When you open a scene and a linked datablock has been removed from the linked .blend file, a little warning shows up in the console:
Info: LIB: Material: 'MyCoolMaterial' missing from 'C:\Whatever\SomeFile.blend'
How would one go about finding that same information via python (without uber hacky crap like scraping the console stdout, please don’t suggest it )
In this hypothetical example, the library itself is still valid. The file still exists, and has plenty of valid data being linked into the scene- only a specific material was removed (and is still being used in the other scene). Looking at the outliner in “File mode” there’s even a special icon for data blocks with broken links, so that information exists somewhere, but I cannot find it.
There doesn’t appear to be anything in bpy.types.id that would check a linked datablock against its original location on demand, and the library class itself is a really bare bones container for loading/reloading the library itself. It’s possible to figure out which datablocks came from a library by checking library.users_id() but that just points you to the local data, which then points you back to the library (there is nothing on the datablock itself as far as I can tell).
So yeah, any suggestions would be great. So far the best I’ve got is pretty hacky (though not nearly as hacky as scraping stdout)- using ctypes to capture the linked data state based on the icon shown in the outliner, but I’m 99.9% sure that will fail if the outliner is not in file explorer mode so I really don’t want to head down that path unless it’s necessary.