Accept relative paths in SDF files

[chapulina]

Closes #2718 .

The main motivation for this is to allow more models to be reused between Gazebo-classic and Ignition to ease migration. Comments on this issue are relevant: gazebosim/gz-fuel-tools#77 (comment). I'm planning to document all this on a proper tutorial once the implementation is in.

This is a port of this ign-gazebo PR.

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What's working

I locally modified the Dumpster model to use relative URIs for visual and collision geometries, as well as materials. Then I verified that it's possible to insert it from the left menu and both client and server load all assets correctly.

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TODO

• Add tests
• Check that directly calling ~/factory with sdf_filename works
• Check that actor skins and animations work

[azeey]

Just FYI, #2734 changes how models are spawned from ModelMaker.

[ahcorde]

I added the testdb directory to my GAZEBO_MODEL_PATH and I'm not able to include the model with the interface. I'm getting a segfault

[Wrn] [SystemPaths.cc:464] File or path does not exist [""] [meshes/test.dae]
[Err] [Visual.cc:3043] No mesh specified

[ahcorde]

@chapulina, I added these lines and there is no segfault:

  if (_elem->GetParent() == nullptr){
    return;
  }

here https://github.com/osrf/gazebo/blob/737037fb3b302a20b64d596f120b350aed84e0c6/gazebo/common/CommonIface.cc#L513

[ahcorde]

I made some comments

[chapulina]

@ahcorde , all your comments have been addressed on e041cca

[ahcorde]

LGTM. Tested locally too

[chapulina]

There are way too many failing tests, I'll take a look at them next week.

[chapulina]

Ok, so from the 24 test failures on Ubuntu, 17 claim to be the 1st failure, but they're almost all flaky:

• https://build.osrfoundation.org/job/gazebo-ci-pr_any-ubuntu_auto-amd64-gpu-none/336/testReport/(root)/UNIT_SonarSensor_TEST/test_ran/history/
• https://build.osrfoundation.org/job/gazebo-ci-pr_any-ubuntu_auto-amd64-gpu-none/336/testReport/(root)/ActorTest/TrajectoryFromSDF/history/
• https://build.osrfoundation.org/job/gazebo-ci-pr_any-ubuntu_auto-amd64-gpu-none/336/testReport/(root)/SpeedThreadIslandsTest/DualPR2QuickStep/history/
• https://build.osrfoundation.org/job/gazebo-ci-pr_any-ubuntu_auto-amd64-gpu-none/336/testReport/(root)/SpeedThreadIslandsTest/MultiplePendulumWorldStep/history/
• https://build.osrfoundation.org/job/gazebo-ci-pr_any-ubuntu_auto-amd64-gpu-none/336/testReport/(root)/SpeedThreadIslandsTest/MultiplePendulumQuickStep/history/
• https://build.osrfoundation.org/job/gazebo-ci-pr_any-ubuntu_auto-amd64-gpu-none/336/testReport/(root)/PhysicsEngines_Pioneer2dx/StraightLine_1/history/
• https://build.osrfoundation.org/job/gazebo-ci-pr_any-ubuntu_auto-amd64-gpu-none/336/testReport/(root)/PhysicsEngines_PhysicsLinkTest/AddForce_3/history/
• https://build.osrfoundation.org/job/gazebo-ci-pr_any-ubuntu_auto-amd64-gpu-none/336/testReport/(root)/PhysicsEngines_PhysicsFrictionTest/FrictionDemo_3/history/
• https://build.osrfoundation.org/job/gazebo-ci-pr_any-ubuntu_auto-amd64-gpu-none/336/testReport/(root)/INTEGRATION_joint_spawn/test_ran/history/
• https://build.osrfoundation.org/job/gazebo-ci-pr_any-ubuntu_auto-amd64-gpu-none/336/testReport/(root)/PhysicsEngines_JointForceTorqueTest/GetForceTorqueWithAppliedForceReset_3/history/
• https://build.osrfoundation.org/job/gazebo-ci-pr_any-ubuntu_auto-amd64-gpu-none/336/testReport/(root)/PhysicsEngines_JointForceTorqueTest/GetForceTorqueWithAppliedForce_3/history/
• https://build.osrfoundation.org/job/gazebo-ci-pr_any-ubuntu_auto-amd64-gpu-none/336/testReport/(root)/PhysicsEngines_ImuTest/ImuSensorTestWorld_0/history/
• https://build.osrfoundation.org/job/gazebo-ci-pr_any-ubuntu_auto-amd64-gpu-none/336/testReport/(root)/PhysicsEngines_ContactSensor/ModelRemoval_3/history/
• https://build.osrfoundation.org/job/gazebo-ci-pr_any-ubuntu_auto-amd64-gpu-none/336/testReport/(root)/EXAMPLE_examples_build/test_ran/history/

This is the only one which seems to be new, but it passes for me locally and I can't think of how it's related:

• https://build.osrfoundation.org/job/gazebo-ci-pr_any-ubuntu_auto-amd64-gpu-none/336/testReport/(root)/INTEGRATION_speed/test_ran/history/
• https://build.osrfoundation.org/job/gazebo-ci-pr_any-ubuntu_auto-amd64-gpu-none/345/testReport/(root)/PhysicsEngines_SpeedTest/history/

[scpeters]

I think the windows build is failing since this was merged:

LINK : fatal error LNK1181: cannot open input file 'stdc++fs.lib'

https://build.osrfoundation.org/job/gazebo-ci-gazebo11-windows7-amd64/29/