Deno: Panic when sending UDP without awaiting

[hazae41]

When sending an UDP in a for-loop, Rust panics and the app crashes

for(const buffer of buffers){
    listener.send(buffer, ...);
}

thread 'main' panicked at 'already borrowed: BorrowMutError', /rustc/4fb7144ed159f94491249e86d5bbd033b5d60550\src\libcore\cell.rs:878:9

note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace`

Sending an UDP in a for-loop, but awaiting, works

for(const buffer of buffers){
    await listener.send(buffer, ...);
}

[ry]

Is this using version 1.0.5 ?

Can you provide some concrete example script which demonstrates this error?

[pevers]

@ry I debugged it and found this

Example code that breaks:

const listener = Deno.listenDatagram({
  port: 8080,
  transport: "udp"
});

const encoder = new TextEncoder();
const buffer = encoder.encode('test data');

const buffers = [buffer, buffer];

for(const buffer of buffers){
  listener.send(buffer, {
    hostname: "127.0.0.1",
    port: 8080,
    transport: "udp"
  });
}

Output:

thread 'main' panicked at 'already borrowed: BorrowMutError', /rustc/8d69840ab92ea7f4d323420088dd8c9775f180cd/src/libcore/cell.rs:878:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

At this borrow:
https://github.com/denoland/deno/blob/bad6f2b224a92e3bf5cdaf5f8cbc700cb0f9de04/cli/ops/net.rs#L213

It seems like a multi-threading issue. But I lack the knowledge of Rust to come up with a fix.

[ry]

@pevers Thank you - I can repeat it now. I'm working on a fix.