fix: validate BEACON_MODE, cache process info, handle missing plugins

Address self-review findings:
- Warn and fall back to 'none' for invalid BEACON_MODE values
- Warn and fall back to 'none' when beacon plugin .so is not found
- Cache pid and hostname in constructor instead of calling syscalls
  on every publish
- Add explicit <chrono> include
- Null-terminate hostname buffer to prevent truncation UB

Author: bburda

demos/sensor_diagnostics/launch/demo.launch.py

@@ -57,6 +57,16 @@ def generate_launch_description():
 
     # Beacon mode from environment (controls both plugin loading and node behavior)
     beacon_mode = os.environ.get('BEACON_MODE', 'none')
+    valid_beacon_modes = ('none', 'topic', 'param')
+    if beacon_mode not in valid_beacon_modes:
+        import sys
+        print(
+            f"WARNING: Invalid BEACON_MODE='{beacon_mode}'. "
+            f"Valid values: {', '.join(valid_beacon_modes)}. "
+            "Falling back to 'none'.",
+            file=sys.stderr,
+        )
+        beacon_mode = 'none'
 
     # Resolve plugin paths
     graph_provider_path = _resolve_plugin_path(
@@ -84,6 +94,14 @@ def generate_launch_description():
             plugin_overrides['plugins.topic_beacon.topic'] = \
                 '/ros2_medkit/discovery'
             plugin_overrides['plugins.topic_beacon.beacon_ttl_sec'] = 10.0
+        else:
+            import sys
+            print(
+                "WARNING: BEACON_MODE=topic but topic_beacon plugin not "
+                "found. Falling back to none.",
+                file=sys.stderr,
+            )
+            beacon_mode = 'none'
     elif beacon_mode == 'param':
         param_beacon_path = _resolve_plugin_path(
             'ros2_medkit_param_beacon', 'param_beacon_plugin')
@@ -93,6 +111,14 @@ def generate_launch_description():
                 param_beacon_path
             plugin_overrides['plugins.parameter_beacon.poll_interval_sec'] = \
                 5.0
+        else:
+            import sys
+            print(
+                "WARNING: BEACON_MODE=param but param_beacon plugin not "
+                "found. Falling back to none.",
+                file=sys.stderr,
+            )
+            beacon_mode = 'none'
 
     plugin_overrides['plugins'] = active_plugins
 

demos/sensor_diagnostics/src/beacon_helper.hpp

@@ -10,6 +10,7 @@
 
 #pragma once
 
+#include <chrono>
 #include <memory>
 #include <string>
 #include <unordered_map>
@@ -42,10 +43,23 @@ class BeaconHelper
     node_->declare_parameter("beacon_mode", "none");
     mode_ = node_->get_parameter("beacon_mode").as_string();
 
+    // Cache process info (constant during lifetime)
+    pid_ = static_cast<uint32_t>(getpid());
+    char hostname_buf[256];
+    hostname_buf[sizeof(hostname_buf) - 1] = '\0';
+    if (gethostname(hostname_buf, sizeof(hostname_buf) - 1) == 0) {
+      hostname_ = hostname_buf;
+    }
+
     if (mode_ == "topic") {
       init_topic_beacon();
     } else if (mode_ == "param") {
       init_param_beacon();
+    } else if (mode_ != "none") {
+      RCLCPP_WARN(
+        node_->get_logger(),
+        "Unknown beacon_mode '%s', expected none/topic/param. Beacon disabled.",
+        mode_.c_str());
     }
   }
 
@@ -74,12 +88,11 @@ class BeaconHelper
     node_->declare_parameter("ros2_medkit.discovery.component_id", config_.component_id);
     node_->declare_parameter("ros2_medkit.discovery.function_ids", config_.function_ids);
     node_->declare_parameter(
-      "ros2_medkit.discovery.process_id", static_cast<int>(getpid()));
+      "ros2_medkit.discovery.process_id", static_cast<int>(pid_));
     node_->declare_parameter("ros2_medkit.discovery.process_name", node_->get_name());
 
-    char hostname_buf[256];
-    if (gethostname(hostname_buf, sizeof(hostname_buf)) == 0) {
-      node_->declare_parameter("ros2_medkit.discovery.hostname", std::string(hostname_buf));
+    if (!hostname_.empty()) {
+      node_->declare_parameter("ros2_medkit.discovery.hostname", hostname_);
     }
 
     for (const auto & [key, value] : config_.metadata) {
@@ -98,15 +111,11 @@ class BeaconHelper
     msg.display_name = config_.display_name;
     msg.component_id = config_.component_id;
     msg.function_ids = config_.function_ids;
-    msg.process_id = static_cast<uint32_t>(getpid());
+    msg.process_id = pid_;
     msg.process_name = node_->get_name();
+    msg.hostname = hostname_;
     msg.stamp = node_->now();
 
-    char hostname_buf[256];
-    if (gethostname(hostname_buf, sizeof(hostname_buf)) == 0) {
-      msg.hostname = hostname_buf;
-    }
-
     for (const auto & [key, value] : config_.metadata) {
       diagnostic_msgs::msg::KeyValue kv;
       kv.key = key;
@@ -120,6 +129,8 @@ class BeaconHelper
   rclcpp::Node * node_;
   Config config_;
   std::string mode_;
+  uint32_t pid_{0};
+  std::string hostname_;
   rclcpp::Publisher<ros2_medkit_msgs::msg::MedkitDiscoveryHint>::SharedPtr beacon_pub_;
   rclcpp::TimerBase::SharedPtr beacon_timer_;
 };