Synchronous operations are disallowed error with AspNetCore integration when using HttpResponseDataExtensions.WriteString

[kshyju]

Repro steps:

1. Create a new net8 isolated app with one http trigger.

2. Add a reference to the Microsoft.Azure.Functions.Worker.Extensions.Http.AspNetCore NuGet package, version 1.0.0 or later to your project.

3. Update startup code

var host = new HostBuilder()
    .ConfigureFunctionsWebApplication()
    .Build();

host.Run();

Update the HttpTrigger function class to have below 3 functions

[Function("Foo")]
public HttpResponseData Foo([HttpTrigger(AuthorizationLevel.Anonymous, "get", "post")] HttpRequestData req)
{
    var response = req.CreateResponse(HttpStatusCode.OK);
    response.WriteString("Welcome to Azure Functions!");

    return response;
}

[Function("Foo2")]
public async Task<HttpResponseData> Foo2([HttpTrigger(AuthorizationLevel.Anonymous, "get", "post")] HttpRequestData req)
{
    var response = req.CreateResponse(HttpStatusCode.OK);
    await response.WriteStringAsync("Welcome to Azure Functions!");

    return response;
}

[Function("Bar")]
public IActionResult Bar([HttpTrigger(AuthorizationLevel.Anonymous, "get")] HttpRequest req)
{
    return new OkObjectResult($"Welcome to Azure Functions, {req.Query["name"]}!");
}

Run the app and execute all functions. "Bar" and "Foo2" will work. "Foo" will fail with below exception.

System.InvalidOperationException
HResult=0x80131509
Message=Synchronous operations are disallowed. Call WriteAsync or set AllowSynchronousIO to true instead.
Source=Microsoft.AspNetCore.Server.Kestrel.Core
StackTrace:
at Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpResponseStream.Write(Byte[] buffer, Int32 offset, Int32 count)
at Microsoft.Azure.Functions.Worker.Http.HttpResponseDataExtensions.WriteString(HttpResponseData response, String value, Encoding encoding)
at FunctionApp207.Function1.Foo(HttpRequestData req) in C:\Dev\Temp\FunctionApp207\Function1.cs:line 17

Packages used

  <ItemGroup>
    <PackageReference Include="Microsoft.Azure.Functions.Worker" Version="1.20.0" />
    <PackageReference Include="Microsoft.Azure.Functions.Worker.Extensions.Http" Version="3.1.0" />
    <PackageReference Include="Microsoft.Azure.Functions.Worker.Extensions.Http.AspNetCore" Version="1.1.1" />
    <PackageReference Include="Microsoft.Azure.Functions.Worker.Sdk" Version="1.16.4" />
    <PackageReference Include="Microsoft.ApplicationInsights.WorkerService" Version="2.22.0" />
    <PackageReference Include="Microsoft.Azure.Functions.Worker.ApplicationInsights" Version="1.1.0" />
  </ItemGroup>

[jviau]

Synchronous IO is disallowed by default in kestrel:

https://stackoverflow.com/questions/47735133/asp-net-core-synchronous-operations-are-disallowed-call-writeasync-or-set-all

services.Configure<KestrelServerOptions>(options =>
{
    options.AllowSynchronousIO = true;
});

Do we want to set that to true by default? Or leave that to customers to decide?

[fabiocav]

After discussion, here are some of the ideas on what we should do instead of enabling synchronous IO:

• Update all templates to make sure we're using async patterns when working with HttpRequestData
• Create an analyzer that will provide compile time validation and code suggestion to fix/prevent the issue
• Add this information to the migration guides (including highlighting the fact that ASP.NET Core behaviors apply when using the integration)

[fabiocav]

Assigning to the next spring to decompose this into different issues we can track.

[kshyju]

Opened #2286 and Azure/azure-functions-templates#1509