如何在仍被編寫的同時在ASP.NET Core中提供文件服務

Question

我有一個由后台服務不斷寫入的日志文件。 到目前為止，用戶需要能夠下載文件。 當我返回MVC FileResult ，由於Content-Length不匹配，我收到了InvalidOperationException，大概是因為在提供文件時已將某些內容寫入了文件。 提供了一個文件，大多數情況下都可以，但是最后一行通常不完整。

后台服務實際上是在這樣做：

var stream = new FileStream(evidenceFilePath, FileMode.Append, FileAccess.Write, FileShare.Read);
while (true) // obviously it isn't actually this, but it does happen a lot!
{
    var content = "log content\r\n";
    stream.Write(Encoding.UTF8.GetBytes(content);
}

以下是控制器操作的一些變體（所有結果都相同）：

public IActionResult DownloadLog1()
{
    return PhysicalFile("C:\\path\\to\\the\\file.txt", "text/plain", enableRangeProcessing: false); // also tried using true
}

public IActionResult DownloadLog2()
{
    var stream = new FileStream("C:\\path\\to\\the\\file.txt", FileMode.Open, FileAccess.Read, FileShare.ReadWrite);
    return File(stream, "text/plain", enableRangeProcessing: false); // also tried true
}

這是我嘗試以上任一方法時遇到的異常：

System.InvalidOperationException: Response Content-Length mismatch: too many bytes written (216072192 of 216059904).
   at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpProtocol.ThrowTooManyBytesWritten(Int32 count)
   at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpProtocol.VerifyAndUpdateWrite(Int32 count)
   at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpProtocol.WriteAsync(ReadOnlyMemory`1 data, CancellationToken cancellationToken)
   at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpResponseStream.WriteAsync(Byte[] buffer, Int32 offset, Int32 count, CancellationToken cancellationToken)
   at Microsoft.AspNetCore.Http.Extensions.StreamCopyOperation.CopyToAsync(Stream source, Stream destination, Nullable`1 count, Int32 bufferSize, CancellationToken cancel)
   at Microsoft.AspNetCore.Mvc.Infrastructure.FileResultExecutorBase.WriteFileAsync(HttpContext context, Stream fileStream, RangeItemHeaderValue range, Int64 rangeLength)
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.InvokeResultAsync(IActionResult result)
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.InvokeNextResultFilterAsync[TFilter,TFilterAsync]()
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.Rethrow(ResultExecutedContext context)
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.ResultNext[TFilter,TFilterAsync](State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.InvokeResultFilters()
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.InvokeNextResourceFilter()
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.Rethrow(ResourceExecutedContext context)
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.Next(State& next, Scope& scope, Object& state, Boolean& isCompleted)
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.InvokeFilterPipelineAsync()
   at Microsoft.AspNetCore.Mvc.Internal.ResourceInvoker.InvokeAsync()
   at Microsoft.AspNetCore.Routing.EndpointMiddleware.Invoke(HttpContext httpContext)
   at Microsoft.AspNetCore.Routing.EndpointRoutingMiddleware.Invoke(HttpContext httpContext)
   at Microsoft.AspNetCore.Builder.RouterMiddleware.Invoke(HttpContext httpContext)
   at Microsoft.AspNetCore.Session.SessionMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Session.SessionMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.StaticFiles.StaticFileMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.StaticFiles.StaticFileMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Authentication.AuthenticationMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware.Invoke(HttpContext context)
   at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpProtocol.ProcessRequests[TContext](IHttpApplication`1 application)

我不太介意例外，但是如果沒有發生，我更喜歡它。 我確實需要修復不完整的最后一行問題。 對我來說，最明顯的解決方案是跟蹤肯定已寫入文件的字節數，並且以某種方式僅處理前n個字節。 我看不到使用FileResult和構造它的各種幫助程序方法的簡便方法。 該文件可能會變得非常大（最大約500MB），因此在內存中進行緩沖似乎不切實際。

Answer 1

我最終編寫了一個自定義的ActionResult和IActionResultExecutor進行匹配，它們很大程度上基於MVC FileStreamResult和FileStreamResultExecutor ：

public class PartialFileStreamResult : FileResult
{
    Stream stream;
    long bytes;

    /// <summary>
    /// Creates a new <see cref="PartialFileStreamResult"/> instance with
    /// the provided <paramref name="fileStream"/> and the
    /// provided <paramref name="contentType"/>, which will download the first <paramref name="bytes"/>.
    /// </summary>
    /// <param name="stream">The stream representing the file</param>
    /// <param name="contentType">The Content-Type header for the response</param>
    /// <param name="bytes">The number of bytes to send from the start of the file</param>
    public PartialFileStreamResult(Stream stream, string contentType, long bytes)
        : base(contentType)
    {
        this.stream = stream ?? throw new ArgumentNullException(nameof(stream));
        if (bytes == 0)
        {
            throw new ArgumentOutOfRangeException(nameof(bytes), "Invalid file length");
        }
        this.bytes = bytes;
    }

