fix copy buffered write deadlock

cherry-pick of tokio-rs/tokio#4001

src/copy_bidirectional.rs

@@ -17,6 +17,7 @@ use std::task::{Context, Poll};
 #[derive(Debug)]
 struct CopyBuffer {
     read_done: bool,
+    need_flush: bool,
     start_index: usize,
     cache_length: usize,
     size: usize,
@@ -31,6 +32,7 @@ impl CopyBuffer {
         }
         Self {
             read_done: false,
+            need_flush: false,
             start_index: 0,
             cache_length: 0,
             size,
@@ -74,6 +76,13 @@ impl CopyBuffer {
                         }
                     }
                     Poll::Pending => {
+                        // Try flushing when the reader has no progress to avoid deadlock
+                        // when the reader depends on buffered writer.
+                        if self.need_flush {
+                            ready!(writer.as_mut().poll_flush(cx))?;
+                            self.need_flush = false;
+                        }
+
                         read_pending = true;
                     }
                 }