[dnssd] introduce AppendServiceRecords() helper (openthread#11443)

This commit introduces a template helper `AppendServiceRecords()`
designed to append service-related records (SRV, TXT, and host
AAAA addresses) to the appropriate sections within a DNS `Response`
message.

This helper simplifies the codebase by removing repeated patterns.
These patterns occur when resolving queries using either SRP service
data or `ServiceInstanceInfo` retrieved from the platform (when the
platform implements discovery proxy function).

Author: abtink

src/core/net/dnssd_server.cpp

@@ -452,6 +452,15 @@ bool Server::Response::QueryNameMatches(const char *aName) const { return Server
 
 Error Server::Response::AppendQueryName(void) { return Name::AppendPointerLabel(sizeof(Header), *mMessage); }
 
+#if OPENTHREAD_CONFIG_SRP_SERVER_ENABLE
+Error Server::Response::AppendPtrRecord(const Srp::Server::Service &aService)
+{
+    uint32_t ttl = TimeMilli::MsecToSec(aService.GetExpireTime() - TimerMilli::GetNow());
+
+    return AppendPtrRecord(aService.GetInstanceLabel(), ttl);
+}
+#endif
+
 Error Server::Response::AppendPtrRecord(const char *aInstanceLabel, uint32_t aTtl)
 {
     Error     error;
@@ -531,6 +540,11 @@ Error Server::Response::AppendSrvRecord(const char *aHostName,
 }
 
 #if OPENTHREAD_CONFIG_SRP_SERVER_ENABLE
+Error Server::Response::AppendHostAddresses(const Srp::Server::Service &aService)
+{
+    return AppendHostAddresses(aService.GetHost());
+}
+
 Error Server::Response::AppendHostAddresses(const Srp::Server::Host &aHost)
 {
     const Ip6::Address *addrs;
@@ -698,6 +712,41 @@ Error Server::Response::AppendGenericRecord(uint16_t aRrType, const void *aData,
     return error;
 }
 
+template <typename ServiceType> Error Server::Response::AppendServiceRecords(const ServiceType &aService)
+{
+    static const Section kSections[] = {kAnswerSection, kAdditionalDataSection};
+
+    // Append SRV and TXT records along with associated host AAAA addresses
+    // in the proper sections.
+
+    Error error = kErrorNone;
+
+    for (Section section : kSections)
+    {
+        mSection = section;
+
+        if (mSection == kAdditionalDataSection)
+        {
+            VerifyOrExit(!(Get<Server>().mTestMode & kTestModeEmptyAdditionalSection));
+        }
+
+        if (mSection == mQuestions.SectionFor(kRrTypeSrv))
+        {
+            SuccessOrExit(error = AppendSrvRecord(aService));
+        }
+
+        if (mSection == mQuestions.SectionFor(kRrTypeTxt))
+        {
+            SuccessOrExit(error = AppendTxtRecord(aService));
+        }
+    }
+
+    error = AppendHostAddresses(aService);
+
+exit:
+    return error;
+}
+
 void Server::Response::IncResourceRecordCount(void)
 {
     switch (mSection)
@@ -729,8 +778,6 @@ void Server::Response::Log(void) const
 
 Error Server::Response::ResolveBySrp(void)
 {
-    static const Section kSections[] = {kAnswerSection, kAdditionalDataSection};
-
     Error                       error          = kErrorNone;
     const Srp::Server::Service *matchedService = nullptr;
     bool                        found          = false;
@@ -772,9 +819,7 @@ Error Server::Response::ResolveBySrp(void)
             {
                 if (QueryNameMatchesService(service))
                 {
-                    uint32_t ttl = TimeMilli::MsecToSec(service.GetExpireTime() - TimerMilli::GetNow());
-
-                    SuccessOrExit(error = AppendPtrRecord(service.GetInstanceLabel(), ttl));
+                    SuccessOrExit(error = AppendPtrRecord(service));
                     matchedService = &service;
                 }
             }
@@ -815,30 +860,7 @@ Error Server::Response::ResolveBySrp(void)
         VerifyOrExit(mQuestions.IsFor(kRrTypeSrv) || mQuestions.IsFor(kRrTypeTxt));
     }
 
