Improve signal handling

[azeey]

🦟 Bug fix

Summary

This mainly deals with the problem where a signal is ignored if it occurs right before ServerPrivate::Run is called because we generally rely on the running variable to know when to stop, but that variable is set to true at the beginning of ServerPrivate::Run.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
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[iche033]

checking what's a good way to test this? Launch gazebo then verify ctrl + c signal is handled right away?

[azeey]

This mainly improves signal handling when Gazebo starts up. So the best way to test it would be to start Gazebo in server mode (gz sim -s empty.sdf) and type ctrl+c in the first few seconds. Without this PR, I noticed the signal would sometimes be completely ignored.

[iche033]

I did some manual testing and found that there is a specific point in time after launch when I hit ctrl + c, the signal is handled with these changes and ignored without these changes.

I did run into a case that gz sim would still freeze if I ctrl + c right after it starts up:

iche@jammy2:~/code/gz_i_ws$ gz sim -v 4 -s shapes.sdf
[Msg] Gazebo Sim Server v9.0.0
[Msg] Loading SDF world file[/home/iche/code/gz_i_ws/install/share/gz/gz-sim9/worlds/shapes.sdf].
^C^C^C^C^C^C^C^C

That can be addressed separately.

[azeey]

That can be addressed separately.

I'm hoping some of the remaining issues are related to gazebosim/gz-common#618, but there might be other instances of timing.