io: add assert in copy_bidirectional that poll_write is sensible
@@ -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 {
