From e1fb2a10ba9827fded510952ac3040ec89ca15e2 Mon Sep 17 00:00:00 2001
From: Kieran Klukas <me@dunkirk.sh>
Date: Wed, 4 Mar 2026 10:13:25 -0500
Subject: [PATCH] chore: drain uart

---
 src/protocol.rs | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/src/protocol.rs b/src/protocol.rs
index d009ab4..26f8bac 100644
--- a/src/protocol.rs
+++ b/src/protocol.rs
@@ -1,5 +1,6 @@
 use std::fs::File;
 use std::io::{Read, Write};
+use std::os::fd::AsRawFd;
 
 use anyhow::{Result, bail};
 
@@ -81,7 +82,10 @@ impl HSMIntf {
     fn write_all(&mut self, data: &[u8]) -> Result<()> {
         log::trace_hex("TX", data);
         self.file.write_all(data)?;
-        self.file.flush()?;
+        // tcdrain ensures all written data is transmitted before returning.
+        // File::flush() is a no-op for std::fs::File, so without this,
+        // macOS CDC-ACM may buffer writes and cause protocol desync.
+        unsafe { libc::tcdrain(self.file.as_raw_fd()); }
         Ok(())
     }