Proper handling of root relative hrefs

[soxofaan]

We're struggling with a use case where items have "root relative" hrefs to assets and pystac does not provide a valid absolute href:

>>> import pystac
>>> pystac.__version__
'1.14.1'
>>> item = pystac.read_file("https://eoresults.esa.int/stac/collections/FCM-AGB-100m/items/FCM-AGB-100m-2017")
>>> asset = item.assets["PRODUCT"]
>>> asset.href
'/d/FCM-AGB-100m/2017/01/01/FCM-AGB-100m-2017/FCM_Europe_demo_2017_AGB.tif'
>>> asset.get_absolute_href()
'/d/FCM-AGB-100m/2017/01/01/FCM-AGB-100m-2017/FCM_Europe_demo_2017_AGB.tif'

The expectation is to get absolute href https://eoresults.esa.int/d/FCM-AGB-100m/2017/01/01/FCM-AGB-100m-2017/FCM_Europe_demo_2017_AGB.tif

Relevant parts of item STAC:

{
  "id": "FCM-AGB-100m-2017",
  "type": "Feature",
  "links": [
    {
      "rel": "root",
      "type": "application/json",
      "href": "https://eoresults.esa.int/stac/"
    },
    {
      "rel": "self",
      "type": "application/geo+json",
      "href": "https://eoresults.esa.int/stac/collections/FCM-AGB-100m/items/FCM-AGB-100m-2017"
    }
  ],
  "assets": {
    "PRODUCT": {
      "href": "/d/FCM-AGB-100m/2017/01/01/FCM-AGB-100m-2017/FCM_Europe_demo_2017_AGB.tif",
      "type": "image/tiff; application=geotiff",
      "roles": ["data"],
      "title": "Product",
    }
  },

note: STAC browser does produce the expected asset href: https://radiantearth.github.io/stac-browser/#/external/eoresults.esa.int/stac/collections/FCM-AGB-100m/items/FCM-AGB-100m-2017

[gadomski]

Yeah, this is a tricky one. Because pystac is designed to work on filesystems and in network-aware systems, we treat "root relative" hrefs as absolute filesystem paths. stac-browser doesn't have to worry about filesystem paths, which is why it's able to handle the root relative href correctly by default.

It would be reasonable to add a "network mode" to pystac read (probably via changing the default StacIO, but I'm not sure), which would resolve hrefs as "root relative uris" instead of filesystem paths. I'll mark this issue as an enhancement, and I'd be happy to review a PR implementing.