From cdc46a9dedc5210198ccec44b4e8ce5a8c90f02b Mon Sep 17 00:00:00 2001
From: Suika <32665912+suikammd@users.noreply.github.com>
Date: Wed, 22 Sep 2021 16:06:51 +0800
Subject: [PATCH] io: add assert in copy_bidirectional that poll_write is
 sensible (#4125)

---
 tokio/src/io/util/copy.rs | 8 ++++++++
 1 file changed, 8 insertions(+)

diff --git a/tokio/src/io/util/copy.rs b/tokio/src/io/util/copy.rs
index fbd77b5f2ea..d7fca2c3778 100644
--- a/tokio/src/io/util/copy.rs
+++ b/tokio/src/io/util/copy.rs
@@ -84,6 +84,14 @@ impl CopyBuffer {
                 }
             }
 
+            // If pos larger than cap, this loop will never stop.
+            // In particular, user's wrong poll_write implementation returning
+            // incorrect written length may lead to thread blocking.
+            debug_assert!(
+                self.pos <= self.cap,
+                "writer returned length larger than input slice"
+            );
+
             // If we've written all the data and we've seen EOF, flush out the
             // data and finish the transfer.
             if self.pos == self.cap && self.read_done {