tower-memlim 0.3.0

tower-memlim is a Tower based middleware layer to limit requests based on the host's computer memory usage.
Documentation 
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use tokio::time::{Instant, Sleep};
use tower_service::Service;

use std::{
    future::Future,
    pin::Pin,
    task::{Context, Poll},
    time::Duration,
};

use crate::{
    error::{BoxError, MemCheckFailure},
    future::ResponseFuture,
    memory::{AvailableMemory, Threshold},
};

/// Enforces a limit on the underlying service when a memory threshold is met.
#[derive(Debug)]
pub struct MemoryLimit<T, M>
where
    M: AvailableMemory,
{
    inner: T,
    threshold: Threshold,
    mem_checker: M,
    retry_interval: Duration,
    is_ready: bool,
    sleep: Pin<Box<Sleep>>,
}

impl<T, M> MemoryLimit<T, M>
where
    M: AvailableMemory,
{
    /// Create a new memory limiter.
    pub fn new(inner: T, threshold: Threshold, mem_checker: M, retry_interval: Duration) -> Self {
        Self {
            inner,
            threshold,
            mem_checker,
            retry_interval,
            is_ready: false,
            sleep: Box::pin(tokio::time::sleep(retry_interval)),
        }
    }

    /// Get a reference to the inner service
    pub fn get_ref(&self) -> &T {
        &self.inner
    }

    /// Get a mutable reference to the inner service
    pub fn get_mut(&mut self) -> &mut T {
        &mut self.inner
    }

    /// Consume `self`, returning the inner service
    pub fn into_inner(self) -> T {
        self.inner
    }
}

impl<S, Request, M> Service<Request> for MemoryLimit<S, M>
where
    S: Service<Request>,
    M: AvailableMemory,
    S::Error: Into<BoxError>,
{
    type Response = S::Response;
    type Error = BoxError;
    type Future = ResponseFuture<S::Future>;

    fn poll_ready(&mut self, cx: &mut Context<'_>) -> Poll<Result<(), Self::Error>> {
        // Check current memory usage
        match self.threshold {
            Threshold::MinAvailableBytes(min_m) => match self.mem_checker.available_memory() {
                Ok(v) => {
                    if v < min_m as usize {
                        // Reset sleep
                        self.sleep
                            .as_mut()
                            .reset(Instant::now() + self.retry_interval);

                        // Wake up after sleep
                        match self.sleep.as_mut().poll(cx) {
                            Poll::Ready(_r) => {
                                // Unlikely as we just reset the Sleep but we handle it
                                cx.waker().wake_by_ref();
                            }
                            Poll::Pending => (),
                        }

                        Poll::Pending
                    } else {
                        self.is_ready = true;
                        self.inner.poll_ready(cx).map_err(Into::into)
                    }
                }
                Err(e) => Poll::Ready(Err(MemCheckFailure::new(e).into())),
            },
        }
    }

    fn call(&mut self, request: Request) -> Self::Future {
        if self.is_ready {
            ResponseFuture::called(self.inner.call(request))
        } else {
            panic!("service not ready; poll_ready must be called first")
        }
    }
}

impl<S: Clone, M> Clone for MemoryLimit<S, M>
where
    M: AvailableMemory,
{
    fn clone(&self) -> Self {
        Self {
            inner: self.inner.clone(),
            threshold: self.threshold.clone(),
            mem_checker: self.mem_checker.clone(),
            retry_interval: self.retry_interval,
            is_ready: false,
            sleep: Box::pin(tokio::time::sleep(self.retry_interval)),
        }
    }
}