I was updating the https://github.com/querqy/querqy-solr repo for Solr 10, and
then tests started due to issues with ExternalPaths.
I asked claude for help and it said:
The `ExternalPaths` class assumed it was running within the Solr source tree
and attempted to resolve paths like `webapp/web`,
`server/solr/configsets/_default/conf`, etc. on the `SOURCE_HOME` path. When
`SOURCE_HOME` was `null` (as in external projects), calling `.resolve()` on it
caused an NPE.
I dug a bit more, and it was caused when we did the File --> Path migation in
SOLR-16903 (commit 86f2afded1252b69b8478a2c795a7657585ef5c2).
In Java's `java.io.File` implementation:
```javaFile file = new File(null, "webapp/web"); // This is LEGALString path =
file.getAbsolutePath(); // Returns a valid path (relative)```but with
path:
```javaPath path = null;path.resolve("webapp/web"); // ❌
NullPointerException!```
I have a potential patch here:
https://gist.github.com/epugh/beaf36329593aa879af2d26eaa9030fd
Thoughts? Am I understanding things correctly?
Eric
