specification: add ros2idl to MCAP spec appendix

docs/specification/appendix.md

@@ -67,13 +67,19 @@ Schema `encoding` may only be omitted for self-describing message encodings such
 
 - `name`: A valid [package resource name](http://wiki.ros.org/Names#Package_Resource_Names), e.g. `sensor_msgs/PointCloud2`
 - `encoding`: `ros1msg`
-- `data`: Concatenated ROS1 .msg files
+- `data`: ROS1 .msg files, concatenated with [`gendeps`-style message definition delimiting](#gendeps-style-message-definition-delimiting)
 
 ### ros2msg
 
 - `name`: A valid [package resource name](http://wiki.ros.org/Names#Package_Resource_Names), e.g. `sensor_msgs/msg/PointCloud2`
 - `encoding`: `ros2msg`
-- `data`: Concatenated ROS2 .msg files
+- `data`: ROS2 .msg files, concatenated with [`gendeps`-style message definition delimiting](#gendeps-style-message-definition-delimiting)
+
+### ros2idl
+
+- `name`: A valid [package resource name](http://wiki.ros.org/Names#Package_Resource_Names), e.g. `sensor_msgs/msg/PointCloud2`
+- `encoding`: `ros2idl`
+- `data`: ROS2 .idl files, concatenated with [`gendeps`-style message definition delimiting](#gendeps-style-message-definition-delimiting)
 
 ### jsonschema
 
@@ -118,4 +124,68 @@ The `ros2` profile describes how to create MCAP files for [ROS 2](https://docs.r
 
 #### Schema
 
-- `encoding`: MUST contain `ros2msg`
+- `encoding`: MUST contain either `ros2msg` or `ros2idl`
+
+## `gendeps`-style message definition delimiting
+
+For ROS1 and ROS2 message types, the message definition is stored in the [Schema record](./README.md#schema-op0x03) along with the definitions of any non-primitive field types. These definitions are concatenated together using a simple delimiting scheme:
+
+- The full content of the top-level message definition file is stored first.
+- for each dependent definition, first a "separator" is inserted. A separator consists of:
+  - a line containing only `=` characters, with more than one `=` character, then
+  - A line containing only `MSG: <package resource name>` for the following message definition.
+- The full content of the next message definition file is inserted after that.
+
+This scheme has its roots in [ROS1 gentools](http://wiki.ros.org/roslib/gentools). The scheme described here can be produced for ROS1 `.msg` definitions using `gendeps --cat`.
+
+### Example
+
+The common `diagnostic_msgs/DiagnosticArray` message definition would be stored in a schema record as follows:
+
+- `name`: `diagnostic_msgs/DiagnosticArray`
+- `encoding`: `ros1msg`
+- `data`:
+
+```
+# This message is used to send diagnostic information about the state of the robot
+Header header #for timestamp
+DiagnosticStatus[] status # an array of components being reported on
+================================================================================
+MSG: std_msgs/Header
+# Standard metadata for higher-level stamped data types.
+# This is generally used to communicate timestamped data
+# in a particular coordinate frame.
+#
+# sequence ID: consecutively increasing ID
+uint32 seq
+#Two-integer timestamp that is expressed as:
+# * stamp.sec: seconds (stamp_secs) since epoch (in Python the variable is called 'secs')
+# * stamp.nsec: nanoseconds since stamp_secs (in Python the variable is called 'nsecs')
+# time-handling sugar is provided by the client library
+time stamp
+#Frame this data is associated with
+string frame_id
+
+================================================================================
+MSG: diagnostic_msgs/DiagnosticStatus
+# This message holds the status of an individual component of the robot.
+#
+
+# Possible levels of operations
+byte OK=0
+byte WARN=1
+byte ERROR=2
+byte STALE=3
+
+byte level # level of operation enumerated above
+string name # a description of the test/component reporting
+string message # a description of the status
+string hardware_id # a hardware unique string
+KeyValue[] values # an array of values associated with the status
+
+
+================================================================================
+MSG: diagnostic_msgs/KeyValue
+string key # what to label this value when viewing
+string value # a value to track over time
+```

python/mcap/mcap/mcap0/well_known.py

@@ -22,6 +22,7 @@ class SchemaEncoding:
     Flatbuffer = "flatbuffer"
     ROS1 = "ros1msg"
     ROS2 = "ros2msg"
+    ROS2IDL = "ros2idl"
     JSONSchema = "jsonschema"