'BUG': get_loader() returns None when loading bst project

[harrysarson]

Hi we have seen a bug whilst loading a buildstream project that uses junctions. The bug ends up with AttributeError: 'NoneType' object has no attribute '_load_file' in buildstream/_loader/loader.py presumably because get_loader() returns None here

buildstream/src/buildstream/_loader/loader.py

Line 189 in 6d16e3a

def get_loader(self, name, provenance_node, *, load_subprojects=True):

.

I have seen #1975 which seems to make get_loader() returning None more robust. However, we are struggling to reproduce so are not able to confirm whether that issue fixes the bug.

[harrysarson]

I have managed to reproduce this bug (unfortunately I cannot share the code). I have confirmed that applying the fix in #1975 fixes the issue.

To reproduce I needed:

1. Four bst projects A, B, C, D. A depends on B via a junction, B depends on C via a junction and C depends on D via a junction.
2. A uses link elements to refer to C and D.
3. A overrides an element in D

I haven't managed to reproduce on anything simpler than four projects (so three junctions).

[harrysarson]

Closing as #1975 is merged