uorb: add message format compatibility check

This can be used by DDS/ROS 2 to check for matching message definitions.

Tools/msg/px_generate_uorb_topic_helper.py

@@ -171,6 +171,61 @@ def get_children_fields(base_type, search_path):
     return spec_temp.parsed_fields()
 
 
+def get_message_fields_str_for_message_hash(msg_fields, search_path):
+    """
+    Get all fields (including for nested types) in the form of:
+'''
+uint64 timestamp
+uint8 esc_count
+uint8 esc_online_flags
+EscReport[8] esc
+uint64 timestamp
+uint32 esc_errorcount
+int32 esc_rpm
+float32 esc_voltage
+uint16 failures
+int8 esc_power
+'''
+    """
+    all_fields_str = ''
+    for field in msg_fields:
+        if field.is_header:
+            continue
+
+        type_name = field.type
+        # detect embedded types
+        sl_pos = type_name.find('/')
+        if sl_pos >= 0:
+            type_name = type_name[sl_pos + 1:]
+
+        all_fields_str += type_name + ' ' + field.name + '\n'
+
+        if sl_pos >= 0: # nested type, add all nested fields
+            children_fields = get_children_fields(field.base_type, search_path)
+            all_fields_str += get_message_fields_str_for_message_hash(children_fields, search_path)
+
+    return all_fields_str
+
+
+def hash_32_fnv1a(data: str):
+    hash_val = 0x811c9dc5
+    prime = 0x1000193
+    for i in range(len(data)):
+        value = ord(data[i])
+        hash_val = hash_val ^ value
+        hash_val *= prime
+        hash_val &= 0xffffffff
+    return hash_val
+
+
+def get_message_hash(msg_fields, search_path):
+    """
+    Get a 32 bit message hash over all fields
+    """
+    all_fields_str = get_message_fields_str_for_message_hash(msg_fields, search_path)
+    return hash_32_fnv1a(all_fields_str)
+
+
 def add_padding_bytes(fields, search_path):
     """
     Add padding fields before the embedded types, at the end and calculate the

Tools/msg/templates/uorb/msg.cpp.em

@@ -58,6 +58,7 @@ from px_generate_uorb_topic_helper import * # this is in Tools/
 
 uorb_struct = '%s_s'%name_snake_case
 
+message_hash = get_message_hash(spec.parsed_fields(), search_path)
 sorted_fields = sorted(spec.parsed_fields(), key=sizeof_field_type, reverse=True)
 struct_size, padding_end_size = add_padding_bytes(sorted_fields, search_path)
 }@
@@ -74,7 +75,7 @@ struct_size, padding_end_size = add_padding_bytes(sorted_fields, search_path)
 
 @[for topic in topics]@
 static_assert(static_cast<orb_id_size_t>(ORB_ID::@topic) == @(all_topics.index(topic)), "ORB_ID index mismatch");
-ORB_DEFINE(@topic, struct @uorb_struct, @(struct_size-padding_end_size), static_cast<orb_id_size_t>(ORB_ID::@topic));
+ORB_DEFINE(@topic, struct @uorb_struct, @(struct_size-padding_end_size), @(message_hash)u, static_cast<orb_id_size_t>(ORB_ID::@topic));
 @[end for]
 
 void print_message(const orb_metadata *meta, const @uorb_struct& message)