Creating a standalone server executable

[sauk2]

🎉 New feature

Related to gazebosim/gz-tools#7, gazebo-release/gz-sim8-release#8 and #2737

Summary

This PR extends on the recent migration of gz sim to use a standalone executable, gz-sim-main. This executable also launches a GUI executable when built using -DENABLE_GUI=ON. This change updates the CMake configuration to allow the gz-sim-main executable to be reused for building a standalone server executable, gz-sim-server irrespective of the -DENABLE_GUI=ON argument. Additionally, GUI-related CLI flags are now conditionally compiled using WITH_GUI, ensuring that GUI-specific options do not appear in gz-sim-server or in gz-sim-main when built without GUI support.

Test it

After compilation, check for a new executable named gz-sim-server-client inside libexec/gz/sim10. Calling it should launch the server without any GUI.

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
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• All tests passed (See test coverage)
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[j-rivero]

to be reused for building a standalone server executable, gz-sim-server-client

I think that we can simply call it gz-sim-server

Before continue reviewing the code, note that the validation step that we need for the official docker images coming from our .deb packages is that a single build WITH_GUI produces a server binary that does not use any X / Qt / other Xorg related library. We are linking the new server against the Qt libraries:

gz-sim/build on  standalone-server-exe [?] via C v13.3.0-gcc via △ v3.28.3 ❯ ldd ./bin/gz-sim-server-client  | grep Qt
	libQt6Core.so.6 => /lib/x86_64-linux-gnu/libQt6Core.so.6 (0x000074d147e00000)
	libQt6Quick.so.6 => /lib/x86_64-linux-gnu/libQt6Quick.so.6 (0x000074d146e00000)
	libQt6Qml.so.6 => /lib/x86_64-linux-gnu/libQt6Qml.so.6 (0x000074d146800000)
	libQt6Widgets.so.6 => /lib/x86_64-linux-gnu/libQt6Widgets.so.6 (0x000074d146000000)
	libQt6Gui.so.6 => /lib/x86_64-linux-gnu/libQt6Gui.so.6 (0x000074d145800000)
	libQt6QmlModels.so.6 => /lib/x86_64-linux-gnu/libQt6QmlModels.so.6 (0x000074d146d40000)
	libQt6Network.so.6 => /lib/x86_64-linux-gnu/libQt6Network.so.6 (0x000074d144869000)
	libQt6OpenGL.so.6 => /lib/x86_64-linux-gnu/libQt6OpenGL.so.6 (0x000074d1474a4000)
	libQt6DBus.so.6 => /lib/x86_64-linux-gnu/libQt6DBus.so.6 (0x000074d145f4b000)

This probably comes from the fact of llnking it to gz.

[j-rivero]

Let's try to fix the X / Qt issue

[sauk2]

Let's try to fix the X / Qt issue

Thanks for catching this! I have made changes in CMake to not link the gz library to gz-sim-server. I think that should solve this issue.