- Implemented

  - rcl_get_publishers_info_by_topic
  - rcl_get_subscriptions_info_by_topic
- Wrote tests for the same

Note: colcon build --packages-up-to rcl && colcon test --packages-select rcl && colcon test-result --verbose --test-result-base build/rcl
runs successfully without any errors or failures.

Signed-off-by: Jaison Titus <jaisontj92@gmail.com>

rcl/include/rcl/graph.h

@@ -525,22 +525,23 @@ rcl_count_subscribers(
   size_t * count);
 
 /// Returns a list of all publishers to a topic.
-/// Each element in the list will contain the publisher's name and its respective qos profile.
+/// Each element in the list will contain the node name, node namespace, topic type,
+/// gid and the qos profile of the publisher.
 /**
  * The `node` parameter must point to a valid node.
  *
  * The `topic_name` parameter must not be `NULL`.
  *
- * The `publishers` parameter must point to a valid struct of type rmw_participants_t.
- * The `count` field inside the struct must be set to 0
- * The `participants` field inside the struct must be set to null.
- * The `publishers` parameter is the output for this function and will be set.
+ * The `no_mangle` parameter determines if the provided topic_name should be
+ * expanded to its fully qualified name.
  *
- * The topic name is not automatically remapped by this function.
- * If there is a publisher created with topic name `foo` and remap rule `foo:=bar` then calling
- * this with `topic_name` set to `bar` will return a list with 1 publisher, and with `topic_name` set to `foo`
- * will return a list with 0 publishers.
- * /sa rcl_remap_topic_name()
+ * It is the responsibility of the caller to ensure that `publishers_info` parameter points
+ * to a valid struct of type rmw_topic_info_array_t. The `count` field inside the struct
+ * must be set to 0 and the `info_array` field inside the struct must be set to null.
+ *
+ * The `allocator` will be used to allocate memory to the `info_array` member
+ * inside of `publishers_info`. However, it is the responsibility of the caller to
+ * reclaim any allocated resources to `publishers_info` to avoid leaking memory.
  *
  * <hr>
  * Attribute          | Adherence
@@ -552,38 +553,46 @@ rcl_count_subscribers(
  * <i>[1] implementation may need to protect the data structure with a lock</i>
  *
  * \param[in] node the handle to the node being used to query the ROS graph
+ * \param[in] allocator allocator to be used when allocating space for
+ *            the array inside publishers_info
  * \param[in] topic_name the name of the topic in question
- * \param[out] publishers a struct representing a list of publishers with their qos profile.
+ * \param[in] no_mangle if true, the topic_name will be expanded to its fully qualified name.
+ * \param[out] publishers_info a struct representing a list of publisher information.
  * \return `RCL_RET_OK` if the query was successful, or
  * \return `RCL_RET_NODE_INVALID` if the node is invalid, or
  * \return `RCL_RET_INVALID_ARGUMENT` if any arguments are invalid, or
+ * \return `RCL_RET_BAD_ALLOC` if memory allocation fails, or
  * \return `RCL_RET_ERROR` if an unspecified error occurs.
  */
 RCL_PUBLIC
 RCL_WARN_UNUSED
 rcl_ret_t
-rcl_get_qos_for_publishers(
+rcl_get_publishers_info_by_topic(
   const rcl_node_t * node,
+  rcutils_allocator_t * allocator,
   const char * topic_name,
-  rmw_participants_t * publishers);
+  bool no_mangle,
+  rmw_topic_info_array_t * publishers_info);
+
 
-/// Returns a list of all subscribers to a topic.
-/// Each element in the list will contain the subscriber's name and its respective qos profile.
+/// Returns a list of all subscriptions to a topic.
+/// Each element in the list will contain the node name, node namespace, topic type,
+/// gid and the qos profile of the subscription.
 /**
  * The `node` parameter must point to a valid node.
  *
  * The `topic_name` parameter must not be `NULL`.
  *
- * The `subscriber` parameter must point to a valid struct of type rmw_participants_t.
- * The `count` field inside the struct must be set to 0
- * The `participants` field inside the struct must be set to null.
- * The `subscribers` parameter is the output for this function and will be set.
+ * The `no_mangle` parameter determines if the provided topic_name should be
+ * expanded to its fully qualified name.
  *
- * The topic name is not automatically remapped by this function.
- * If there is a subscriber created with topic name `foo` and remap rule `foo:=bar` then calling
- * this with `topic_name` set to `bar` will return a list with 1 subscriber , and with `topic_name` set to `foo`
- * will return a list with 0 subscribers.
- * /sa rcl_remap_topic_name()
+ * It is the responsibility of the caller to ensure that `subscriptions_info` parameter points
+ * to a valid struct of type rmw_topic_info_array_t. The `count` field inside the struct
+ * must be set to 0 and the `info_array` field inside the struct must be set to null.
+ *
+ * The `allocator` will be used to allocate memory to the `info_array` member
+ * inside of `subscriptions_info`.However, it is the responsibility of the caller to
+ * reclaim any allocated resources to `subscriptions_info` to avoid leaking memory.
  *
  * <hr>
  * Attribute          | Adherence
@@ -595,20 +604,26 @@ rcl_get_qos_for_publishers(
  * <i>[1] implementation may need to protect the data structure with a lock</i>
  *
  * \param[in] node the handle to the node being used to query the ROS graph
+ * \param[in] allocator allocator to be used when allocating space for
+ *            the array inside publishers_info
  * \param[in] topic_name the name of the topic in question
- * \param[out] subscribers a struct representing a list of subscribers with their qos profile.
+ * \param[in] no_mangle if true, the topic_name will be expanded to its fully qualified name.
+ * \param[out] subscriptions_info a struct representing a list of subscriptions information.
  * \return `RCL_RET_OK` if the query was successful, or
  * \return `RCL_RET_NODE_INVALID` if the node is invalid, or
  * \return `RCL_RET_INVALID_ARGUMENT` if any arguments are invalid, or
+ * \return `RCL_RET_BAD_ALLOC` if memory allocation fails, or
  * \return `RCL_RET_ERROR` if an unspecified error occurs.
  */
 RCL_PUBLIC
 RCL_WARN_UNUSED
 rcl_ret_t
-rcl_get_qos_for_subscribers(
+rcl_get_subscriptions_info_by_topic(
   const rcl_node_t * node,
+  rcutils_allocator_t * allocator,
   const char * topic_name,
-  rmw_participants_t * subscribers);
+  bool no_mangle,
+  rmw_topic_info_array_t * subscriptions_info);
 
