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了一些第三方下载管理器应用程序，而我们提供文件的方式目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：

恢复下载的基本思路是什么？是否有某个 HTTP GET 请求告诉我开始的偏移量？

Answer 1

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二千字节，它可能会发送标头 Range: bytes=1024-2048。

您可以查看the RFC for HTTP/1.1 的第 139 页了解更多信息。

Answer 2

感谢 icktoofay 让我开始，这是一个完整的示例，可以为其他开发人员节省一些时间：

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);
}

辅助方法

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 });
}

这演示了一种从磁盘或数据库中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，这会浪费资源。

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