Rust Async Internals

Rust Async Internals -- RIPDPI

Purpose

Guide agents through async patterns specific to RIPDPI: JNI-to-async bridging, the io_loop event-driven architecture, tokio runtime configuration for Android NDK, CancellationToken-based shutdown, and common select!/join! pitfalls.

Triggers

• "Why is the tunnel blocking / slow?"
• "How does JNI call into async Rust here?"
• "How does select! work and what are the pitfalls?"
• "How is the tokio runtime configured?"
• "How does cancellation / shutdown work?"

Project async architecture

RIPDPI has two independent tokio runtimes bridged from JNI:

1. Tunnel runtime (ripdpi-tunnel-android): shared multi_thread(2) runtime stored in a OnceCell<Arc<Runtime>>. A dedicated std::thread calls runtime.block_on(run_tunnel(...)) so the JNI thread is not blocked.

2. Proxy runtime (ripdpi-android): run_proxy_with_embedded_control blocks the calling JNI thread directly (the Android Service thread). Uses an IdleGuard drop-safety pattern to reset state on panic.

JNI-to-async bridge pattern

// ripdpi-tunnel-android/src/session/lifecycle.rs -- the canonical pattern
//
// JNI create_session() returns a jlong handle.
// JNI start_session() spawns a std::thread that calls runtime.block_on():
let worker = std::thread::Builder::new()
    .name("ripdpi-tunnel-worker".into())
    .spawn(move || {
        let result = std::panic::catch_unwind(AssertUnwindSafe(|| {
            runtime.block_on(run_tunnel(config, fd, cancel, stats))
        }));
        // Handle Ok/Err/panic, update last_error and telemetry
    });
// The JNI thread returns immediately; stop_session() cancels via token.

Key rules for this pattern:

• Never call block_on from a JNI callback thread directly for long-running work -- spawn a dedicated thread so the JNI call returns promptly.
• Duplicate file descriptors (nix::unistd::dup) before passing to async -- Android VpnService can revoke the original fd at any time.
• Wrap block_on in catch_unwind -- a panic must not unwind through JNI.

Required tokio version floor

Do not downgrade tokio below these versions:

• ≥ 1.42.1 — fixes a broadcast::Sender::clone() soundness bug (missing synchronization for Send + !Sync payloads) and a CancellationToken race where futures that polled to Ready before the token fired were not cancelled. RIPDPI's connection-level abort paths rely on the cancellation fix. See the tokio CHANGELOG and PR #7462.
• ≥ 1.51.1 — fixes a file-descriptor leak when an io_uring open operation is cancelled before completion. ripdpi-tunnel-core cancels in-flight FS/IO operations on session teardown; below this version the leaked fds accumulate until the process exits. See PR #7983.

If cargo tree -i tokio shows a version below the floor, promote it in native/rust/Cargo.toml workspace dependencies (never downgrade a transitive dep to work around a breaking change — open an upstream issue instead).

Runtime configuration for Android NDK

// ripdpi-tunnel-android/src/session/registry.rs
tokio::runtime::Builder::new_multi_thread()
    .worker_threads(2)          // constrained for mobile battery/CPU
    .thread_stack_size(1024 * 1024) // 1 MiB -- Android default is small
    .thread_name("ripdpi-tunnel-tokio")
    .enable_all()
    .build()

Android-specific considerations:

• worker_threads(2) balances throughput vs battery drain on mobile.
• Explicit thread_stack_size(1 MiB) -- Android NDK default stack is often too small for deep async state machines.
• The runtime is stored in OnceCell<Arc<Runtime>> and shared across sessions to avoid repeated thread pool creation.
• current_thread runtime is used only in tests and the DNS resolver's synchronous resolve_blocking() path.

io_loop event-driven architecture

The core tunnel runs a single-task 6-phase loop (io_loop_task in ripdpi-tunnel-core/src/io_loop.rs):

1. Drain TUN fd -- read raw IP packets via AsyncFd::try_io, classify
2. smoltcp poll -- advance TCP state machines in userspace
3. New sessions -- detect ESTABLISHED sockets, spawn TcpSession tasks
4. Duplex bridge -- pump data between smoltcp sockets and session tasks
5. Flush tx_queue -- write smoltcp packets back to TUN fd
6. Wait -- select! on TUN readable / smoltcp timer / UDP / DNS / cancel

