Fix MapStaticAssets returning incorrect Content-Length for Range requests

src/StaticAssets/src/Development/StaticAssetDevelopmentRuntimeHandler.cs

@@ -191,11 +191,22 @@ public Task SendFileAsync(string path, long offset, long? count, CancellationTok
                 _context.Response.Headers.ETag = "";
                 // Compute the new ETag, if this is a compressed asset, RuntimeStaticAssetResponseBodyFeature will update it.
                 _context.Response.Headers.ETag = GetETag(fileInfo);
-                _context.Response.Headers.ContentLength = fileInfo.Length;
                 _context.Response.Headers.LastModified = fileInfo.LastModified.ToString("ddd, dd MMM yyyy HH:mm:ss 'GMT'", CultureInfo.InvariantCulture);
 
-                // Send the modified asset as is.
-                return _original.SendFileAsync(fileInfo.PhysicalPath!, 0, fileInfo.Length, cancellationToken);
+                // For range requests (206 Partial Content), we preserve the original offset and count to send the correct range.
+                // For normal requests (200 OK), we send the entire file.
+                var isRangeRequest = _context.Response.StatusCode == StatusCodes.Status206PartialContent;
+                if (isRangeRequest)
+                {
+                    // Send the requested range from the modified file.
+                    return _original.SendFileAsync(fileInfo.PhysicalPath!, offset, count, cancellationToken);
+                }
+                else
+                {
+                    // Send the entire modified file.
+                    _context.Response.Headers.ContentLength = fileInfo.Length;
+                    return _original.SendFileAsync(fileInfo.PhysicalPath!, 0, fileInfo.Length, cancellationToken);
+                }
             }
         }
 

src/StaticAssets/test/StaticAssetsIntegrationTests.cs

@@ -393,6 +393,72 @@ public async Task CanModifyAssetsOnTheFlyInDevelopment()
         Directory.Delete(webRoot, true);
     }
 
+    [Fact]
+    public async Task RangeRequestReturnsCorrectContentLengthForModifiedAssets()
+    {
+        // Arrange
+        var appName = nameof(RangeRequestReturnsCorrectContentLengthForModifiedAssets);
+        var (contentRoot, webRoot) = ConfigureAppPaths(appName);
+
+        CreateTestManifest(
+            appName,
+            webRoot,
+            [
+                new TestResource("sample.txt", "Hello, World!", false),
+            ]);
+
+        var builder = WebApplication.CreateEmptyBuilder(new WebApplicationOptions
+        {
+            ApplicationName = appName,
+            ContentRootPath = contentRoot,
+            EnvironmentName = "Development",
+            WebRootPath = webRoot
+        });
+        builder.WebHost.UseSetting(StaticAssetDevelopmentRuntimeHandler.ReloadStaticAssetsAtRuntimeKey, "true");
+        builder.WebHost.ConfigureServices(services =>
+        {
+            services.AddRouting();
+        });
+        builder.WebHost.UseTestServer();
+
+        var app = builder.Build();
+        app.UseRouting();
+        app.UseEndpoints(endpoints =>
+        {
+            endpoints.MapStaticAssets();
+        });
+
+        await app.StartAsync();
+
+        var client = app.GetTestClient();
+
+        File.WriteAllText(Path.Combine(webRoot, "sample.txt"), "Hello, World! Modified");
+
+        // Act - Request first 5 bytes (Range: bytes=0-4)
+        var message = new HttpRequestMessage(HttpMethod.Get, "/sample.txt");
+        message.Headers.Range = new RangeHeaderValue(0, 4);
+        var response = await client.SendAsync(message);
+
+        // Assert - Should return 206 Partial Content with correct Content-Length of 5 bytes
+        Assert.NotNull(response);
+        Assert.Equal(HttpStatusCode.PartialContent, response.StatusCode);
+        Assert.Equal(5, response.Content.Headers.ContentLength);
+        Assert.Equal("Hello", await response.Content.ReadAsStringAsync());
+
+        // Act - Request bytes 7-11 (Range: bytes=7-11)
+        var message2 = new HttpRequestMessage(HttpMethod.Get, "/sample.txt");
+        message2.Headers.Range = new RangeHeaderValue(7, 11);
+        var response2 = await client.SendAsync(message2);
+
+        // Assert - Should return 206 Partial Content with correct Content-Length of 5 bytes
+        Assert.NotNull(response2);
+        Assert.Equal(HttpStatusCode.PartialContent, response2.StatusCode);
+        Assert.Equal(5, response2.Content.Headers.ContentLength);
+        Assert.Equal("World", await response2.Content.ReadAsStringAsync());
+
+        Directory.Delete(webRoot, true);
+    }
+
     [Fact]
     public async Task CanModifyAssetsWithCompressedVersionsOnTheFlyInDevelopment()
     {