How to handle a user abort by Deno.serve

Question

The Deno website has an example at: https://deno.land/[email protected]/runtime/http_server_apis#inspecting-the-incoming-request

I have shortened it to the following code:

const abortController = new AbortController();
Deno.addSignalListener('SIGINT', () => {
    abortController.abort();
});

Deno.serve(
    {
        port:3000,
        signal: abortController.signal
    },
    async (req) => {
        if (req.body) {
            try {
                console.log('Start');
                const body = await req.formData();
                for (const key of body.keys()) {
                    console.log(key);
                }
                console.log('Never reached, when user abort.');
            } catch {
                // Will not be triggered on abort.
                console.log('Error on await req.formData()');
            }
        }

        return new Response("Hello, World!");
    }
);

There is the note below it: Be aware that the req.text() call can fail if the user hangs up the connection before the body is fully received. Make sure to handle this case.

However, I have not found how to catch such a case. Can Deno detect that the connection has been terminated?

By the way, I added the "AbortHandler" for the test. If await req.formData() passes successfully, the server can be terminated with Ctrl + C in the terminal (Linux Mint). If the user cancels the "upload", after the Ctrl + C command is still running a process until I close the terminal.

I think that is why Deno writes on the website, you have to catch this case. How to do this?

Answer 1

From Deno version 1.37 onwards, if a client closes the connection while the body is being consumed (using methods like .json, .arrayBuffer, .text, etc.), the promise will now appropriately reject.

---

OLD ANSWER

You can test that case with the following snippet.

const body = new ReadableStream({
        start(controller) {
                controller.enqueue(new Uint8Array([97]));
        }
});

const abort = new AbortController();
setTimeout(() => abort.abort(), 1500);

const res = await fetch('http://localhost:3000', { 
   body, 
   method: 'POST', 
   signal: abort.signal 
});

console.log(res.status);
console.log(await res.text());

Unfortunately Deno does not detect connection drops, see: https://github.com/denoland/deno/issues/16246

Deno.serve(
    {
        port:3000,
    },
    async (req) => {
        console.log(req.body);
        if (req.body) {
            try {
                for await(const chunk of req.body) { 
                    console.log(chunk);
                }
                console.log('Never reached, when user abort.');
            } catch {
                // Will not be triggered on abort.
                console.log('never reached, bug in Deno');
            }
        }

        return new Response("Hello, World!");
    }
);

You can see that the first chunk is logged, but there's no error at all when the connection is dropped.

ReadableStream { locked: false }
Uint8Array(1) [ 97 ]

You can see more about this here: Deno connection close event