Async HTTP Trigger Function Does Not Cancel Cleanly

[david-maw]

An isolated async function with a CancellationToken parameter correctly sets IsCancelRequested when a client disconnects while the function is running but it also:

1. Throws a TaskCancelled exception (which can be caught in the function)
2. Even if it is caught, reports "An exception was thrown by the invocation"

I'm new to Azure Functions so I may have missed something obvious, but I'd expected to see IsCancelRequested set and nothing else (no exception unless I threw an uncaught one).

The test is simply to invoke the sample function below from a browser then cancel the request before it has chance to complete.

My sample Function code below is just the default code generated by VS with some minor additions.

using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Azure.Functions.Worker;
using Microsoft.Extensions.Logging;

namespace FunctionAppCancel
{
    public class Function1
    {
        private readonly ILogger<Function1> _logger;

        public Function1(ILogger<Function1> logger)
        {
            _logger = logger;
        }

        [Function("Function1")]
        public async Task<IActionResult> Run([HttpTrigger(AuthorizationLevel.Anonymous, "get", "post")] HttpRequest req, CancellationToken cancellationToken)
        {
            try
            {
                _logger.LogInformation("C# HTTP trigger function processed a request.");
                await Task.Delay(5000, cancellationToken);

                if (cancellationToken.IsCancellationRequested)
                {
                    _logger.LogError("C# HTTP trigger function canceled.");
                    return new OkResult();
                }
                return new OkObjectResult("Welcome to Azure Functions!");
            }
            catch (Exception ex)
            {
                _logger.LogError($"C# HTTP trigger function threw exception {ex.Message}");
                return new BadRequestResult();
            }
        }
    }
}

The local emulator shows:

Azure Functions Core Tools
Core Tools Version:       4.0.6610 Commit hash: N/A +0d55b5d7efe83d85d2b5c6e0b0a9c1b213e96256 (64-bit)
Function Runtime Version: 4.1036.1.23224

[2025-01-14T00:56:41.615Z] Found H:\Develop\Samples\FunctionAppCancel\FunctionAppCancel\FunctionAppCancel.csproj. Using for user secrets file configuration.
[2025-01-14T00:56:43.806Z] Azure Functions .NET Worker (PID: 107448) initialized in debug mode. Waiting for debugger to attach...
[2025-01-14T00:56:43.841Z] Worker process started and initialized.

Functions:

        Function1: [GET,POST] http://localhost:7193/api/Function1

For detailed output, run func with --verbose flag.
[2025-01-14T00:56:48.886Z] Host lock lease acquired by instance ID '0000000000000000000000001BD189B9'.
[2025-01-14T00:56:52.633Z] Executing 'Functions.Function1' (Reason='This function was programmatically called via the host APIs.', Id=c0813456-e99e-4828-8b43-c2c6b28c97e6)
[2025-01-14T00:56:52.894Z] C# HTTP trigger function processed a request.
[2025-01-14T00:56:54.987Z] C# HTTP trigger function threw exception A task was canceled.
[2025-01-14T00:56:55.036Z] Function 'Function1', Invocation id 'c0813456-e99e-4828-8b43-c2c6b28c97e6': An exception was thrown by the invocation.
[2025-01-14T00:56:55.037Z] Result: Function 'Function1', Invocation id 'c0813456-e99e-4828-8b43-c2c6b28c97e6': An exception was thrown by the invocation.
Exception: System.ObjectDisposedException: Request has finished and HttpContext disposed.
[2025-01-14T00:56:55.038Z] Object name: 'HttpContext'.
[2025-01-14T00:56:55.039Z]    at Microsoft.AspNetCore.Http.DefaultHttpContext.ThrowContextDisposed()
[2025-01-14T00:56:55.040Z]    at Microsoft.AspNetCore.Http.DefaultHttpContext.get_Features()
[2025-01-14T00:56:55.041Z]    at Microsoft.AspNetCore.Routing.RoutingHttpContextExtensions.GetRouteData(HttpContext httpContext)
[2025-01-14T00:56:55.042Z]    at Microsoft.Azure.Functions.Worker.Extensions.Http.AspNetCore.FunctionsHttpProxyingMiddleware.TryHandleHttpResult(Object result, FunctionContext context, HttpContext httpContext, Boolean isInvocationResult) in D:\a\_work\1\s\extensions\Worker.Extensions.Http.AspNetCore\src\FunctionsMiddleware\FunctionsHttpProxyingMiddleware.cs:line 77
[2025-01-14T00:56:55.043Z]    at Microsoft.Azure.Functions.Worker.Extensions.Http.AspNetCore.FunctionsHttpProxyingMiddleware.Invoke(FunctionContext context, FunctionExecutionDelegate next) in D:\a\_work\1\s\extensions\Worker.Extensions.Http.AspNetCore\src\FunctionsMiddleware\FunctionsHttpProxyingMiddleware.cs:line 56
[2025-01-14T00:56:55.044Z]    at Microsoft.Azure.Functions.Worker.FunctionsApplication.InvokeFunctionAsync(FunctionContext context) in D:\a\_work\1\s\src\DotNetWorker.Core\FunctionsApplication.cs:line 96
Stack:    at Microsoft.AspNetCore.Http.DefaultHttpContext.ThrowContextDisposed()
[2025-01-14T00:56:55.045Z]    at Microsoft.AspNetCore.Http.DefaultHttpContext.get_Features()
[2025-01-14T00:56:55.046Z]    at Microsoft.AspNetCore.Routing.RoutingHttpContextExtensions.GetRouteData(HttpContext httpContext)
[2025-01-14T00:56:55.046Z]    at Microsoft.Azure.Functions.Worker.Extensions.Http.AspNetCore.FunctionsHttpProxyingMiddleware.TryHandleHttpResult(Object result, FunctionContext context, HttpContext httpContext, Boolean isInvocationResult) in D:\a\_work\1\s\extensions\Worker.Extensions.Http.AspNetCore\src\FunctionsMiddleware\FunctionsHttpProxyingMiddleware.cs:line 77
[2025-01-14T00:56:55.047Z]    at Microsoft.Azure.Functions.Worker.Extensions.Http.AspNetCore.FunctionsHttpProxyingMiddleware.Invoke(FunctionContext context, FunctionExecutionDelegate next) in D:\a\_work\1\s\extensions\Worker.Extensions.Http.AspNetCore\src\FunctionsMiddleware\FunctionsHttpProxyingMiddleware.cs:line 56
[2025-01-14T00:56:55.048Z]    at Microsoft.Azure.Functions.Worker.FunctionsApplication.InvokeFunctionAsync(FunctionContext context) in D:\a\_work\1\s\src\DotNetWorker.Core\FunctionsApplication.cs:line 96.
[2025-01-14T00:56:55.070Z] Executed 'Functions.Function1' (Failed, Id=c0813456-e99e-4828-8b43-c2c6b28c97e6, Duration=2463ms)
[2025-01-14T00:56:55.071Z] System.Private.CoreLib: Exception while executing function: Functions.Function1. Microsoft.Azure.WebJobs.Script.Grpc: Failed to proxy request with ForwarderError: RequestCanceled. System.Net.Http: The operation was canceled. System.Net.Sockets: Unable to read data from the transport connection: The I/O operation has been aborted because of either a thread exit or an application request.. The I/O operation has been aborted because of either a thread exit or an application request.