Add lazy subscription to republisher (backport #325)

image_transport/include/image_transport/republish.hpp

@@ -30,6 +30,7 @@
 #define IMAGE_TRANSPORT__REPUBLISH_HPP_
 
 #include <memory>
+#include <mutex>
 
 #include "image_transport/image_transport.hpp"
 #include "image_transport/visibility_control.hpp"
@@ -54,6 +55,7 @@ class Republisher : public rclcpp::Node
   bool initialized_{false};
   image_transport::Subscriber sub;
   image_transport::Publisher pub;
+  std::mutex pub_matched_mutex;
   pluginlib::UniquePtr<image_transport::PublisherPlugin> instance;
   std::shared_ptr<pluginlib::ClassLoader<image_transport::PublisherPlugin>> loader;
 };

image_transport/src/republish.cpp

@@ -111,18 +111,30 @@ void Republisher::initialize()
   if (out_transport.empty()) {
     // Use all available transports for output
 
-    this->pub = image_transport::create_publisher(
-      this, out_topic,
-      rmw_qos_profile_default, pub_options);
-
     // Use Publisher::publish as the subscriber callback
     typedef void (image_transport::Publisher::* PublishMemFn)(
       const sensor_msgs::msg::Image::ConstSharedPtr &) const;
     PublishMemFn pub_mem_fn = &image_transport::Publisher::publish;
 
-    this->sub = image_transport::create_subscription(
-      this, in_topic, std::bind(pub_mem_fn, &pub, std::placeholders::_1),
-      in_transport, rmw_qos_profile_default, sub_options);
+    pub_options.event_callbacks.matched_callback =
+      [this, in_topic, in_transport, pub_mem_fn, sub_options](rclcpp::MatchedInfo &)
+      {
+        std::scoped_lock<std::mutex> lock(this->pub_matched_mutex);
+        if (this->pub.getNumSubscribers() == 0) {
+          this->sub.shutdown();
+        } else if (!this->sub) {
+          this->sub = image_transport::create_subscription(
+            this, in_topic,
+            std::bind(pub_mem_fn, &this->pub, std::placeholders::_1),
+            in_transport,
+            rmw_qos_profile_default,
+            sub_options);
+        }
+      };
+
+    this->pub = image_transport::create_publisher(
+      this, out_topic,
+      rmw_qos_profile_default, pub_options);
   } else {
     // Use one specific transport for output
     // Load transport plugin
@@ -132,17 +144,28 @@ void Republisher::initialize()
       "image_transport::PublisherPlugin");
     std::string lookup_name = Plugin::getLookupName(out_transport);
 
-    instance = loader->createUniqueInstance(lookup_name);
-    instance->advertise(this, out_topic, rmw_qos_profile_default, pub_options);
-
-    // Use PublisherPlugin::publish as the subscriber callback
+    // Use PublisherPlugin::publishPtr as the subscriber callback
     typedef void (Plugin::* PublishMemFn)(const sensor_msgs::msg::Image::ConstSharedPtr &) const;
     PublishMemFn pub_mem_fn = &Plugin::publishPtr;
-    this->sub = image_transport::create_subscription(
-      this, in_topic,
-      std::bind(
-        pub_mem_fn,
-        instance.get(), std::placeholders::_1), in_transport, rmw_qos_profile_default, sub_options);
+
+    this->instance = loader->createUniqueInstance(lookup_name);
+
+    pub_options.event_callbacks.matched_callback =
+      [this, in_topic, in_transport, pub_mem_fn, sub_options](rclcpp::MatchedInfo & matched_info)
+      {
+        if (matched_info.current_count == 0) {
+          this->sub.shutdown();
+        } else if (!this->sub) {
+          this->sub = image_transport::create_subscription(
+            this, in_topic,
+            std::bind(
+              pub_mem_fn,
+              this->instance.get(), std::placeholders::_1), in_transport, rmw_qos_profile_default,
+            sub_options);
+        }
+      };
+
+    this->instance->advertise(this, out_topic, rmw_qos_profile_default, pub_options);
   }
 }