SOLR-9616 Solr throws exception when expand=true on empty index

solr/CHANGES.txt

@@ -218,6 +218,8 @@ Bug Fixes
 
 * SOLR-9768: RecordingJsonParser produces incomplete json (Wojciech Stryszyk via ab)
 
+* SOLR-9616: Solr throws exception when expand=true on empty index (Timo Hund via Ishan Chattopadhyaya)
+
 Other Changes
 ----------------------
 

solr/core/src/java/org/apache/solr/handler/component/ExpandComponent.java

@@ -265,6 +265,12 @@ public class ExpandComponent extends SearchComponent implements PluginInfoInitia
     * This code gathers the group information for the current page.
     */
     List<LeafReaderContext> contexts = searcher.getTopReaderContext().leaves();
+
+    if(contexts.size() == 0) {
+      //When no context is available we can skip the expanding
+      return;
+    }
+
     int currentContext = 0;
     int currentDocBase = contexts.get(currentContext).docBase;
     int nextDocBase = (currentContext+1)<contexts.size() ? contexts.get(currentContext+1).docBase : Integer.MAX_VALUE;

solr/core/src/test/org/apache/solr/handler/component/TestExpandComponent.java

@@ -314,4 +314,19 @@ public class TestExpandComponent extends SolrTestCaseJ4 {
     );
   }
 
+  @Test
+  public void testExpandWithEmptyIndexReturnsZeroResults() {
+    //We make sure the index is cleared
+
+    clearIndex();
+    assertU(commit());
+
+    ModifiableSolrParams params = new ModifiableSolrParams();
+    params.add("q", "*:*");
+    params.add("fq", "{!collapse field=group_s}");
+    params.add("expand" ,"true");
+    params.add("expand.rows", "10");
+
+    assertQ(req(params), "*[count(//doc)=0]");
+  }
 }