From ca470ea18069410bb6a63115709617bc94ed6fc6 Mon Sep 17 00:00:00 2001
From: Sergey Polyakov <sergeuz@gmail.com>
Date: Fri, 9 Aug 2024 11:01:43 +0200
Subject: [PATCH] Bugfix

---
 communication/src/coap_util.cpp | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/communication/src/coap_util.cpp b/communication/src/coap_util.cpp
index 54e62c402f..116ec38990 100644
--- a/communication/src/coap_util.cpp
+++ b/communication/src/coap_util.cpp
@@ -30,16 +30,25 @@
 #include "coap_message_decoder.h"
 #include "str_util.h"
 #include "logging.h"
+#include "scope_guard.h"
 #include "check.h"
 
 namespace particle::protocol {
 
 int sendEmptyAckOrRst(MessageChannel& channel, Message& msg, CoapType type) {
+    auto msgLen = msg.length();
+    auto msgCapacity = msg.capacity();
     Message resp;
-    auto err = channel.response(msg, resp, msg.capacity() - msg.length());
+    auto err = channel.response(msg, resp, msgCapacity - msgLen);
     if (err != ProtocolError::NO_ERROR) {
         return toSystemError(err);
     }
+    SCOPE_GUARD({
+        // BufferMessageChannel::response() alters the capacity of the original message. Restore it
+        // so that the calling code could still use the extra space available in the message
+        msg.set_buffer(msg.buf(), msgCapacity);
+        msg.set_length(msgLen);
+    });
     CoapMessageEncoder e((char*)resp.buf(), resp.capacity());
     e.type(type);
     e.code(CoapCode::EMPTY);