SOLR-13821 changed the signature of org.apache.solr.client.solrj.SolrResponse, making serialisations of the class incompatible between versions.
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We've been experimenting with doing a rolling in-place upgrade from Solr 8.3.1 to 8.4.0 on a non-production system, but have found that we get this exception for some operations, including when requesting /solr/admin/collections?action=overseerstatus on a node whose version is inconsistent with the overseer:
java.io.InvalidClassException: org.apache.solr.client.solrj.SolrResponse; local class incompatible: stream classdesc serialVersionUID = -7931100103360242645, local class serialVersionUID = 2239939671435624715
As far as I can see, this is due to the change to the SolrResponse class's signature in commit e3bd5a7. My experimentation has shown that if the serialVersionUID of that class is set explicitly to its previous value the exception no longer occurs.
I'm not sure if this is a necessary or good fix, but I wanted to share this issue with you in case it's something that you think needs resolving.