[2209] Skeleton of the getCachedZoneWriter method

It is mostly copy of the reload() method. Currently, it does not contain
the initialization of load_action for the zone writer, so if a real zone
writer is returned, it doesn't work (it throws). But the tests for the
unsuccessful cases succeed.

There's also need to do something about the duplicate code between this
method and the reload(). Later on, we'll probably rewrite reload to use
the ZoneWriter (and even later deprecate reload, maybe? Or keep it as
the thin wrapper for convenience?)

src/lib/datasrc/client_list.cc

@@ -12,14 +12,16 @@
 // OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE USE OR
 // PERFORMANCE OF THIS SOFTWARE.
 
-#include <util/memory_segment_local.h>
 
 #include "client_list.h"
 #include "client.h"
 #include "factory.h"
 #include "memory/memory_client.h"
+#include "memory/zone_table_segment.h"
+#include "memory/zone_writer.h"
 #include "logger.h"
 #include <dns/masterload.h>
+#include <util/memory_segment_local.h>
 
 #include <memory>
 #include <boost/foreach.hpp>
@@ -377,9 +379,45 @@ ConfigurableClientList::reload(const Name& name) {
 }
 
 ConfigurableClientList::ZoneWriterPair
-ConfigurableClientList::getCachedZoneWriter(const Name& ) {
-    // TODO: Just for now.
-    return (ZoneWriterPair(CACHE_DISABLED, ZoneWriterPtr()));
+ConfigurableClientList::getCachedZoneWriter(const Name& name) {
+    if (!allow_cache_) {
+        return (ZoneWriterPair(CACHE_DISABLED, ZoneWriterPtr()));
+    }
+    // Try to find the correct zone.
+    MutableResult result;
+    findInternal(result, name, true, true);
+    if (!result.finder) {
+        return (ZoneWriterPair(ZONE_NOT_FOUND, ZoneWriterPtr()));
+    }
+    // Try to get the in-memory cache for the zone. If there's none,
+    // we can't provide the result.
+    if (!result.info->cache_) {
+        return (ZoneWriterPair(ZONE_NOT_CACHED, ZoneWriterPtr()));
+    }
+    memory::LoadAction load_action;
+    DataSourceClient* client(result.info->data_src_client_);
+    if (client) {
+        // Now finally provide the writer.
+        // If it does not exist in client,
+        // DataSourceError is thrown, which is exactly the result what we
+        // want, so no need to handle it.
+        ZoneIteratorPtr iterator(client->getIterator(name));
+        if (!iterator) {
+            isc_throw(isc::Unexpected, "Null iterator from " << name);
+        }
+        // TODO
+    } else {
+        // The MasterFiles special case
+        const string filename(result.info->cache_->getFileName(name));
+        if (filename.empty()) {
+            isc_throw(isc::Unexpected, "Confused about missing both filename "
+                      "and data source");
+        }
+        // TODO
+    }
+    return (ZoneWriterPair(ZONE_RELOADED,
+                           ZoneWriterPtr(result.info->cache_->getZoneTableSegment().
+                                         getZoneWriter(load_action, name, rrclass_))));
 }
 
 // NOTE: This function is not tested, it would be complicated. However, the