From ec84d0eaa1cf8f9204cd8560b828cc73b0d6838b Mon Sep 17 00:00:00 2001
From: Takeyoshi Kikuchi <kikuchi@centurysys.co.jp>
Date: Sun, 1 Mar 2026 13:37:56 +0900
Subject: [PATCH] Send MQTT packets atomically in a single socket write

Previously the fixed header and payload were sent in
separate async socket writes. Since each write awaits,
other tasks could interleave writes on the same socket,
potentially corrupting packet framing.

This change builds a single contiguous buffer and sends
it in one async write call to ensure packet atomicity.

Signed-off-by: Takeyoshi Kikuchi <kikuchi@centurysys.co.jp>
---
 nmqtt.nim | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/nmqtt.nim b/nmqtt.nim
index 56b74e5..be35500 100644
--- a/nmqtt.nim
+++ b/nmqtt.nim
@@ -355,8 +355,8 @@ proc send(ctx: MqttCtx, pkt: Pkt): Future[bool] {.async.} =
   if ctx.state notin {Connecting, Connected, Disconnecting}:
     return false
 
-  var hdr: seq[uint8]
-  hdr.add (pkt.typ.int shl 4).uint8 or pkt.flags
+  var buf: seq[uint8] = newSeqOfCap[uint8](1 + 4 + pkt.data.len)
+  buf.add (pkt.typ.int shl 4).uint8 or pkt.flags
 
   var len = pkt.data.len
   while true:
@@ -364,15 +364,16 @@ proc send(ctx: MqttCtx, pkt: Pkt): Future[bool] {.async.} =
     len = len div 128
     if len > 0:
       b = b or 128
-    hdr.add b.uint8
+    buf.add b.uint8
     if len == 0:
       break
 
   ctx.dmp "tx> " & $pkt
-  await ctx.s.send(hdr[0].unsafeAddr, hdr.len)
-
   if pkt.data.len > 0:
-    await ctx.s.send(pkt.data[0].unsafeAddr, pkt.data.len)
+    let hdrlen = buf.len
+    buf.setLen(hdrlen + pkt.data.len)
+    copyMem(buf[hdrlen].addr, pkt.data[0].unsafeAddr, pkt.data.len)
+  await ctx.s.send(buf[0].unsafeAddr, buf.len)
 
   return true