From 488ee63b4c170cb5d1789258a69f27aba19c3720 Mon Sep 17 00:00:00 2001
From: Tom Rebbert <109624508+trebbert-lutron@users.noreply.github.com>
Date: Thu, 20 Oct 2022 20:24:05 -0600
Subject: [PATCH] [coap] Bugfix: CoAP not sending 4.04 when URI handler is not
 found (#8297)

Update CoAP base handler to correctly detect when a handler is not
found and send a 4.04 response.

Prior to this change, the error code would be overridden in the case
where a URI handler was not found to kErrorNone. This meant that when
the exit code was reached, no 4.04 message was generated. This fixes
the logical flow to ensure the error message is sent on the link.
---
 src/core/coap/coap.cpp | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/src/core/coap/coap.cpp b/src/core/coap/coap.cpp
index 8730c8e5a..a64d18d57 100644
--- a/src/core/coap/coap.cpp
+++ b/src/core/coap/coap.cpp
@@ -1302,7 +1302,7 @@ void CoapBase::ProcessReceivedRequest(Message &aMessage, const Ip6::MessageInfo
 {
     char     uriPath[Message::kMaxReceivedUriPath + 1];
     Message *cachedResponse = nullptr;
-    Error    error          = kErrorNotFound;
+    Error    error          = kErrorNone;
 #if OPENTHREAD_CONFIG_COAP_BLOCKWISE_TRANSFER_ENABLE
     Option::Iterator iterator;
     char *           curUriPath         = uriPath;
@@ -1320,10 +1320,10 @@ void CoapBase::ProcessReceivedRequest(Message &aMessage, const Ip6::MessageInfo
     case kErrorNone:
         cachedResponse->Finish();
         error = Send(*cachedResponse, aMessageInfo);
-
-        OT_FALL_THROUGH;
+        ExitNow();
 
     case kErrorNoBufs:
+        error = kErrorNoBufs;
         ExitNow();
 
     case kErrorNotFound:
@@ -1453,8 +1453,11 @@ void CoapBase::ProcessReceivedRequest(Message &aMessage, const Ip6::MessageInfo
     {
         mDefaultHandler(mDefaultHandlerContext, &aMessage, &aMessageInfo);
         error = kErrorNone;
+        ExitNow();
     }
 
+    error = kErrorNotFound;
+
 exit:
 
     if (error != kErrorNone)