Stop hardcoding absolute paths into downloaded models

[chapulina]

When ign-fuel-tools finds a model:// within a file that it's downloading, it replaces that with the full path on disk where the file is being downloaded to. This is done by the LocalCachePrivate::FixPaths* functions.

This is undesirable because it makes those models no longer be relocatable after they've been downloaded.

Moreover, in general, modifying a resource while downloading it makes for poor user experience, because the user is expecting to get the resource as it is on the server. Downloading a resource from the web UI or making direct REST calls to the server doesn't modify resources, and users are using these to circumvent this behaviour (among others) (pit_crew, untouched).

There is a reason for the current behaviour though. Initially Ignition Gazebo didn't support model:// URIs. And even now that it's about to support them as first-class citizens (gazebosim/gz-sim#172), that doesn't map well to models in the Fuel cache, because:

• The cache directory structure, <model name>/version/model.sdf, doesn't match the <model name>/model.sdf pattern used with the environment variable.
• Most of the models in the OpenRobotics org have been transferred from gazebo_models, renamed on the process, but the URIs inside the SDF file remained the same. This is an issue with the resources though, not the library.

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Possible ways forward

Relative paths

Modify all models in Fuel that are using model:// to use relative paths instead.

Problems with this approach:

• We do support model:// though (actually any scheme://), so users may upload that and it has to work without overwriting.
• Relative paths don't support sharing resources across several models.

Fuel cache + model://

Make it possible to find a resource on the cache, such as:

~/.ignition/fuel/fuel.ignitionrobotics.org/OpenRobotics/models/Post\ office/2/meshes/post_office.dae

from

model://Post office/meshes/post_office.dae

The main challenge is, what if multiple owners have models named Post office, which one do we get?

[chapulina]

The more I think about this, I think that any implementation of Fuel cache + model:// will come with so many caveats that we might as well not recommend it.

So I propose circling back to the proposals on this PR. Essentially:

1. ✨ Strongly ✨ recommend the following inside Fuel models:
   • relative paths for files within a resource (i.e. <uri>meshes/mesh.dae</uri> inside model.sdf)
   • full URLs for files from another resource (i.e. <uri>https://fuel.ignitionrobotics.org/...dae</uri> inside model.sdf)
2. ✨ Strongly ✨ discourage the use of any other URI scheme inside Fuel. And if users do, the model will work, but with the caveat that once downloaded, the model:// is converted to a full path Fuel URI. Ignition does support URIs like model://, but they should only be used for projects installing their own models and setting environment variables, not by models uploaded to Fuel.

Action items:

• Update all models in the OpenRobotics org to use relative paths / full URLs
• Document the strong recommendations somewhere (maybe even find a way to warn users when they try to upload a model containing schemed-URIs, like we do if they try to upload without thumbnails)
• Make Gazebo-classic accept relative paths, so that Fuel models can also be used there: Accept relative paths in URIs gazebo-classic#2718 (also consider fixing Fuel support for Gazebo 11 to help with migration to Ignition Improve Ignition Fuel support gazebo-classic#2406)
• 🆕 Change this library to convert model://s to Fuel URIs instead of full paths: Convert model:// to Fuel URLs instead of absolute paths #85

[chapulina]

Closed too quickly 🙂

[chapulina]

Tutorial recommending the use of relative paths: gazebosim/gz-sim#400

[chapulina]

Update all models in the OpenRobotics org to use relative paths / full URLs

Follow up on #194

Closing this one!