Track the incompatibilities introduced in Solr 9

[HoustonPutman]

This is a running list and will be updated overtime.

• Solr modules have moved from contrib/ to modules/ (SOLR-15917)
  • This may not be an issue. We can double-add the modules in an old and new way:
    • <=8: Continue using the sharedLib strategy (adding paths to /contrib//lib and dist/)
    • >=9: Use the new SOLR_MODULES envVar introduced in SOLR-15914
• The Prometheus Exporter is no longer a module (SOLR-15954)
  • This messes up the default paths that we use for the exporter command and default metrics config. (since they will be different from <=8 and >=9.
  • One option for the exporter command is to backport the PATH changes to the docker-solr repo, so that solr-exporter is added to the path for all versions of Solr. That way the operator can merely use the command: solr-exporter
  • The default config is a non-issue in Solr 9+ because it is added to the classpath, and the classpath option is used by default. (SOLR-14957) However we still have to pass the option because it is required in Solr 8-

[colvinco]

Hello, I was just wondering what the likely timeline for formally supporting Solr 9 is as it's not in the matrix yet?
#441 suggests that the items listed in this issue are already resolved. Are there other known issues?

Thanks