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use std::fmt;
use std::future::Future;
use std::pin::Pin;
use std::sync::Arc;

#[cfg(feature = "server")]
use crate::body::Body;
#[cfg(all(feature = "http2", feature = "server"))]
use crate::proto::h2::server::H2Stream;
use crate::rt::Executor;

#[cfg(feature = "server")]
pub trait ConnStreamExec<F, B: Body>: Clone {
    fn execute_h2stream(&mut self, fut: H2Stream<F, B>);
}

pub(crate) type BoxSendFuture = Pin<Box<dyn Future<Output = ()> + Send>>;

// Either the user provides an executor for background tasks, or we panic.
// TODO: with the `runtime`feature, `Exec::Default` used `tokio::spawn`. With the
// removal of the opt-in default runtime, this should be refactored.
#[derive(Clone)]
pub(crate) enum Exec {
    Default,
    Executor(Arc<dyn Executor<BoxSendFuture> + Send + Sync>),
}

// ===== impl Exec =====

impl Exec {
    pub(crate) fn execute<F>(&self, fut: F)
    where
        F: Future<Output = ()> + Send + 'static,
    {
        match *self {
            Exec::Default => {
                panic!("executor must be set");
            }
            Exec::Executor(ref e) => {
                e.execute(Box::pin(fut));
            }
        }
    }
}

impl fmt::Debug for Exec {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.debug_struct("Exec").finish()
    }
}

#[cfg(feature = "server")]
impl<F, B> ConnStreamExec<F, B> for Exec
where
    H2Stream<F, B>: Future<Output = ()> + Send + 'static,
    B: Body,
{
    fn execute_h2stream(&mut self, fut: H2Stream<F, B>) {
        self.execute(fut)
    }
}

// ==== impl Executor =====

#[cfg(feature = "server")]
impl<E, F, B> ConnStreamExec<F, B> for E
where
    E: Executor<H2Stream<F, B>> + Clone,
    H2Stream<F, B>: Future<Output = ()>,
    B: Body,
{
    fn execute_h2stream(&mut self, fut: H2Stream<F, B>) {
        self.execute(fut)
    }
}

// If http2 is not enable, we just have a stub here, so that the trait bounds
// that *would* have been needed are still checked. Why?
//
// Because enabling `http2` shouldn't suddenly add new trait bounds that cause
// a compilation error.
#[cfg(not(feature = "http2"))]
#[allow(missing_debug_implementations)]
pub struct H2Stream<F, B>(std::marker::PhantomData<(F, B)>);

#[cfg(not(feature = "http2"))]
impl<F, B, E> Future for H2Stream<F, B>
where
    F: Future<Output = Result<http::Response<B>, E>>,
    B: crate::body::Body,
    B::Error: Into<Box<dyn std::error::Error + Send + Sync>>,
    E: Into<Box<dyn std::error::Error + Send + Sync>>,
{
    type Output = ();

    fn poll(
        self: Pin<&mut Self>,
        _cx: &mut std::task::Context<'_>,
    ) -> std::task::Poll<Self::Output> {
        unreachable!()
    }
}