Dart Functions aren't cancel safe

[sampaioletti]

Sorry i was unable to use the bug template..it kept saying could not create issue.

Calling an Async Dart Fn in a cancelable context causes a panic.

Clone and modify the https://github.com/fzyzcjy/flutter_rust_bridge/tree/master/frb_example/dart_minimal example as follows.

1. Add tokio
2. Add rust sleep fn in rust/src/api/minimal.rs

pub async fn sleep_fn( sleep_cb: impl Fn(i32) -> DartFnFuture<()> + Send + Sync + 'static){
    tokio::select!{
        _=(sleep_cb)(1000)=>{
            println!("sleep 1000 returning")
        }
        _=(sleep_cb)(500)=>{
            println!("sleep 500 returning")
        }
    }
}

4. Call from dart in lib/src/main.dart

  await sleepFn(sleepCb: (ms) async {
    print("sleeping $ms");
    await Future.delayed(Duration(milliseconds: ms));
    print("returning $ms");
  });

Short sleep returns first and successfully returns from select function

When longer delay returns it causes a panic..I think because the future has been dropped.

sleeping 1000
sleeping 500
returning 500
sleep 500 returning
returning 1000
thread '<unnamed>' panicked at C:\Code\playground\flutter_rust_bridge\frb_rust\src\dart_fn\handler.rs:61:45:
called `Result::unwrap()` on an `Err` value: Dart2RustMessageSse { vec: [0, 0, 0, 0, 0, 0, 0, 0], data_len: 1 }

Fork available at https://github.com/sampaioletti/flutter_rust_bridge/tree/cancel-async-bug

[fzyzcjy]

Hi, could you please show full stacktrace to see what is going on? (e.g. by https://cjycode.com/flutter_rust_bridge/guides/how-to/stack-trace)

[sampaioletti]

Sorry had to get setup in WSL since the backtrace in windows is junk

Call Rust and get: 100+200 = 300
sleeping 500
sleeping 1000
returinging 500
sleep 500 returning
returinging 1000
thread '<unnamed>' panicked at /mnt/c/Code/playground/flutter_rust_bridge/frb_rust/src/dart_fn/handler.rs:61:45:
called `Result::unwrap()` on an `Err` value: Dart2RustMessageSse { vec: [0, 0, 0, 0, 0, 0, 0, 0], data_len: 1 }
stack backtrace:
   0:     0x7ff36b93083a - <std::sys::backtrace::BacktraceLock::print::DisplayBacktrace as core::fmt::Display>::fmt::ha4a311b32f6b4ad8
   1:     0x7ff36b94d943 - core::fmt::write::h1866771663f62b81
   2:     0x7ff36b92e153 - std::io::Write::write_fmt::hb549e7444823135e
   3:     0x7ff36b930682 - std::sys::backtrace::BacktraceLock::print::hddd3a9918ce29aa7
   4:     0x7ff36b9319cd - std::panicking::default_hook::{{closure}}::h791f75256b902d7d
   5:     0x7ff36b931813 - std::panicking::default_hook::h82cc572fcb0d8cd7
   6:     0x7ff36b8ee5f4 - flutter_rust_bridge::misc::panic_backtrace::PanicBacktrace::setup::{{closure}}::h375890a8d6440541
   7:     0x7ff36b932078 - std::panicking::rust_panic_with_hook::he21644cc2707f2c4
   8:     0x7ff36b931e3a - std::panicking::begin_panic_handler::{{closure}}::h42f7c414fed3cad9
   9:     0x7ff36b930d19 - std::sys::backtrace::__rust_end_short_backtrace::ha26cf5766b4e8c65
  10:     0x7ff36b931acc - rust_begin_unwind
  11:     0x7ff36b8dc1e0 - core::panicking::panic_fmt::h74866b78e934b1c0
  12:     0x7ff36b8dc5b6 - core::result::unwrap_failed::h899ed7ab2ccb8159
  13:     0x7ff36b8ecd7f - flutter_rust_bridge::dart_fn::handler::DartFnHandler::handle_output::hab78b2daa3dac4e5
  14:     0x7ff36b8e986f - frb_dart_fn_deliver_output
  15:     0x7ff3a9d0718b - <unknown>
Error when dart_fn_handle_output: Any { .. }

[sampaioletti]

Looks to me like because of the dropped future the completor needs to ignore send errors

This commit fixes the issue in my code.

sampaioletti@0ef0b9d

[fzyzcjy]

Looks reasonable and feel free to PR for this!

Btw, instead of ignoring the error, it may be reasonable to use log_warn_or_println to print it.

[sampaioletti]

Cool thanks. I'll throw a PR at this, but the more I've thought about it, it might be more 'correct' to wrap the dart fn in a CancelableOperation https://api.flutter.dev/flutter/async/CancelableOperation-class.html in a future PR so that we can also stop execution when it's dropped on the rust side.

[fzyzcjy]

Looks pretty reasonable!