[2459] do not throw when getZoneWriter result in "not cached".

instead, explicitly log the event at the debug level.  behavior-wise there's
no difference, but the corresponding log message can be misleading.

src/bin/auth/auth_messages.mes

@@ -93,6 +93,16 @@ This debug message is issued when the separate thread for maintaining data
 source clients successfully loaded the named zone of the named class as a
 result of the 'loadzone' command.
 
+% AUTH_DATASRC_CLIENTS_BUILDER_LOAD_ZONE_NOCACHE skipped loading zone %1/%2 due to no in-memory cache
+This debug message is issued when the separate thread for maintaining data
+source clients received a command to reload a zone but skipped it because
+the specified zone is not loaded in-memory (but served from an underlying
+data source).  This could happen if the loadzone command is manually issued
+by a user but the zone name is misspelled, but a more likely cause is
+that the command is sent from another BIND 10 module (such as xfrin or DDNS).
+In the latter case it can be simply ignored because there is no need
+for explicit reloading.
+
 % AUTH_DATASRC_CLIENTS_BUILDER_RECONFIGURE_CONFIG_ERROR Error in data source configuration: %1
 The thread for maintaining data source clients has received a command to
 reconfigure, but the parameter data (the new configuration) contains an

src/bin/auth/datasrc_clients_mgr.h

@@ -581,6 +581,9 @@ DataSrcClientsBuilderBase<MutexType, CondVarType>::doLoadZone(
     try {
         boost::shared_ptr<datasrc::memory::ZoneWriter> zwriter =
             getZoneWriter(*client_list, rrclass, origin);
+        if (!zwriter) {
+            return;
+        }
 
         zwriter->load(); // this can take time but doesn't cause a race
         {   // install() can cause a race and must be in a critical section
@@ -632,8 +635,10 @@ DataSrcClientsBuilderBase<MutexType, CondVarType>::getZoneWriter(
                   << "/" << rrclass << ": not found in any configured "
                   "data source.");
     case datasrc::ConfigurableClientList::ZONE_NOT_CACHED:
-        isc_throw(InternalCommandError, "failed to load zone " << origin
-                  << "/" << rrclass << ": not served from memory");
+        LOG_DEBUG(auth_logger, DBG_AUTH_OPS,
+                  AUTH_DATASRC_CLIENTS_BUILDER_LOAD_ZONE_NOCACHE)
+            .arg(origin).arg(rrclass);
+        break;                  // return NULL below
     case datasrc::ConfigurableClientList::CACHE_DISABLED:
         // This is an internal error. Auth server must have the cache
         // enabled.
@@ -642,8 +647,6 @@ DataSrcClientsBuilderBase<MutexType, CondVarType>::getZoneWriter(
                   "is somehow disabled");
     }
 
-    // all cases above should either return or throw, but some compilers
-    // still need a return statement
     return (boost::shared_ptr<datasrc::memory::ZoneWriter>());
 }
 } // namespace datasrc_clientmgr_internal

src/bin/auth/tests/datasrc_clients_builder_unittest.cc

@@ -385,7 +385,10 @@ TEST_F(DataSrcClientsBuilderTest,
               find(Name("example.org")).finder_->
               find(Name("example.org"), RRType::SOA())->code);
 
-    // attempt of reloading a zone but in-memory cache is disabled.
+    // attempt of reloading a zone but in-memory cache is disabled.  In this
+    // case the command is simply ignored.
+    const size_t orig_lock_count = map_mutex.lock_count;
+    const size_t orig_unlock_count = map_mutex.unlock_count;
     const ConstElementPtr config2(Element::fromJSON("{"
         "\"IN\": [{"
         "    \"type\": \"sqlite3\","
@@ -394,11 +397,13 @@ TEST_F(DataSrcClientsBuilderTest,
         "    \"cache-zones\": [\"example.org\"]"
         "}]}"));
     clients_map = configureDataSource(config2);
-    EXPECT_THROW(builder.handleCommand(
+    builder.handleCommand(
                      Command(LOADZONE, Element::fromJSON(
                                  "{\"class\": \"IN\","
-                                 " \"origin\": \"example.org\"}"))),
-                 TestDataSrcClientsBuilder::InternalCommandError);
+                                 " \"origin\": \"example.org\"}")));
+    // Only one mutex was needed because there was no actual reload.
+    EXPECT_EQ(orig_lock_count + 1, map_mutex.lock_count);
+    EXPECT_EQ(orig_unlock_count + 1, map_mutex.unlock_count);
 
     // basically impossible case: in-memory cache is completely disabled.
     // In this implementation of manager-builder, this should never happen,