Url.ActionLink() is not thread-safe

I have an API that allows users to upload documents. To optimize the performance I save the documents to the database in parallel. After uploading has completed I return a list of URI's pointing to the document locations.

Here is what the code looks like:

	[HttpPost]
	[ProducesResponseType(StatusCodes.Status200OK)]
	public async Task<IActionResult> UploadDocuments(List<IFormFile> documents)
	{
	var tasks = new List<Task<string>>();
	foreach (var document in documents)
	{
	var task=UploadDocument(document);
	tasks.Add(task);
	}
	var documentUris = await Task.WhenAll(tasks);
	return Ok(documentUris);
	async Task<string> UploadDocument(IFormFile document)
	{
	var data = await document.GetData();
	var createdDocument = await _documentService.CreateDocument(data, document.FileName, document.ContentType);
	return Url.ActionLink("GetDocumentContent", "Document", new { createdDocument.Id, filename = createdDocument.Metadata.FileName });
	}
	}

view raw UploadDocuments.cs hosted with ❤ by GitHub

As you can see in the code above, I combine a Task.WhenAll with a local function. Nothing special.

However when I executed this code under high load, I started to get errors back from the API. A look at the logs, showed the following exception:

Index was out of range. Must be non-negative and less than the size of the collection. Parameter name: chunkLength

at System.Text.StringBuilder.ToString()

  at Microsoft.AspNetCore.Mvc.Routing.UrlHelperBase.GenerateUrl(String protocol, String host, String path)

  at Microsoft.AspNetCore.Mvc.Routing.EndpointRoutingUrlHelper.Action(UrlActionContext urlActionContext)

  at Microsoft.AspNetCore.Mvc.UrlHelperExtensions.ActionLink(IUrlHelper helper, String action, String controller, Object values, String protocol, String host, String fragment)

  at DocumentStorage.API.Controllers.DocumentsController.<UploadDocuments>g__UploadDocument|4_0(IFormFile document) in D:\b\3\_work\129\s\DocumentStorage\DocumentStorage.API\Controllers\DocumentsController.cs:line 53

  at DocumentStorage.API.Controllers.DocumentsController.UploadDocuments(List`1 documents)

It turns out that the Url.ActionLink() method is using a StringBuilder behind the scenes. And the StringBuilder is not thread-safe which brings us into trouble when we start using it in combination with the TPL.

To fix the threading issue I moved the url generation logic outside the parallel execution path:

	[HttpPost]
	[ProducesResponseType(StatusCodes.Status200OK)]
	public async Task<IActionResult> UploadDocuments(List<IFormFile> documents)
	{
	var tasks = new List<Task<(string key, string fileName)>>();
	foreach (var document in documents)
	{
	var task=UploadDocument(document);
	tasks.Add(task);
	}
	var uploadedDocuments=await Task.WhenAll(tasks);
	//Url.ActionLink turns out not to be thread-safe. Therefore we do the URL generation outside the UploadDocument inline function.
	var documentUris = uploadedDocuments
	.Select(document => Url.ActionLink("GetDocumentContent", "Document", new { document.id, document.fileName }))
	.ToArray();
	return Ok(documentUris);
	async Task<(string id, string filename)> UploadDocument(IFormFile document)
	{
	var data = await document.GetData();
	var createdDocument = await _documentService.CreateDocument(data, document.FileName, document.ContentType);
	return (createdDocument.Id, createdDocument.Metadata.FileName);
	}
	}

view raw UploadDocumentsFixed.cs hosted with ❤ by GitHub