Turn get_conn waiting queue into FIFO queue.

src/conn/pool/futures/disconnect_pool.rs

@@ -16,7 +16,7 @@ use futures_core::ready;
 use tokio::sync::mpsc::UnboundedSender;
 
 use crate::{
-    conn::pool::{Inner, Pool},
+    conn::pool::{Inner, Pool, QueuedWaker, QUEUE_END_ID},
     error::Error,
     Conn,
 };
@@ -50,7 +50,9 @@ impl Future for DisconnectPool {
         self.pool_inner.close.store(true, atomic::Ordering::Release);
         let mut exchange = self.pool_inner.exchange.lock().unwrap();
         exchange.spawn_futures_if_needed(&self.pool_inner);
-        exchange.waiting.push_back(cx.waker().clone());
+        exchange
+            .waiting
+            .push(QueuedWaker::new(QUEUE_END_ID, cx.waker().clone()));
         drop(exchange);
 
         if self.pool_inner.closed.load(atomic::Ordering::Acquire) {

src/conn/pool/futures/get_conn.rs

@@ -16,7 +16,10 @@ use std::{
 use futures_core::ready;
 
 use crate::{
-    conn::{pool::Pool, Conn},
+    conn::{
+        pool::{Pool, QueueId},
+        Conn,
+    },
     error::*,
 };
 
@@ -58,13 +61,15 @@ impl GetConnInner {
 #[derive(Debug)]
 #[must_use = "futures do nothing unless you `.await` or poll them"]
 pub struct GetConn {
+    pub(crate) queue_id: Option<QueueId>,
     pub(crate) pool: Option<Pool>,
     pub(crate) inner: GetConnInner,
 }
 
 impl GetConn {
     pub(crate) fn new(pool: &Pool) -> GetConn {
         GetConn {
+            queue_id: None,
             pool: Some(pool.clone()),
             inner: GetConnInner::New,
         }
@@ -91,23 +96,26 @@ impl Future for GetConn {
     fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
         loop {
             match self.inner {
-                GetConnInner::New => match ready!(Pin::new(self.pool_mut()).poll_new_conn(cx))?
-                    .inner
-                    .take()
-                {
-                    GetConnInner::Connecting(conn_fut) => {
-                        self.inner = GetConnInner::Connecting(conn_fut);
-                    }
-                    GetConnInner::Checking(conn_fut) => {
-                        self.inner = GetConnInner::Checking(conn_fut);
-                    }
-                    GetConnInner::Done => unreachable!(
-                        "Pool::poll_new_conn never gives out already-consumed GetConns"
-                    ),
-                    GetConnInner::New => {
-                        unreachable!("Pool::poll_new_conn never gives out GetConnInner::New")
+                GetConnInner::New => {
+                    let queued = self.queue_id.is_some();
+                    let queue_id = *self.queue_id.get_or_insert_with(QueueId::next);
+                    let next =
+                        ready!(Pin::new(self.pool_mut()).poll_new_conn(cx, queued, queue_id))?;
+                    match next {
+                        GetConnInner::Connecting(conn_fut) => {
+                            self.inner = GetConnInner::Connecting(conn_fut);
+                        }
+                        GetConnInner::Checking(conn_fut) => {
+                            self.inner = GetConnInner::Checking(conn_fut);
+                        }
+                        GetConnInner::Done => unreachable!(
+                            "Pool::poll_new_conn never gives out already-consumed GetConns"
+                        ),
+                        GetConnInner::New => {
+                            unreachable!("Pool::poll_new_conn never gives out GetConnInner::New")
+                        }
                     }
-                },
+                }
                 GetConnInner::Done => {
                     unreachable!("GetConn::poll polled after returning Async::Ready");
                 }

src/conn/pool/mod.rs

@@ -10,7 +10,8 @@ use futures_util::FutureExt;
 use tokio::sync::mpsc;
 
 use std::{
-    collections::VecDeque,
+    cmp::{Ordering, Reverse},
+    collections::{BinaryHeap, VecDeque},
     convert::TryFrom,
     pin::Pin,
     str::FromStr,
@@ -62,7 +63,7 @@ impl From<Conn> for IdlingConn {
 /// This is fine as long as we never do expensive work while holding the lock!
 #[derive(Debug)]
 struct Exchange {
-    waiting: VecDeque<Waker>,
+    waiting: BinaryHeap<QueuedWaker>,
     available: VecDeque<IdlingConn>,
     exist: usize,
     // only used to spawn the recycler the first time we're in async context
@@ -87,6 +88,51 @@ impl Exchange {
     }
 }
 
