GzOdeCollisionDetector: Use static mutex in create method to avoid problems with non-thread-safe ODE functions

[traversaro]

🦟 Bug fix

Summary

This is done to avoid that non-thread-safe ODE functions are called at the same time, causing errors such as

ODE INTERNAL ERROR 1: assertion "!colliders_initialized" failed in dInitColliders() [collision_kernel.cpp:168]

Why this happens? As the GzOdeCollisionDetector constructor calls the DART's OdeCollisionDetector constructor, that instead calls the non-thread-safe dInitODE2(0) ode function. If dInitODE2(0) is called at different times, everything seems to be working fine, so we enforce this by adding a static std::mutex in the GzOdeCollisionDetector::create.

This fixes part of https://github.com/gazebosim/gz-sim/issues/18 . Note that this does not solve all the ODE-related multi-threading problems, but at least allows to set a different collision manager without having errors such as:

ODE INTERNAL ERROR 1: assertion "!colliders_initialized" failed in dInitColliders() [collision_kernel.cpp:168]

That happens as GzOdeCollisionDetector is allocated before the requested collision detector is set by WorldFeatures::SetWorldCollisionDetector, so it is not possible to avoid to construct `GzOdeCollisionDetector by specifying a custom collision detector in the SDF, unless we want to change the default collision detector, that is something we want to avoid (at least at the moment) as discussed in https://github.com/gazebosim/gz-sim/issues/18 .

Checklist

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

I consider adding tests for this, but the race condition that results in errors such as:

ODE INTERNAL ERROR 1: assertion "!colliders_initialized" failed in dInitColliders() [collision_kernel.cpp:168]

is extremely complex to reproduce (I was able to do so just with apt dependencies and in Debug mode) so I am not sure it is useful to try to reproduce this in a test.