[dnssd] smaller enhancements in Server (#11439)

This commit contains a few smaller enhancements in the DNS-SD server
implementation:
- `ShouldForwardToUpstream()` now checks the `mEnableUpstreamQuery`.
- The `ResolveByUpstream()` method now handles its own error logging.
- Comments are added/updated to improve code readability.

src/core/net/dnssd_server.cpp

@@ -157,17 +157,13 @@ void Server::ProcessQuery(Request &aRequest)
     Response     response(GetInstance());
 
 #if OPENTHREAD_CONFIG_DNS_UPSTREAM_QUERY_ENABLE
-    if (mEnableUpstreamQuery && ShouldForwardToUpstream(aRequest))
+    if (ShouldForwardToUpstream(aRequest))
     {
-        Error error = ResolveByUpstream(aRequest);
+        if (ResolveByUpstream(aRequest) == kErrorNone)
-
-        if (error == kErrorNone)
         {
             ExitNow();
         }
 
-        LogWarnOnError(error, "forwarding to upstream");
-
         rcode = Header::kResponseServerFailure;
 
         // Continue to allocate and prepare the response message
@@ -207,6 +203,10 @@ void Server::ProcessQuery(Request &aRequest)
     }
 #endif
 
+    // `ResolveByProxy` may take ownership of `response.mMessage` and
+    // setting it to `nullptr`. In such a case, the `response.Send()`
+    // call will effectively do nothing.
+
     ResolveByProxy(response, *aRequest.mMessageInfo);
 
 exit:
@@ -386,8 +386,9 @@ exit:
 
 Error Server::Response::ParseQueryName(void)
 {
-    // Parses and validates the query name and updates
+    // Parses the query name, determines name compression
-    // the name compression offsets.
+    // offsets, and validates that the query name is for
+    // `kDefaultDomainName` ("default.service.arpa.").
 
     Error        error = kErrorNone;
     Name::Buffer name;
@@ -865,12 +866,14 @@ exit:
 #endif // OPENTHREAD_CONFIG_SRP_SERVER_ENABLE
 
 #if OPENTHREAD_CONFIG_DNS_UPSTREAM_QUERY_ENABLE
-bool Server::ShouldForwardToUpstream(const Request &aRequest)
+bool Server::ShouldForwardToUpstream(const Request &aRequest) const
 {
     bool         shouldForward = false;
     uint16_t     readOffset;
     Name::Buffer name;
 
+    VerifyOrExit(mEnableUpstreamQuery);
+
     VerifyOrExit(aRequest.mHeader.IsRecursionDesiredFlagSet());
     readOffset = sizeof(Header);
 
@@ -949,6 +952,7 @@ Error Server::ResolveByUpstream(const Request &aRequest)
     mCounters.mUpstreamDnsCounters.mQueries++;
 
 exit:
+    LogWarnOnError(error, "forward to upstream");
     return error;
 }
 #endif // OPENTHREAD_CONFIG_DNS_UPSTREAM_QUERY_ENABLE

src/core/net/dnssd_server.hpp

@@ -573,7 +573,7 @@ private:
 #endif
 
 #if OPENTHREAD_CONFIG_DNS_UPSTREAM_QUERY_ENABLE
-    static bool               ShouldForwardToUpstream(const Request &aRequest);
+    bool                      ShouldForwardToUpstream(const Request &aRequest) const;
     UpstreamQueryTransaction *AllocateUpstreamQueryTransaction(const Ip6::MessageInfo &aMessageInfo);
     void                      ResetUpstreamQueryTransaction(UpstreamQueryTransaction &aTxn, Error aError);
     Error                     ResolveByUpstream(const Request &aRequest);