+const QUEUE_END_ID: QueueId = QueueId(Reverse(u64::MAX));
+
+#[derive(Debug, Copy, Clone, Eq, PartialEq, Ord, PartialOrd)]
+pub(crate) struct QueueId(Reverse<u64>);
+
+impl QueueId {
+    fn next() -> Self {
+        static NEXT_QUEUE_ID: atomic::AtomicU64 = atomic::AtomicU64::new(0);
+        let id = NEXT_QUEUE_ID.fetch_add(1, atomic::Ordering::SeqCst);
+        QueueId(Reverse(id))
+    }
+}
+
+#[derive(Debug)]
+struct QueuedWaker {
+    queue_id: QueueId,
+    waker: Waker,
+}
+
+impl QueuedWaker {
+    fn new(queue_id: QueueId, waker: Waker) -> Self {
+        QueuedWaker { queue_id, waker }
+    }
+}
+
+impl Eq for QueuedWaker {}
+
+impl PartialEq for QueuedWaker {
+    fn eq(&self, other: &Self) -> bool {
+        self.queue_id == other.queue_id
+    }
+}
+
+impl Ord for QueuedWaker {
+    fn cmp(&self, other: &Self) -> Ordering {
+        self.queue_id.cmp(&other.queue_id)
+    }
+}
+
+impl PartialOrd for QueuedWaker {
+    fn partial_cmp(&self, other: &Self) -> Option<Ordering> {
+        Some(self.cmp(other))
+    }
+}
+
 /// Connection pool data.
 #[derive(Debug)]
 pub struct Inner {
@@ -131,7 +177,7 @@ impl Pool {
                 closed: false.into(),
                 exchange: Mutex::new(Exchange {
                     available: VecDeque::with_capacity(pool_opts.constraints().max()),
-                    waiting: VecDeque::new(),
+                    waiting: BinaryHeap::new(),
                     exist: 0,
                     recycler: Some((rx, pool_opts)),
                 }),
@@ -181,8 +227,8 @@ impl Pool {
             let mut exchange = self.inner.exchange.lock().unwrap();
             if exchange.available.len() < self.opts.pool_opts().active_bound() {
                 exchange.available.push_back(conn.into());
-                if let Some(w) = exchange.waiting.pop_front() {
-                    w.wake();
+                if let Some(qw) = exchange.waiting.pop() {
+                    qw.waker.wake();
                 }
                 return;
             }
@@ -216,17 +262,27 @@ impl Pool {
         let mut exchange = self.inner.exchange.lock().unwrap();
         exchange.exist -= 1;
         // we just enabled the creation of a new connection!
-        if let Some(w) = exchange.waiting.pop_front() {
-            w.wake();
+        if let Some(qw) = exchange.waiting.pop() {
+            qw.waker.wake();
         }
     }
 
     /// Poll the pool for an available connection.
-    fn poll_new_conn(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<GetConn>> {
-        self.poll_new_conn_inner(cx)
-    }
-
-    fn poll_new_conn_inner(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Result<GetConn>> {
+    fn poll_new_conn(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        queued: bool,
+        queue_id: QueueId,
+    ) -> Poll<Result<GetConnInner>> {
+        self.poll_new_conn_inner(cx, queued, queue_id)
+    }
+
+    fn poll_new_conn_inner(
+        self: Pin<&mut Self>,
+        cx: &mut Context<'_>,
+        queued: bool,
+        queue_id: QueueId,
+    ) -> Poll<Result<GetConnInner>> {
         let mut exchange = self.inner.exchange.lock().unwrap();
 
         // NOTE: this load must happen while we hold the lock,
@@ -238,18 +294,23 @@ impl Pool {
 
         exchange.spawn_futures_if_needed(&self.inner);
 
+        // Check if others are waiting and we're not queued.
+        if !exchange.waiting.is_empty() && !queued {
+            exchange
+                .waiting
+                .push(QueuedWaker::new(queue_id, cx.waker().clone()));
+            return Poll::Pending;
+        }
+
         while let Some(IdlingConn { mut conn, .. }) = exchange.available.pop_back() {
             if !conn.expired() {
-                return Poll::Ready(Ok(GetConn {
-                    pool: Some(self.clone()),
-                    inner: GetConnInner::Checking(
-                        async move {
-                            conn.stream_mut()?.check().await?;
-                            Ok(conn)
-                        }
-                        .boxed(),
-                    ),
-                }));
+                return Poll::Ready(Ok(GetConnInner::Checking(
+                    async move {
+                        conn.stream_mut()?.check().await?;
+                        Ok(conn)
+                    }
+                    .boxed(),
+                )));
             } else {
                 self.send_to_recycler(conn);
             }
@@ -261,14 +322,15 @@ impl Pool {
             // we are allowed to make a new connection, so we will!
             exchange.exist += 1;
 
-            return Poll::Ready(Ok(GetConn {
-                pool: Some(self.clone()),
-                inner: GetConnInner::Connecting(Conn::new(self.opts.clone()).boxed()),
-            }));
+            return Poll::Ready(Ok(GetConnInner::Connecting(
+                Conn::new(self.opts.clone()).boxed(),
+            )));
         }
 
-        // no go -- we have to wait
-        exchange.waiting.push_back(cx.waker().clone());
+        // Polled, but no conn available? Back into the queue.
+        exchange
+            .waiting
+            .push(QueuedWaker::new(queue_id, cx.waker().clone()));
         Poll::Pending
     }
 }

src/conn/pool/recycler.rs

@@ -76,8 +76,8 @@ impl Future for Recycler {
                         $self.discard.push($conn.close_conn().boxed());
                     } else {
                         exchange.available.push_back($conn.into());
-                        if let Some(w) = exchange.waiting.pop_front() {
-                            w.wake();
+                        if let Some(qw) = exchange.waiting.pop() {
+                            qw.waker.wake();
                         }
                     }
                 }
@@ -163,8 +163,8 @@ impl Future for Recycler {
             let mut exchange = self.inner.exchange.lock().unwrap();
             exchange.exist -= self.discarded;
             for _ in 0..self.discarded {
-                if let Some(w) = exchange.waiting.pop_front() {
-                    w.wake();
+                if let Some(qw) = exchange.waiting.pop() {
+                    qw.waker.wake();
                 }
             }
             drop(exchange);
@@ -197,8 +197,8 @@ impl Future for Recycler {
         if self.inner.closed.load(Ordering::Acquire) {
             // `DisconnectPool` might still wait to be woken up.
             let mut exchange = self.inner.exchange.lock().unwrap();
-            while let Some(w) = exchange.waiting.pop_front() {
-                w.wake();
+            while let Some(qw) = exchange.waiting.pop() {
+                qw.waker.wake();
             }
             // we're about to exit, so there better be no outstanding connections
             assert_eq!(exchange.exist, 0);