Fix form files when request buffering is enabled

src/Http/Http/src/Features/FormFeature.cs

@@ -251,9 +251,11 @@ private async Task<IFormCollection> InnerReadFormAsync(CancellationToken cancell
                         var fileSection = new FileMultipartSection(section, contentDisposition);
 
                         // Enable buffering for the file if not already done for the full body
+                        // Only first file can be a reference to buffered body to avoid concurrency issues
                         section.EnableRewind(
                             _request.HttpContext.Response.RegisterForDispose,
-                            _options.MemoryBufferThreshold, _options.MultipartBodyLengthLimit);
+                            _options.MemoryBufferThreshold, _options.MultipartBodyLengthLimit,
+                            forceBuffering: files is not null);
 
                         // Find the end
                         await section.Body.DrainAsync(cancellationToken);

src/Http/Http/src/Internal/BufferingHelper.cs

@@ -25,16 +25,17 @@ public static HttpRequest EnableRewind(this HttpRequest request, int bufferThres
     }
 
     public static MultipartSection EnableRewind(this MultipartSection section, Action<IDisposable> registerForDispose,
-        int bufferThreshold = DefaultBufferThreshold, long? bufferLimit = null)
+        int bufferThreshold = DefaultBufferThreshold, long? bufferLimit = null, bool forceBuffering = false)
     {
         ArgumentNullException.ThrowIfNull(section);
         ArgumentNullException.ThrowIfNull(registerForDispose);
 
         var body = section.Body;
-        if (!body.CanSeek)
+        if (!body.CanSeek || forceBuffering)
         {
             var fileStream = new FileBufferingReadStream(body, bufferThreshold, bufferLimit, AspNetCoreTempDirectory.TempDirectoryFactory);
             section.Body = fileStream;
+            section.BaseStreamOffset = null;
             registerForDispose(fileStream);
         }
         return section;

src/Http/Http/test/Features/FormFeatureTests.cs

@@ -176,6 +176,11 @@ private class MockRequestBodyPipeFeature : IRequestBodyPipeFeature
         MultipartFormFile +
         MultipartFormEnd;
 
+    private const string MultipartFormWithFiles =
+        MultipartFormFile +
+        MultipartFormFile +
+        MultipartFormEnd;
+
     private const string MultipartFormWithFieldAndFile =
         MultipartFormField +
         MultipartFormFile +
@@ -313,6 +318,38 @@ public async Task ReadFormAsync_MultipartWithFile_ReturnsParsedFormCollection(bo
         await responseFeature.CompleteAsync();
     }
 
+    [Theory]
+    [InlineData(true)]
+    [InlineData(false)]
+    public async Task ReadFormAsync_MultipartWithFiles_ReturnsParsedFormCollection(bool bufferRequest)
+    {
+        var formContent = Encoding.UTF8.GetBytes(MultipartFormWithFiles);
+        var context = new DefaultHttpContext();
+        var responseFeature = new FakeResponseFeature();
+        context.Features.Set<IHttpResponseFeature>(responseFeature);
+        context.Request.ContentType = MultipartContentType;
+        context.Request.Body = new NonSeekableReadStream(formContent);
+
+        IFormFeature formFeature = new FormFeature(context.Request, new FormOptions() { BufferBody = bufferRequest });
+        context.Features.Set<IFormFeature>(formFeature);
+
+        var formCollection = await context.Request.ReadFormAsync();
+
+        Assert.NotNull(formCollection);
+        Assert.NotNull(formCollection.Files);
+        Assert.Equal(2, formCollection.Files.Count);
+
+        Stream[] streams = [formCollection.Files[0].OpenReadStream(), formCollection.Files[1].OpenReadStream()];
+        foreach (var stream in streams.Reverse())
+        {
+            using var reader = new StreamReader(stream);
+            Assert.True(stream.CanSeek);
+            await reader.ReadToEndAsync();
+        }
+
+        await responseFeature.CompleteAsync();
+    }
+
     [Theory]
     [InlineData(true)]
     [InlineData(false)]