 
 /// Check if a service server is available for the given service client.

rcl/src/rcl/graph.c

@@ -28,6 +28,7 @@ extern "C"
 #include "rmw/get_topic_names_and_types.h"
 #include "rmw/names_and_types.h"
 #include "rmw/rmw.h"
+#include "rmw/topic_info_array.h"
 #include "rmw/validate_namespace.h"
 #include "rmw/validate_node_name.h"
 
@@ -375,6 +376,81 @@ rcl_count_subscribers(
   return rcl_convert_rmw_ret_to_rcl_ret(rmw_ret);
 }
 
+
+typedef rmw_ret_t (* get_topic_info_func)(
+  const rmw_node_t * node,
+  rcutils_allocator_t * allocator,
+  const char * topic_name,
+  bool no_mangle,
+  rmw_topic_info_array_t * info_array);
+
+rcl_ret_t
+__rcl_get_info_by_topic(
+  const rcl_node_t * node,
+  rcutils_allocator_t * allocator,
+  const char * topic_name,
+  bool no_mangle,
+  rmw_topic_info_array_t * info_array,
+  get_topic_info_func get_topic_info)
+{
+  if (!rcl_node_is_valid(node)) {
+    RCL_SET_ERROR_MSG("Invalid node provided.");
+    return RCL_RET_NODE_INVALID;
+  }
+  const rcl_node_options_t * node_options = rcl_node_get_options(node);
+  if (!node_options) {
+    RCL_SET_ERROR_MSG("Node options are invalid.");
+    return RCL_RET_NODE_INVALID;  // shouldn't happen, but error is already set if so
+  }
+  RCL_CHECK_ALLOCATOR_WITH_MSG(allocator, "invalid allocator", return RCL_RET_INVALID_ARGUMENT);
+  RCL_CHECK_ARGUMENT_FOR_NULL(topic_name, RCL_RET_INVALID_ARGUMENT);
+  rcl_ret_t rcl_ret = rcl_convert_rmw_ret_to_rcl_ret(rmw_topic_info_array_check_zero(info_array));
+  if (rcl_ret != RCL_RET_OK) {
+    RCL_SET_ERROR_MSG_WITH_FORMAT_STRING(
+      "rmw_topic_info_array_t must be zero initialized: %s,\n"
+      "Use rmw_get_zero_initialized_topic_info_array",
+      rmw_get_error_string().str);
+    rmw_reset_error();
+    return rcl_ret;
+  }
+  rmw_ret_t rmw_ret = get_topic_info(
+    rcl_node_get_rmw_handle(node),
+    allocator,
+    topic_name,
+    no_mangle,
+    info_array);
+  rcl_ret = rcl_convert_rmw_ret_to_rcl_ret(rmw_ret);
+  if (rcl_ret != RCL_RET_OK) {
+    RCL_SET_ERROR_MSG(rmw_get_error_string().str);
+    rmw_reset_error();
+  }
+  return rcl_ret;
+}
+
+rcl_ret_t
+rcl_get_publishers_info_by_topic(
+  const rcl_node_t * node,
+  rcutils_allocator_t * allocator,
+  const char * topic_name,
+  bool no_mangle,
+  rmw_topic_info_array_t * publishers_info)
+{
+  return __rcl_get_info_by_topic(node, allocator, topic_name, no_mangle, publishers_info,
+           rmw_get_publishers_info_by_topic);
+}
+
+rcl_ret_t
+rcl_get_subscriptions_info_by_topic(
+  const rcl_node_t * node,
+  rcutils_allocator_t * allocator,
+  const char * topic_name,
+  bool no_mangle,
+  rmw_topic_info_array_t * subscriptions_info)
+{
+  return __rcl_get_info_by_topic(node, allocator, topic_name, no_mangle, subscriptions_info,
+           rmw_get_subscriptions_info_by_topic);
+}
+
 rcl_ret_t
 rcl_service_server_is_available(
   const rcl_node_t * node,