Phase 6 select pattern:

tokio::select! {
    _ = tun.readable() => {},
    _ = tokio::time::sleep(smol_delay) => {},
    udp_event = udp_rx.recv() => { handle_udp_event(...) }
    dns_result = async { ... }, if dns_resp_rx.is_some() => { ... }
    _ = cancel.cancelled() => { break; }
}

This is NOT a typical "spawn per connection" design. One task owns the entire smoltcp stack. Individual TCP/UDP sessions ARE spawned as separate tokio tasks that communicate back via mpsc channels and tokio::io::duplex pairs.

The smoltcp ↔ duplex bridge uses a NoopWaker-based manual poll pattern: see the ## io_loop event-driven architecture section above for the full treatment of try_read_duplex / try_write_duplex in io_loop/bridge.rs:19-45. The short version: the bridge calls poll_read / poll_write directly from the io_loop tick with a discarded waker. Consequence: the try_*_duplex family must NEVER be called from inside an async await — a Poll::Pending under the NoopWaker stalls the task permanently. If you find yourself writing async fn wrappers around duplex streams in this crate, stop and consult the io_loop architecture notes.

CancellationToken pattern (used throughout RIPDPI)

// Parent creates token, passes child tokens to spawned work
let cancel = CancellationToken::new();
let child_cancel = cancel.child_token();

tokio::spawn(async move {
    tokio::select! {
        _ = child_cancel.cancelled() => { /* clean shutdown */ }
        _ = do_work() => {}
    }
});

// Later: cancel.cancel() propagates to all child tokens

RIPDPI uses tokio_util::sync::CancellationToken (not tokio::sync::Notify) for structured shutdown. The tunnel session state machine transitions through Ready -> Starting -> Running -> Destroyed, with the token stored in the Starting and Running variants.

Debugging async issues

• Task starvation: Look for blocking calls in async context. The io_loop is especially sensitive -- a single blocked poll starves all TCP sessions.
• Deadlock at shutdown: Check that cancel.cancelled() is in every select! loop. Missing it causes tasks that never terminate.
• fd leaks: Verify OwnedFd cleanup on all error paths in JNI lifecycle functions. The dup'd fd must be closed even if start_session fails.

Note: tokio-console is not practical for this project -- it requires the tokio_unstable cfg flag and console-subscriber, which add overhead and complexity to Android NDK cross-compilation. Use tracing spans and RUST_LOG filtering instead.

Pitfall catalog

The following async-Rust pitfalls have their own catalog page so this skill stays focused on RIPDPI's architecture. Read references/async-pitfall-catalog.md when authoring or reviewing async code touched by any of them:

• Blocking syscalls inside async fn
• tokio::select! / tokio::join! semantics and cancellation surprises
• Cancel-safety annotation discipline + library method cancel-safety table
• Spawn-and-join firewall for non-cancellable critical sections
• Extended CancellationToken patterns: child tokens, DropGuard, run_until_cancelled
• Structured concurrency status as of Rust 1.94
• Async-Drop contracts of pooled resource libraries (sqlx, deadpool, tokio::fs::File)
• Async closures and the AsyncFn family (Rust 1.85+)
• HRTB pitfalls in Fn callbacks
• Async + shared &mut State in event loops
• Pin necessity in FFI types
• impl Trait (RPIT) lifetime overcapture in edition 2024
• tokio::time::timeout cooperative — never fires on non-yielding futures
• JoinSet drop cannot abort spawn_blocking threads
• spawn_blocking pool exhaustion from long-lived tasks
• block_in_place panics on current_thread runtime
• broadcast receiver silently drops messages on Lagged
• std::sync::Mutex guard across .await deadlocks silently
• async fn in traits: not object-safe, no Send bound by default

Related skills

• .claude/skills/rust-debugging/ -- GDB/LLDB debugging of async Rust
• .claude/skills/rust-profiling/ -- cargo-flamegraph with async stack frames
• .claude/skills/memory-model/ -- memory ordering in async contexts

Source: po4yka/RIPDPI — distributed by TomeVault.