My recent changes to buffered tcp send broke this. [1]
One of my local apps using non-blocking tcp is working
again with this fix.
[1]
```
commit 837e1a72a47b4e5a874124e316192b2172301d30
Author: YAMAMOTO Takashi <yamamoto@midokura.com>
Date: Mon Mar 15 16:19:42 2021 +0900
tcp_send_buffered.c: improve tcp write buffering
```