Fix poll_flush after poll_shutdown (#37)

* tests: simplify window_size_decremented_past_zero (#829)

This helps finding the root cause of why the assertion
in FlowControl::dec_send_window failed in the first place.

See #692 for more details.

* Fix reclaiming reserved capacity (#832)

Reclaiming requested capacity that has not been actually reserved yet
is wrong, as this capacity never existed to begin with.

Fixes #607

* fix: busy loop on shutdown (#834)

When in a slow shutdown, this was busy looping since no more streams were left. The change here now only makes sure we poll one extra time if the state changed during the poll.

Fixes #831

* Fix window size decrement of send-closed streams (#830)

Send-closed streams should be skipped when decreasing window size,
as they are skipped when increasing it.

* fix: handle implicit resets at the right time (#833)

A stream whose ref count reaches zero while open should
not immediately decrease the number of active streams,
otherwise MAX_CONCURRENT_STREAMS isn't respected anymore.

* Fix poll_flush after poll_shutdown (#836)

---------

Co-authored-by: Anthony Ramine <123095+nox@users.noreply.github.com>
Co-authored-by: Sean McArthur <sean@seanmonstar.com>
Co-authored-by: 包布丁 <htbai1998m@hotmail.com>

src/codec/framed_write.rs

@@ -23,6 +23,7 @@ macro_rules! limited_write_buf {
 pub struct FramedWrite<T, B> {
     /// Upstream `AsyncWrite`
     inner: T,
+    final_flush_done: bool,
 
     encoder: Encoder<B>,
 }
@@ -89,6 +90,7 @@ where
         };
         FramedWrite {
             inner,
+            final_flush_done: false,
             encoder: Encoder {
                 hpack: hpack::Encoder::default(),
                 buf: Cursor::new(BytesMut::with_capacity(DEFAULT_BUFFER_CAPACITY)),
@@ -170,7 +172,10 @@ where
 
     /// Close the codec
     pub fn shutdown(&mut self, cx: &mut Context) -> Poll<io::Result<()>> {
-        ready!(self.flush(cx))?;
+        if !self.final_flush_done {
+            ready!(self.flush(cx))?;
+            self.final_flush_done = true;
+        }
         Pin::new(&mut self.inner).poll_shutdown(cx)
     }
 }