Convert poll_new_conn to an async function

I want to move the manual async bits to the Waitlist implementation,
but for that I need to change the locks, which should be done in its
own patch.

This at least already makes the manual async code quite a bit smaller.

Author: espindola

src/conn/pool/futures/get_conn.rs

@@ -6,7 +6,7 @@
 // option. All files in the project carrying such notice may not be copied,
 // modified, or distributed except according to those terms.
 
-use std::{fmt, future::poll_fn, task::Context};
+use std::fmt;
 
 use crate::{
     conn::{
@@ -79,8 +79,7 @@ pub(crate) async fn get_conn(pool: Pool) -> Result<Conn> {
         match state.inner {
             GetConnInner::New => {
                 let pool = state.pool_mut();
-                let poll_new = |cx: &mut Context<'_>| pool.poll_new_conn(cx, queue_id);
-                let next = poll_fn(poll_new).await?;
+                let next = pool.new_conn(queue_id).await?;
                 match next {
                     GetConnInner::Connecting(conn_fut) => {
                         state.inner = GetConnInner::Connecting(conn_fut);

src/conn/pool/mod.rs

@@ -14,6 +14,7 @@ use std::{
     borrow::Borrow,
     cmp::Reverse,
     collections::VecDeque,
+    future::poll_fn,
     future::Future,
     hash::{Hash, Hasher},
     str::FromStr,
@@ -336,12 +337,34 @@ impl Pool {
         }
     }
 
-    /// Poll the pool for an available connection.
-    fn poll_new_conn(
-        &mut self,
-        cx: &mut Context<'_>,
-        queue_id: QueueId,
-    ) -> Poll<Result<GetConnInner>> {
+    fn queue(&mut self, cx: &mut Context<'_>, queue_id: QueueId) -> Poll<()> {
+        let mut exchange = self.inner.exchange.lock().unwrap();
+        exchange.waiting.push(cx.waker().clone(), queue_id);
+        Poll::Ready(())
+    }
+
+    fn poll_higher_priority(&mut self, cx: &mut Context<'_>, queue_id: QueueId) -> Poll<()> {
+        let mut exchange = self.inner.exchange.lock().unwrap();
+        let highest = if let Some(cur) = exchange.waiting.peek_id() {
+            queue_id > cur
+        } else {
+            true
+        };
+        if highest {
+            Poll::Ready(())
+        } else {
+            // to make sure the waker is updated
+            exchange.waiting.push(cx.waker().clone(), queue_id);
+            Poll::Pending
+        }
+    }
+
+    async fn queue_and_wait(&mut self, queue_id: QueueId) {
+        poll_fn(|cx| self.queue(cx, queue_id)).await;
+        poll_fn(|cx| self.poll_higher_priority(cx, queue_id)).await;
+    }
+
+    fn try_new_conn(&mut self, queue_id: QueueId) -> Result<Option<GetConnInner>> {
         let mut exchange = self.inner.exchange.lock().unwrap();
 
         // NOTE: this load must happen while we hold the lock,
@@ -362,8 +385,7 @@ impl Pool {
 
         // If we are not, just queue
         if !highest {
-            exchange.waiting.push(cx.waker().clone(), queue_id);
-            return Poll::Pending;
+            return Ok(None);
         }
 
         #[allow(unused_variables)] // `since` is only used when `hdrhistogram` is enabled
@@ -379,7 +401,7 @@ impl Pool {
                 #[cfg(feature = "hdrhistogram")]
                 let metrics = self.metrics();
                 conn.inner.active_since = Instant::now();
-                return Poll::Ready(Ok(GetConnInner::Checking(
+                return Ok(Some(GetConnInner::Checking(
                     async move {
                         conn.stream_mut()?.check().await?;
                         #[cfg(feature = "hdrhistogram")]
@@ -419,7 +441,7 @@ impl Pool {
             #[cfg(feature = "hdrhistogram")]
             let metrics = self.metrics();
 
-            return Poll::Ready(Ok(GetConnInner::Connecting(
+            return Ok(Some(GetConnInner::Connecting(
                 async move {
                     let conn = Conn::new(opts).await;
                     #[cfg(feature = "hdrhistogram")]
@@ -437,10 +459,17 @@ impl Pool {
                 .boxed(),
             )));
         }
+        Ok(None)
+    }
 
-        // Polled, but no conn available? Back into the queue.
-        exchange.waiting.push(cx.waker().clone(), queue_id);
-        Poll::Pending
+    /// Get a new connection from the pool.
+    async fn new_conn(&mut self, queue_id: QueueId) -> Result<GetConnInner> {
+        loop {
+            if let Some(conn) = self.try_new_conn(queue_id)? {
+                return Ok(conn);
+            }
+            self.queue_and_wait(queue_id).await;
+        }
     }
 
     fn unqueue(&self, queue_id: QueueId) {