snapdir-stores 1.1.0

//! Transfer configuration, rate limiting, and bounded-concurrency driver.
//!
//! This module is the foundation for concurrent object transfers and bandwidth
//! limiting. It provides:
//!
//! - [`TransferConfig`] — how many objects to transfer in parallel and an
//!   optional aggregate byte-rate cap.
//! - [`RateLimiter`] — a zero-dependency async token bucket built on
//!   [`tokio::time`], shareable across tasks via [`Arc`].
//! - [`run_concurrent`] — a generic bounded-concurrency driver that runs up to
//!   `concurrency` async operations in flight and returns the first error.
//!
//! Nothing here changes the existing (sequential) push / fetch loops yet; the
//! stores merely carry a [`TransferConfig`] so later gates can wire these
//! primitives into their transfer loops.

use std::num::NonZeroUsize;
use std::sync::Arc;
use std::time::Duration;

use futures::stream::{self, StreamExt, TryStreamExt};
use snapdir_core::store::StoreError;
use tokio::sync::Mutex;

/// Upper bound on the auto-detected default concurrency.
const DEFAULT_CONCURRENCY_CAP: usize = 16;

/// Configuration for object transfers: how many to run in parallel and an
/// optional aggregate byte-rate cap.
///
/// `Default` auto-detects the available parallelism (capped at
/// [`DEFAULT_CONCURRENCY_CAP`]) and leaves bandwidth unlimited.
#[derive(Debug, Clone)]
pub struct TransferConfig {
    /// Maximum number of object transfers to run concurrently.
    pub concurrency: NonZeroUsize,
    /// Optional aggregate bandwidth cap, in bytes per second. `None` means
    /// unlimited.
    pub max_bytes_per_sec: Option<u64>,
}

impl TransferConfig {
    /// Builds a config, clamping `concurrency` to at least 1.
    #[must_use]
    pub fn new(concurrency: usize, max_bytes_per_sec: Option<u64>) -> Self {
        Self {
            concurrency: NonZeroUsize::new(concurrency.max(1)).unwrap_or(NonZeroUsize::MIN),
            max_bytes_per_sec,
        }
    }
}

impl Default for TransferConfig {
    fn default() -> Self {
        let detected = std::thread::available_parallelism()
            .map_or(1, NonZeroUsize::get)
            .clamp(1, DEFAULT_CONCURRENCY_CAP);
        Self {
            // `detected` is >= 1, so the NonZeroUsize is always Some.
            concurrency: NonZeroUsize::new(detected).unwrap_or(NonZeroUsize::MIN),
            max_bytes_per_sec: None,
        }
    }
}

/// Shared token-bucket state, guarded by an async mutex.
#[derive(Debug)]
struct Bucket {
    /// Currently available tokens (bytes).
    tokens: f64,
    /// Last time the bucket was refilled.
    last_refill: tokio::time::Instant,
}

/// Inner state of a [`RateLimiter`].
#[derive(Debug)]
struct Inner {
    /// Refill rate in bytes per second. `0` is impossible here (unlimited is
    /// modelled by `bucket = None`).
    rate: f64,
    /// Maximum burst capacity, in bytes (~1 second's worth of budget).
    capacity: f64,
    /// `None` when unlimited; otherwise the live bucket state.
    bucket: Option<Mutex<Bucket>>,
}

/// An async token-bucket rate limiter that throttles aggregate transfer
/// throughput.
///
/// Construct with [`RateLimiter::new`]. When `max_bytes_per_sec` is `None` (or
/// `Some(0)`), the limiter is unlimited and [`acquire`](RateLimiter::acquire)
/// returns immediately. Otherwise tokens refill at `max_bytes_per_sec` per
/// second, allowing a burst of up to ~1 second's worth of budget.
///
/// The limiter is [`Arc`]-shareable and [`Clone`] (cloning shares the same
/// underlying bucket).
#[derive(Debug, Clone)]
pub struct RateLimiter {
    inner: Arc<Inner>,
}

