HttpJettySolrClient: don't throw IllegalStateException if connection lost

changelog/unreleased/PR#4490-HttpJettySolrClient-IllegalStateException.yml

@@ -0,0 +1,9 @@
+title: >
+  HttpJettySolrClient could throw IllegalStateException on connection lost,
+  which foiled LBSolrClient's attempts to classify a request as retry-able.
+type: fixed
+authors:
+  - name: David Smiley
+links:
+  - name: PR#4490
+    url: https://github.com/apache/solr/pull/4490

solr/solrj-jetty/src/java/org/apache/solr/client/solrj/jetty/HttpJettySolrClient.java

@@ -507,6 +507,10 @@ public NamedList<Object> request(SolrRequest<?> solrRequest, String collection)
             "IOException occurred when talking to server at: " + url, cause);
       }
       throw new SolrServerException(cause.getMessage(), cause);
+    } catch (IllegalStateException e) {
+      // Jetty HTTP/2 throws IllegalStateException ("session closed") when the connection is lost.
+      abortCause = e;
+      throw new SolrServerException("Connection lost at: " + url, new IOException(e));
     } catch (SolrServerException | RuntimeException sse) {
       abortCause = sse;
       throw sse;