relay: cache as passive subscriber of primary forwarder

src/MoqxCache.cpp

@@ -7,6 +7,7 @@
  */
 
 #include "MoqxCache.h"
+#include "relay/NullConsumers.h"
 #include <folly/logging/xlog.h>
 #include <moxygen/MoQTrackProperties.h>
 
@@ -1293,6 +1294,10 @@ class MoqxCache::FetchWriteback : public FetchConsumer {
   }
 };
 
+std::shared_ptr<TrackConsumer> MoqxCache::makePassiveConsumer(const FullTrackName& ftn) {
+  return getSubscribeWriteback(ftn, std::make_shared<moxygen::NullTrackConsumer>());
+}
+
 std::shared_ptr<TrackConsumer> MoqxCache::getSubscribeWriteback(
     const FullTrackName& ftn,
     std::shared_ptr<TrackConsumer> consumer

src/MoqxCache.h

@@ -48,6 +48,11 @@ class MoqxCache {
       std::shared_ptr<moxygen::TrackConsumer> consumer
   );
 
+  // Returns a TrackConsumer suitable for use as a passive forwarder subscriber.
+  // Objects delivered to it are written to the cache; nothing is forwarded
+  // downstream (the inner consumer is a NullTrackConsumer).
+  std::shared_ptr<moxygen::TrackConsumer> makePassiveConsumer(const moxygen::FullTrackName& ftn);
+
   // Serves objects from the cache to the consumer.  If objects in the range are
   // not in cache, issue one-or-more FETCH'es upstream.  Objects fetched from
   // upstream are written back to the cache and passed to the consumer.