    /// <summary>
    /// Gets or sets the stream representing the file to download.
    /// </summary>
    public Stream Stream
    {
        get => stream;
        set => stream = value ?? throw new ArgumentNullException(nameof(stream));
    }

    /// <summary>
    /// Gets or sets the number of bytes to send from the start of the file.
    /// </summary>
    public long Bytes
    {
        get => bytes;
        set
        {
            if (value == 0)
            {
                throw new ArgumentOutOfRangeException(nameof(bytes), "Invalid file length");
            }
            bytes = value;
        }
    }

    /// <inheritdoc />
    public override Task ExecuteResultAsync(ActionContext context)
    {
        if (context == null)
        {
            throw new ArgumentNullException(nameof(context));
        }
        var executor = context.HttpContext.RequestServices.GetRequiredService<IActionResultExecutor<PartialFileStreamResult>>();
        return executor.ExecuteAsync(context, this);
    }
}

public class PartialFileStreamResultExecutor : FileResultExecutorBase, IActionResultExecutor<PartialFileStreamResult>
{
    public PartialFileStreamResultExecutor(ILoggerFactory loggerFactory)
        : base(CreateLogger<PartialFileStreamResultExecutor>(loggerFactory))
    {
    }

    public async Task ExecuteAsync(ActionContext context, PartialFileStreamResult result)
    {
        if (context == null)
        {
            throw new ArgumentNullException(nameof(context));
        }

        if (result == null)
        {
            throw new ArgumentNullException(nameof(result));
        }

        using (result.Stream)
        {
            long length = result.Bytes;
            var (range, rangeLength, serveBody) = SetHeadersAndLog(context, result, length, result.EnableRangeProcessing);
            if (!serveBody) return;

            try
            {
                var outputStream = context.HttpContext.Response.Body;
                if (range == null)
                {
                    await StreamCopyOperation.CopyToAsync(result.Stream, outputStream, length, bufferSize: BufferSize, cancel: context.HttpContext.RequestAborted);
                }
                else
                {
                    result.Stream.Seek(range.From.Value, SeekOrigin.Begin);
                    await StreamCopyOperation.CopyToAsync(result.Stream, outputStream, rangeLength, BufferSize, context.HttpContext.RequestAborted);
                }
            }
            catch (OperationCanceledException)
            {
                // Don't throw this exception, it's most likely caused by the client disconnecting.
                // However, if it was cancelled for any other reason we need to prevent empty responses.
                context.HttpContext.Abort();
            }
        }
    }
}

我本來可以做更多的工作來添加其他構造函數重載來設置一些可選參數（例如下載文件名等），但這足以滿足我的需要。

您需要在Startup.ConfigureServices中添加IActionResultExecutor：

services.AddTransient<IActionResultExecutor<PartialFileStreamResult>, PartialFileStreamResultExecutor>();

因此，我的控制器操作變為：

[HttpGet]
public IActionResult DownloadLog()
{
    var (path, bytes) = GetThePathAndTheNumberOfBytesIKnowHaveBeenFlushed();

    var stream = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.ReadWrite); // this ensures that the file can be read while it's still being written
    return new PartialFileStreamResult(stream, "text/plain", bytes);
}

Answer 2

文件是非托管資源。

因此，當您訪問非托管資源（如文件）時，將通過句柄打開它。 如果是文件，則為open_file_handle（從內存中回收）。

因此，我可以建議（非常通用）編寫日志條目的最佳方法：

打開文件，

寫文件

關閉文件

處置（如果適用）

簡而言之，不要讓流保持打開狀態。

其次，對於控制器，您可以查看通過控制器提供文件的MSDN示例。

Answer 3

好吧，一般而言，您可能會遇到文件鎖定問題，因此您需要進行計划和補償。 但是，您這里遇到的直接問題更容易解決。 問題歸結為返回流。 該流是在返回響應時寫入的，因此在創建響應主體時，計算出的內容長度是錯誤的。

您需要做的是在某個時間點捕獲日志，即將其讀取為byte[] 。 然后，您可以返回它，而不是返回流，並且可以正確計算content-length，因為byte[]在被讀取后不會改變。

using (var stream = new FileStream("C:\\path\\to\\the\\file.txt", FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
using (var ms = new MemoryStream())
{
    await stream.CopyToAsync(ms);
    return File(ms.ToArray(), "text/plain");
}