParameters != null && i < retrieveBinarySqlParameters.Length; i++)
            {
                command.Parameters.AddWithValue("p" + i, retrieveBinarySqlParameters[i]);
                command.CommandText = command.CommandText.Replace("{" + i + "}", "@p" + i);
            }

            var reader = command.ExecuteReader();
            if (!reader.Read())
            {
                response.StatusCode = (int)HttpStatusCode.NotFound;
                response.End();
            }

            var path = reader.GetString(0);
            var transactionContext = (byte[])reader.GetValue(1);

            using (var fileStream = new SqlFileStream(path, transactionContext, FileAccess.Read, FileOptions.SequentialScan, 0))
            {
                fileStream.Seek(startIndex, SeekOrigin.Begin);
                int bytesRead;
                do
                {
                    bytesRead = fileStream.Read(buffer, 0, buffer.Length);
                    response.OutputStream.Write(buffer, 0, bytesRead);
                }
                while (bytesRead == buffer.Length);
            }

            tran.Commit();
        }
    }
}

public void ProcessRequest(HttpContext context)
{
    var id = Parse<int>(context.Request.QueryString["id"]);
    var db = new DataClassesDataContext();
    var doc = db.Documents.Where(d => d.ID == id).Select(d => new { d.Data.Length, d.Filename, d.Version }).FirstOrDefault();

    if (doc == null)
    {
        context.Response.StatusCode = (int)HttpStatusCode.NotFound;
        context.Response.End();
    }

    context.Response.AddHeader("content-disposition", "filename=" + doc.Filename);
    context.Response.ContentType = GetMimeType(doc.Filename);
    context.Response.TransmitFile("select data from documents where id = {0}", new[] { id }, db.Connection.ConnectionString, doc.Length, doc.Version, false);
}

public static T Parse<T>(object value)
{
    //convert value to string to allow conversion from types like float to int
    //converter.IsValid only works since .NET4 but still returns invalid values for a few cases like NULL for Unit and not respecting locale for date validation
    try { return (T)System.ComponentModel.TypeDescriptor.GetConverter(typeof(T)).ConvertFrom(value.ToString()); }
    catch (Exception) { return default(T); }
}

public string GetMimeType(string fileName)
{
    //note use version 2.0.0.0 if .NET 4 is not installed, in .NET 4.5 this method has now been made public, this method apparently stores a list of mime types which would be more complete then using registry
    return (string)Assembly.Load("System.Web, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a")
        .GetType("System.Web.MimeMapping")
        .GetMethod("GetMimeMapping", BindingFlags.NonPublic | BindingFlags.Public | BindingFlags.Static)
        .Invoke(null, new object[] { fileName });
}

帮助方法 从磁盘或数据库读取部分文件并作为响应输出的方法，而不是将整个文件加载到内存中，如果下载中途暂停或恢复，则会浪费资源。

编辑：添加了 etag 以在 IE9 中启用可恢复下载，感谢 EricLaw 的帮助，使其在 IE9 中正常工作。

Answer 2

恢复下载通常通过 HTTP Range 标头进行。例如，如果客户端只需要文件的第二个千字节，它可能会发送标头 Range: bytes=1024-2048。

您可以参阅 HTTP/1.1 的 RFC 第 139 页了解更多信息。