AdrianVovk commented on issue #1851:
URL: https://github.com/apache/buildstream/issues/1851#issuecomment-1668363193
OK I think I've figured this one out. Here's a minimal reproducer:
```yaml
sources:
- kind: local
path: project.conf
- kind: remote
url: https://bing.com
```
The crash happens when any non-trackable element (i.e. `kind: local`)
precedes an element that actually implements tracking. The problem is here:
https://github.com/apache/buildstream/blob/master/src/buildstream/_projectrefs.py#L149.
If `project.refs` is empty, then we append a blank dict to the list and then
get the specified index. However, a `kind: local` element never reaches this
code path. Thus, we end up with one less empty dictionary than expected!
In the above example:
```
element_list = []
# Here, we start processing the kind:local source
(nothing happens, since there's nothing to track)
# Here, we start processing the kind:remote source
element_list.mapping_at(1) -> raises IndexError
element_list.append({}) -> element_list is now [{}]
element_list.mapping_at(1) -> STILL AN INDEX ERROR!
```
Not sure how to fix
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