impl RateLimiter {
    /// Builds a limiter. `None` (or `Some(0)`) yields an unlimited, no-op
    /// limiter whose [`acquire`](RateLimiter::acquire) never waits.
    #[must_use]
    pub fn new(max_bytes_per_sec: Option<u64>) -> Self {
        let inner = match max_bytes_per_sec {
            Some(rate) if rate > 0 => {
                #[allow(clippy::cast_precision_loss)]
                let rate = rate as f64;
                Inner {
                    rate,
                    capacity: rate,
                    bucket: Some(Mutex::new(Bucket {
                        tokens: rate,
                        last_refill: tokio::time::Instant::now(),
                    })),
                }
            }
            _ => Inner {
                rate: 0.0,
                capacity: 0.0,
                bucket: None,
            },
        };
        Self {
            inner: Arc::new(inner),
        }
    }

    /// Blocks until `n` bytes of budget are available, refilling the bucket at
    /// the configured rate. Unlimited limiters return immediately.
    ///
    /// A single request larger than the bucket capacity is still satisfied: the
    /// bucket is allowed to go negative and the caller waits out the deficit,
    /// so throttling is correct even for objects bigger than one second's
    /// worth of budget.
    pub async fn acquire(&self, n: u64) {
        let Some(bucket) = self.inner.bucket.as_ref() else {
            return; // unlimited fast path
        };
        if n == 0 {
            return;
        }
        #[allow(clippy::cast_precision_loss)]
        let need = n as f64;

        loop {
            let wait = {
                let mut state = bucket.lock().await;
                let now = tokio::time::Instant::now();
                let elapsed = now.duration_since(state.last_refill).as_secs_f64();
                state.tokens = (state.tokens + elapsed * self.inner.rate).min(self.inner.capacity);
                state.last_refill = now;

                if state.tokens >= need {
                    state.tokens -= need;
                    return;
                }
                // Not enough budget: compute how long until the deficit is
                // covered, then sleep (releasing the lock first).
                let deficit = need - state.tokens;
                deficit / self.inner.rate
            };
            tokio::time::sleep(Duration::from_secs_f64(wait)).await;
        }
    }
}

/// Runs `op` over `items` with at most `concurrency` operations in flight,
/// collecting their results in completion-independent order and returning the
/// first error encountered (remaining in-flight work is cancelled).
///
/// This is the engine later gates use to drive concurrent uploads/downloads.
///
/// # Errors
///
/// Returns the first [`StoreError`] produced by any operation.
pub async fn run_concurrent<I, T, F, Fut>(
    items: I,
    concurrency: NonZeroUsize,
    op: F,
) -> Result<Vec<T>, StoreError>
where
    I: IntoIterator,
    F: Fn(I::Item) -> Fut,
    Fut: std::future::Future<Output = Result<T, StoreError>>,
{
    stream::iter(items)
        .map(op)
        .buffer_unordered(concurrency.get())
        .try_collect()
        .await
}

#[cfg(test)]
mod tests {
    use super::*;
    use std::sync::atomic::{AtomicUsize, Ordering};

    /// Builds a current-thread tokio runtime with time enabled, avoiding a
    /// dependency on the `#[tokio::test]` macro (keeps tokio's feature set
    /// minimal).
    fn runtime() -> tokio::runtime::Runtime {
        tokio::runtime::Builder::new_current_thread()
            .enable_time()
            .build()
            .expect("build tokio runtime")
    }

    #[test]
    fn transfer_config_default_caps_concurrency() {
        let cfg = TransferConfig::default();
        assert!(cfg.concurrency.get() >= 1, "concurrency must be >= 1");
        assert!(
            cfg.concurrency.get() <= DEFAULT_CONCURRENCY_CAP,
            "default concurrency must be capped at {DEFAULT_CONCURRENCY_CAP}, got {}",
            cfg.concurrency.get()
        );
        assert_eq!(cfg.max_bytes_per_sec, None);

