Building interfaces without Python support

[alsora]

Bug report

Before this PR #346, which introduced parsing of IDL format, I was able to cross-compile ROS2 for platforms which do not have Python3 or any Python dependency installed.

I was basically following the instructions in ros2-for-arm/ros2#5 (comment) updated to the current packages structure

sed -i -r \
    's/<build(.+?py.+?)/<\!\-\-build\1\-\->/' \
    src/ros2/rosidl_defaults/rosidl_default_generators/package.xml

Unfortunately it looks like this is not working anymore since the latest updates.

When I try to build the ROS2 SDK, the cross-compilation fails as soon as I reach some the builtin_interfaces

Starting >>> builtin_interfaces
--- stderr: builtin_interfaces                                   
Traceback (most recent call last):
  File "/root/ws/install/share/rosidl_generator_cpp/cmake/../../../lib/rosidl_generator_cpp/rosidl_generator_cpp", line 8, in <module>
    from rosidl_generator_cpp import generate_cpp
  File "/root/ws/install/lib/python3.6/site-packages/rosidl_generator_cpp/__init__.py", line 17, in <module>
    from rosidl_cmake import generate_files
  File "/root/ws/install/lib/python3.6/site-packages/rosidl_cmake/__init__.py", line 27, in <module>
    from rosidl_parser.parser import parse_idl_file
  File "/root/ws/install/lib/python3.6/site-packages/rosidl_parser/parser.py", line 18, in <module>
    from lark import Lark
ModuleNotFoundError: No module named 'lark'
make[2]: *** [rosidl_generator_cpp/builtin_interfaces/msg/duration.hpp] Error 1
make[1]: *** [CMakeFiles/builtin_interfaces__cpp.dir/all] Error 2
make[1]: *** Waiting for unfinished jobs....
Traceback (most recent call last):
  File "/root/ws/install/share/rosidl_generator_c/cmake/../../../lib/rosidl_generator_c/rosidl_generator_c", line 8, in <module>
    from rosidl_generator_c import generate_c
  File "/root/ws/install/lib/python3.6/site-packages/rosidl_generator_c/__init__.py", line 15, in <module>
    from rosidl_cmake import convert_camel_case_to_lower_case_underscore
  File "/root/ws/install/lib/python3.6/site-packages/rosidl_cmake/__init__.py", line 27, in <module>
    from rosidl_parser.parser import parse_idl_file
  File "/root/ws/install/lib/python3.6/site-packages/rosidl_parser/parser.py", line 18, in <module>
    from lark import Lark
ModuleNotFoundError: No module named 'lark'
make[2]: *** [rosidl_generator_c/builtin_interfaces/msg/duration.h] Error 1
make[1]: *** [CMakeFiles/builtin_interfaces__rosidl_generator_c.dir/all] Error 2
make: *** [all] Error 2
---
Failed   <<< builtin_interfaces	[ Exited with code 2 ]

lark is obviously not present in my sysroot, but, before this change, I was able to build everything without using the rosidl_parser/parser.py script.

In order to make the cross-compilation of the ros2 master succeed, I had to revert the following packages to the commit previous to the aforementioned PR

• rosidl
• rosidl_typesupport
• rosidl_typesupport_fastrtps

Is it possible with the new update to keep building ROS2 without using lark or other python dependencies?

Thank you

[dirk-thomas]

lark is now being used to parse the .idl files. So it is not possible to remove that build dependency.

[sloretz]

Before this PR #346, which introduced parsing of IDL format, I was able to build ROS2 without Python support, for platforms which do not have it installed.

In order to make the cross-compilation of the ros2 master succeed, I had to revert the following packages to the commit previous to the aforementioned PR

Are you cross-compiling for a target system that cannot run python, or cross-compiling on a build machine that does not have python installed?

[alsora]

@sloretz sorry it was not clear.

I'm cross-compiling for a target system that cannot run python

[sloretz]

I'm cross-compiling for a target system that cannot run python

It seems like this should be possible. Lark should only be needed on the machine doing the building, not on the target. I'm not familiar with cross compiling ROS 2 though.

Are you aware of https://index.ros.org/doc/ros2/Tutorials/Cross-compilation/ ? If that tutorial is different from the instructions you're following, does the tutorial work for you?

[alsora]

I'm doing something very similar to the cross-compilation tutorial.
Actually lark is not used at all in that tutorial, neither in the build or the target environment.

The only difference I have is that my sysroot only contains a very minimal set of dependencies.

I installed lark on the machine doing the build, but I get the same error.

Should we move this discussion to cross-compilation?

[alsora]

@sloretz you were right, with lark installed on the machine doing the build I'm able to cross-compile.

Thank you.

We can close this.

[Ryanf55]

Is there a way to disable the python generator in CMake to speed up package build time?

Building px4_msgs takes over an hour on CI.