Symlinking on Windows needs privilege

[wolfv]

As far as I understand, in the default configuration symlinking on Windows needs special privileges.

Here

ament_cmake/ament_cmake_python/cmake/ament_python_install_package.cmake

Lines 108 to 121 in 84e1cea

	add_custom_target(
	ament_cmake_python_symlink_${package_name}
	COMMAND ${CMAKE_COMMAND} -E create_symlink
	"${ARG_PACKAGE_DIR}" "${build_dir}/${package_name}"
	)
	if(ARG_SETUP_CFG)
	add_custom_target(
	ament_cmake_python_symlink_${package_name}_setup
	COMMAND ${CMAKE_COMMAND} -E create_symlink
	"${ARG_SETUP_CFG}" "${build_dir}/setup.cfg"
	)
	list(APPEND egg_dependencies ament_cmake_python_symlink_${package_name}_setup)
	endif()

the CMake tries to create some symlink which fails when building from an average user:

CUSTOMBUILD : CMake error : failed to create symbolic link 'C:/Users/robostack/miniforge3/conda-bld/ros_1626859624934/work/build/ament_cmake_python/smclib/smclib': A required privilege is not held by the client. [%SRC_DIR%\build\ament_cmake_python_symlink_smclib.vcxproj]

C:\Program Files (x86)\Microsoft Visual Studio\2019\Community\MSBuild\Microsoft\VC\v160\Microsoft.CppCommon.targets(238,5): error MSB8066: Custom build for '%SRC_DIR%\build\CMakeFiles\a56e20129da0512e4208cfc6816c1ca9\ament_cmake_python_symlink_smclib.rule' exited with code 1. [%SRC_DIR%\build\ament_cmake_python_symlink_smclib.vcxproj]

A "hard link" should be fine, though.

cc @ooeygui are you aware of other fixes?

Also PS: for the conda installation one doesn't need admin privileges, and it would be quite annoying to change that for symlinks :)

There is apparently a way to allow users to create symlinks: https://community.perforce.com/s/article/3472#:~:text=Create%20Symbolic%20Links%20Privilege&text=Open%20Control%20Panel%2D%3EAdministrative%20Tools,Administrators%20group%2C%20add%20the%20user.

[wolfv]

I tried with create_hardlink but that also fails as it's not possible to hardlink directories 🤦

So now I'll go with copy_directory and copy. I guess that's not desirable for an "interactive" scenario where cmake would probably not re-execute the copy step. Am I getting that right?

I haven't yet dug very deeply into the functionality here.

Could this also be implemented without symlinking?

[ooeygui]

This is a bit of a head scratcher. The Windows command line tool mklink does not require elevation to create a symbolic link within the user context. However, it seems like cmake is calling the system API directly and failing. I'll need to dig in a bit more, but a quick workaround would be to call mklink as suggested here - https://stackoverflow.com/questions/61243174/replacement-of-create-symlink-in-windows.

[wolfv]

I also needed privileges for a symbolic link using mklink, unfortunately.

[wolfv]

Note: hardlink doesn't need privileges (mklink /h ...) works fine

[hidmic]

So now I'll go with copy_directory and copy. I guess that's not desirable for an "interactive" scenario where cmake would probably not re-execute the copy step. Am I getting that right?

Yeah, a copy step would defeat the purpose of a symlink install.

---

https://gitlab.kitware.com/cmake/cmake/-/merge_requests/2144 suggests cmake -E create_symlink should work when Developer Mode is enabled on Windows 10. I haven't verified it myself though. If requiring Developer Mode is a no-go (although a symlink install makes sense for development only, really), why not falling back to a merged install?

[hidmic]

FYI @wolfv, same efforts have been made in the past: #6.

[gavanderhoorn]

I just ran into this. I'm migrating a toolchain from foxy to galactic branches.

Could it be #327 now also complicates / requires this (ie: symlinks)?

What's puzzling is that I'm asking Colcon to do a --merge-install. Is that supposed to prevent things from using symlinks or am I being too hopeful?

I'm not sure I completely understand ament_cmake_python/cmake/ament_python_install_package.cmake, but is it unconditionally trying to use symlinks?

[hidmic]

Could it be #327 now also complicates / requires this?

Hmm, yeah, it does. It is unconditionally creating symlinks to build the egg in the build tree off of sources that are in the source tree (w/o affecting the source tree). This ensures symlink installs work, but breaks things on Windows... (I feel like a man yelling at clouds).

What's puzzling is that I'm asking Colcon to do a --merge-install. Is that supposed to prevent things from using symlinks or am I being too hopeful?

colcon can't do much if cmake starts creating symlinks here and there.

---

@gavanderhoorn perhaps the solution is to fallback to a copy if creating a symlink fails. Symlink installs are not possible anyways if that's the case.

[gavanderhoorn]

What would be the way forward?

This is currently completely blocking my efforts to migrate to Galactic. But I must admit I'm easily getting lost in all the ament_* packages, which prevents me from offering to patch this myself and submit a PR right now.

[hidmic]

Let me see if I can put together something quickly.