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>

Author: centurysys

nmqtt.nim

@@ -355,24 +355,25 @@ 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:
     var b = len mod 128
     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