async-io-model

Async I/O Model Guide

Turso uses cooperative yielding with explicit state machines instead of Rust async/await.

Core Types

pub enum IOCompletions {
    Single(Completion),
}

#[must_use]
pub enum IOResult<T> {
    Done(T),      // Operation complete, here's the result
    IO(IOCompletions),  // Need I/O, call me again after completions finish
}

Functions returning IOResult must be called repeatedly until Done.

Completion and CompletionGroup

A Completion tracks a single I/O operation:

pub struct Completion { /* ... */ }

impl Completion {
    pub fn finished(&self) -> bool;
    pub fn succeeded(&self) -> bool;
    pub fn get_error(&self) -> Option<CompletionError>;
}

To wait for multiple I/O operations, use CompletionGroup:

let mut group = CompletionGroup::new(|_| {});

// Add individual completions
group.add(&completion1);
group.add(&completion2);

// Build into single completion that finishes when all complete
let combined = group.build();
io_yield_one!(combined);

CompletionGroup features:

• Aggregates multiple completions into one
• Calls callback when all complete (or any errors)
• Can nest groups (add a group's completion to another group)
• Cancellable via group.cancel()

Helper Macros

return_if_io!

Unwraps IOResult, propagates IO variant up the call stack:

let result = return_if_io!(some_io_operation());
// Only reaches here if operation returned Done

io_yield_one!

Yields a single completion:

io_yield_one!(completion);  // Returns Ok(IOResult::IO(Single(completion)))

State Machine Pattern

Operations that may yield use explicit state enums:

enum MyOperationState {
    Start,
    WaitingForRead { page: PageRef },
    Processing { data: Vec<u8> },
    Done,
}

The function loops, matching on state and transitioning:

fn my_operation(&mut self) -> Result<IOResult<Output>> {
    loop {
        match &mut self.state {
            MyOperationState::Start => {
                let (page, completion) = start_read();
                self.state = MyOperationState::WaitingForRead { page };
                io_yield_one!(completion);
            }
            MyOperationState::WaitingForRead { page } => {
                let data = page.get_contents();
                self.state = MyOperationState::Processing { data: data.to_vec() };
                // No yield, continue loop
            }
            MyOperationState::Processing { data } => {
                let result = process(data);
                self.state = MyOperationState::Done;
                return Ok(IOResult::Done(result));
            }
            MyOperationState::Done => unreachable!(),
        }
    }
}

Re-Entrancy: The Critical Pitfall

State mutations before yield points cause bugs on re-entry.

Wrong

fn bad_example(&mut self) -> Result<IOResult<()>> {
    self.counter += 1;  // Mutates state
    return_if_io!(something_that_might_yield());  // If yields, re-entry will increment again!
    Ok(IOResult::Done(()))
}

If something_that_might_yield() returns IO, caller waits for completion, then calls bad_example() again. counter gets incremented twice (or more).

Correct: Mutate After Yield

fn good_example(&mut self) -> Result<IOResult<()>> {
    return_if_io!(something_that_might_yield());
    self.counter += 1;  // Only reached once, after IO completes
    Ok(IOResult::Done(()))
}

Correct: Use State Machine

enum State { Start, AfterIO }

fn good_example(&mut self) -> Result<IOResult<()>> {
    loop {
        match self.state {
            State::Start => {
                // Don't mutate shared state here
                self.state = State::AfterIO;
                return_if_io!(something_that_might_yield());
            }
            State::AfterIO => {
                self.counter += 1;  // Safe: only entered once
                return Ok(IOResult::Done(()));
            }
        }
    }
}

Common Re-Entrancy Bugs

State Enum Design

Encode progress in state variants:

// Good: index is part of state, preserved across yields
enum ProcessState {
    Start,
    ProcessingItem { idx: usize, items: Vec<Item> },
    Done,
}

// Loop advances idx only when transitioning states
ProcessingItem { idx, items } => {
    return_if_io!(process_item(&items[idx]));
    if idx + 1 < items.len() {
        self.state = ProcessingItem { idx: idx + 1, items };
    } else {
        self.state = Done;
    }
}

Turso Implementation

Key files:

• core/types.rs - IOResult, IOCompletions, return_if_io!, return_and_restore_if_io!
• core/io/completions.rs - Completion, CompletionGroup
• core/util.rs - io_yield_one! macro
• core/state_machine.rs - Generic StateMachine wrapper
• core/storage/btree.rs - Many state machine examples
• core/storage/pager.rs - CompletionGroup usage examples

Testing Async Code

Re-entrancy bugs often only manifest under specific IO timing. Use:

• Deterministic simulation (testing/simulator/)
• Whopper concurrent DST (testing/concurrent-simulator/)
• Fault injection to force yields at different points

References

• docs/manual.md section on I/O