-    // Append SRV and TXT records along with associated host AAAA addresses
-    // in the proper sections.
-
-    for (Section section : kSections)
-    {
-        mSection = section;
-
-        if (mSection == kAdditionalDataSection)
-        {
-            VerifyOrExit(!(Get<Server>().mTestMode & kTestModeEmptyAdditionalSection));
-        }
-
-        if (mSection == mQuestions.SectionFor(kRrTypeSrv))
-        {
-            SuccessOrExit(error = AppendSrvRecord(*matchedService));
-        }
-
-        if (mSection == mQuestions.SectionFor(kRrTypeTxt))
-        {
-            SuccessOrExit(error = AppendTxtRecord(*matchedService));
-        }
-    }
-
-    SuccessOrExit(error = AppendHostAddresses(matchedService->GetHost()));
+    error = AppendServiceRecords(*matchedService);
 
 exit:
     return error;
@@ -1168,8 +1190,6 @@ void Server::Response::InitFrom(ProxyQuery &aQuery, const ProxyQueryInfo &aInfo)
 
 void Server::Response::Answer(const ServiceInstanceInfo &aInstanceInfo, const Ip6::MessageInfo &aMessageInfo)
 {
-    static const Section kSections[] = {kAnswerSection, kAdditionalDataSection};
-
     Error error = kErrorNone;
 
     if (mQuestions.IsFor(kRrTypePtr))
@@ -1181,27 +1201,7 @@ void Server::Response::Answer(const ServiceInstanceInfo &aInstanceInfo, const Ip
         SuccessOrExit(error = AppendPtrRecord(instanceLabel, aInstanceInfo.mTtl));
     }
 
-    for (Section section : kSections)
-    {
-        mSection = section;
-
-        if (mSection == kAdditionalDataSection)
-        {
-            VerifyOrExit(!(Get<Server>().mTestMode & kTestModeEmptyAdditionalSection));
-        }
-
-        if (mSection == mQuestions.SectionFor(kRrTypeSrv))
-        {
-            SuccessOrExit(error = AppendSrvRecord(aInstanceInfo));
-        }
-
-        if (mSection == mQuestions.SectionFor(kRrTypeTxt))
-        {
-            SuccessOrExit(error = AppendTxtRecord(aInstanceInfo));
-        }
-    }
-
-    error = AppendHostAddresses(aInstanceInfo);
+    error = AppendServiceRecords(aInstanceInfo);
 
 exit:
     if (error != kErrorNone)

src/core/net/dnssd_server.hpp

@@ -434,8 +434,10 @@ class Server : public InstanceLocator, private NonCopyable
 #if OPENTHREAD_CONFIG_SRP_SERVER_ENABLE
         Error ResolveBySrp(void);
         bool  QueryNameMatchesService(const Srp::Server::Service &aService) const;
+        Error AppendPtrRecord(const Srp::Server::Service &aService);
         Error AppendSrvRecord(const Srp::Server::Service &aService);
         Error AppendTxtRecord(const Srp::Server::Service &aService);
+        Error AppendHostAddresses(const Srp::Server::Service &aService);
         Error AppendHostAddresses(const Srp::Server::Host &aHost);
         Error AppendKeyRecord(const Srp::Server::Host &aHost);
 #endif
@@ -447,6 +449,7 @@ class Server : public InstanceLocator, private NonCopyable
         Error AppendHostIp4Addresses(const ProxyResult &aResult);
         Error AppendGenericRecord(const ProxyResult &aResult);
 #endif
+        template <typename ServiceType> Error AppendServiceRecords(const ServiceType &aService);
 
 #if OT_SHOULD_LOG_AT(OT_LOG_LEVEL_INFO)
         void Log(void) const;