        // The clamping ctor never yields 0.
        assert_eq!(TransferConfig::new(0, None).concurrency.get(), 1);
        assert_eq!(TransferConfig::new(7, Some(99)).concurrency.get(), 7);
        assert_eq!(TransferConfig::new(7, Some(99)).max_bytes_per_sec, Some(99));
    }

    /// Drives `run_concurrent` over N > concurrency items, recording the peak
    /// number of simultaneously-running ops, and asserts the bound is exactly
    /// `min(concurrency, N)` — and strictly 1 (sequential) when concurrency=1.
    fn max_in_flight_for(concurrency: usize, items: usize) -> usize {
        let in_flight = Arc::new(AtomicUsize::new(0));
        let high_water = Arc::new(AtomicUsize::new(0));

        let rt = runtime();
        let result = rt.block_on(async {
            let in_flight = Arc::clone(&in_flight);
            let high_water = Arc::clone(&high_water);
            run_concurrent(
                0..items,
                NonZeroUsize::new(concurrency).unwrap(),
                move |_item| {
                    let in_flight = Arc::clone(&in_flight);
                    let high_water = Arc::clone(&high_water);
                    async move {
                        let cur = in_flight.fetch_add(1, Ordering::SeqCst) + 1;
                        high_water.fetch_max(cur, Ordering::SeqCst);
                        tokio::time::sleep(Duration::from_millis(20)).await;
                        in_flight.fetch_sub(1, Ordering::SeqCst);
                        Ok::<_, StoreError>(())
                    }
                },
            )
            .await
        });
        assert!(result.is_ok());
        high_water.load(Ordering::SeqCst)
    }

    #[test]
    fn transfer_config_run_concurrent_max_in_flight() {
        // concurrency=4 over 12 items: peak in-flight is exactly 4.
        assert_eq!(max_in_flight_for(4, 12), 4);
        // concurrency=1 over 5 items: strictly sequential, peak in-flight is 1.
        assert_eq!(max_in_flight_for(1, 5), 1);
        // concurrency greater than item count is bounded by the item count.
        assert_eq!(max_in_flight_for(8, 3), 3);
    }

    #[test]
    fn transfer_config_run_concurrent_propagates_error() {
        let rt = runtime();
        let result: Result<Vec<()>, StoreError> = rt.block_on(async {
            run_concurrent(0..10, NonZeroUsize::new(3).unwrap(), |item| async move {
                if item == 5 {
                    Err(StoreError::Backend {
                        message: "boom".to_owned(),
                        source: None,
                    })
                } else {
                    tokio::time::sleep(Duration::from_millis(5)).await;
                    Ok(())
                }
            })
            .await
        });
        let err = result.expect_err("must surface the failing op's error");
        assert!(
            matches!(err, StoreError::Backend { ref message, .. } if message == "boom"),
            "unexpected error: {err:?}"
        );
    }

    #[test]
    fn transfer_config_rate_limiter() {
        let rt = runtime();
        rt.block_on(async {
            // Unlimited: acquiring a large amount returns essentially instantly.
            let unlimited = RateLimiter::new(None);
            let start = tokio::time::Instant::now();
            unlimited.acquire(1_000_000).await;
            assert!(
                start.elapsed() < Duration::from_millis(200),
                "unlimited acquire should not block"
            );

            // Limited to 1000 bytes/sec. The bucket starts full (1000), so the
            // first 1000 bytes are free; acquiring another ~2000 bytes total
            // must wait for the deficit to refill — at least ~1s.
            let limiter = RateLimiter::new(Some(1000));
            let start = tokio::time::Instant::now();
            limiter.acquire(1000).await; // drains the initial burst
            limiter.acquire(1000).await; // must wait ~1s to refill
            let elapsed = start.elapsed();
            assert!(
                elapsed >= Duration::from_millis(900),
                "throttled acquire should take ~1s, took {elapsed:?}"
            );
        });
    }
}