CCBC-1664: register read event watcher for KV socket if duplex is enabled

When the cluster is 7.6.5+ and support Faster Failover, libcouchbase
would automatically disable config polling mechanism and eventually
remove all event watchers for KV sockets, if there is no KV traffic
(e.g. the application only doing Query or Search requests).

If the cluster will be reconfigured during this time, the libcouchbase
will not pick up new topology and continue using previous set of the
endpoints.

Change-Id: I4f0460bd6697c7b7987992a53cadde5b1e1d9a44
Reviewed-on: https://review.couchbase.org/c/libcouchbase/+/228199
Tested-by: Build Bot <build@couchbase.com>
Reviewed-by: Dimitris Christodoulou <dimitris.christodoulou@couchbase.com>

Author: avsej

src/mcserver/mcserver.cc

@@ -746,6 +746,9 @@ static void on_read(lcbio_CTX *ctx, unsigned)
 
     while (server->try_read(ctx, ior) == Server::PKT_READ_COMPLETE)
         ;
+    if (server->supports_duplex()) {
+        lcbio_ctx_rwant(ctx, 24);
+    }
     lcbio_ctx_schedule(ctx);
     lcb_maybe_breakout(server->instance);
 }
@@ -1127,6 +1130,7 @@ void Server::handle_connected(lcbio_SOCKET *sock, lcb_STATUS err, lcbio_OSERR sy
         } else if (settings->conntype == LCB_TYPE_BUCKET && settings->bucket) {
             try_to_select_bucket = true;
         }
+        duplex = sessinfo->has_feature(PROTOCOL_BINARY_FEATURE_DUPLEX);
         clustermap_change_notification =
             sessinfo->has_feature(PROTOCOL_BINARY_FEATURE_CLUSTERMAP_CHANGE_NOTIFICATION_BRIEF);
         config_with_known_version =

src/mcserver/mcserver.h

@@ -141,6 +141,11 @@ class Server : public mc_PIPELINE
         return clustermap_change_notification;
     }
 
+    bool supports_duplex() const
+    {
+        return duplex;
+    }
+
     /**
      * If true, the library will always send version of current configuration, which allows KV engine skip payload in
      * response, if it does not have newer version.
@@ -258,6 +263,9 @@ class Server : public mc_PIPELINE
     /** Whether GetClusterConfigWithKnownVersion is supported */
     short config_with_known_version{0};
 
+    /** Whether the connection supports unsolicited packets from the server */
+    short duplex{0};
+
     lcbio_CTX *connctx;
     lcb::io::ConnectionRequest *connreq{};