fix(http1): force always-ready connections to yield after a few spins

src/common/task.rs

@@ -0,0 +1,40 @@
+use futures::{Async, Poll, task::Task};
+
+use super::Never;
+
+/// A type to help "yield" a future, such that it is re-scheduled immediately.
+///
+/// Useful for spin counts, so a future doesn't hog too much time.
+#[derive(Debug)]
+pub(crate) struct YieldNow {
+    cached_task: Option<Task>,
+}
+
+impl YieldNow {
+    pub(crate) fn new() -> YieldNow {
+        YieldNow {
+            cached_task: None,
+        }
+    }
+
+    /// Returns `Ok(Async::NotReady)` always, while also notifying the
+    /// current task so that it is rescheduled immediately.
+    ///
+    /// Since it never returns `Async::Ready` or `Err`, those types are
+    /// set to `Never`.
+    pub(crate) fn poll_yield(&mut self) -> Poll<Never, Never> {
+        // Check for a cached `Task` first...
+        if let Some(ref t) = self.cached_task {
+            if t.will_notify_current() {
+                t.notify();
+                return Ok(Async::NotReady);
+            }
+        }
+
+        // No cached task, or not current, so get a new one...
+        let t = ::futures::task::current();
+        t.notify();
+        self.cached_task = Some(t);
+        Ok(Async::NotReady)
+    }
+}