Fix docs

Signed-off-by: Jacob Perron <jacob@openrobotics.org>

rclcpp/include/rclcpp/node_interfaces/node_graph.hpp

@@ -124,6 +124,13 @@ public:
     const std::string & topic_name,
     bool no_mangle = false) const override;
 
+  RCLCPP_PUBLIC
+  bool
+  wait_for_publishers(
+    const std::string & topic_name,
+    size_t count,
+    const std::chrono::nanoseconds & timeout) const override;
+
 private:
   RCLCPP_DISABLE_COPY(NodeGraph)
 

rclcpp/include/rclcpp/node_interfaces/node_graph_interface.hpp

@@ -299,6 +299,29 @@ public:
   virtual
   std::vector<rclcpp::TopicEndpointInfo>
   get_subscriptions_info_by_topic(const std::string & topic_name, bool no_mangle = false) const = 0;
+
+  /// Wait for a number of publishers to be available on a topic.
+  /**
+   * This function will block until the provided number of publishers are available or a timeout occurs.
+   *
+   * TODO(jacobperron): Detect this scenario and throw an exception
+   * This function should not be called concurrently with a running GraphListener.
+   * E.g. Do not call this after calling `rclcpp::GraphListener::get_graph_event()`.
+   * If this happens an exception is thrown.
+   *
+   * \param[in] topic_name the topic to check for publishers. It will not be automatically remapped.
+   * \param[in] count the number of publishers to wait for.
+   * \param[in] timeout elapsed time to wait for publishers.
+   * \return `true` if the number of publishers is greater than or equal to the provided count, or
+   *   `false` if a timeout occurs.
+   */
+  RCLCPP_PUBLIC
+  virtual
+  bool
+  wait_for_publishers(
+    const std::string & topic_name,
+    size_t count,
+    const std::chrono::nanoseconds & timeout) const = 0;
 };
 
 }  // namespace node_interfaces

rclcpp/src/rclcpp/node_interfaces/node_graph.cpp

@@ -533,6 +533,31 @@ NodeGraph::get_subscriptions_info_by_topic(
     rcl_get_subscriptions_info_by_topic);
 }
 
+bool
+NodeGraph::wait_for_publishers(
+  const std::string & topic_name,
+  size_t count,
+  const std::chrono::nanoseconds & timeout) const
+{
+  // TODO(jacobperron): Guard against concurrent use of graph guard condition
+  // (e.g. with GraphListener)
+  // rcl_wait_for_publishers() also uses the graph guard condition
+  auto rcl_node_handle = node_base_->get_rcl_node_handle();
+  rcl_allocator_t allocator = rcl_get_default_allocator();
+  bool success = false;
+  rcl_ret_t ret = rcl_wait_for_publishers(
+    rcl_node_handle,
+    &allocator,
+    topic_name.c_str(),
+    count,
+    timeout.count(),
+    &success);
+  if (ret != RCL_RET_OK && ret != RCL_RET_TIMEOUT) {
+    throw_from_rcl_error(ret, "error while waiting for publishers");
+  }
+  return success;
+}
+
 std::string &
 rclcpp::TopicEndpointInfo::node_name()
 {