SOLR-13206: Fix AIOOBE when group.facet is specified with group.query

group.facet is supported only for group.field. When group.facet is
used with group.query, then return proper error code

solr/CHANGES.txt

@@ -81,6 +81,11 @@ Improvements
 * SOLR-12368: Support InPlace DV updates for a field that does not yet exist in any documents
 (hossman, Simon Willnauer, Adrien Grand, Munendra S N)
 
+Bug Fixes
+----------------------
+
+* SOLR-13206: Fix AIOOBE when group.facet is specified with group.query and return proper error code. (Marek, Munendra S N)
+
 ==================  8.2.0 ==================
 
 Consult the LUCENE_CHANGES.txt file for additional, low level, changes in this release.

solr/core/src/java/org/apache/solr/request/SimpleFacets.java

@@ -38,6 +38,7 @@ import java.util.concurrent.Semaphore;
 import java.util.function.Predicate;
 import java.util.stream.Stream;
 
+import org.apache.commons.lang3.ArrayUtils;
 import org.apache.lucene.index.ExitableDirectoryReader;
 import org.apache.lucene.index.LeafReader;
 import org.apache.lucene.index.LeafReaderContext;
@@ -705,7 +706,8 @@ public class SimpleFacets {
                                              String prefix,
                                              Predicate<BytesRef> termFilter) throws IOException {
     GroupingSpecification groupingSpecification = rb.getGroupingSpec();
-    final String groupField  = groupingSpecification != null ? groupingSpecification.getFields()[0] : null;
+    String[] groupFields = groupingSpecification != null? groupingSpecification.getFields(): null;
+    final String groupField = ArrayUtils.isNotEmpty(groupFields) ? groupFields[0] : null;
     if (groupField == null) {
       throw new SolrException (
           SolrException.ErrorCode.BAD_REQUEST,

solr/core/src/test/org/apache/solr/request/SimpleFacetsTest.java

@@ -3303,6 +3303,39 @@ public class SimpleFacetsTest extends SolrTestCaseJ4 {
     );
     
   }
+
+  public void testGroupFacetErrors() {
+    ModifiableSolrParams params = params("q", "*:*", "group", "true", "group.query", "myfield_s:*",
+        "facet", "true", "group.facet", "true");
+
+    // with facet.field
+    SolrException ex = expectThrows(SolrException.class, () -> {
+      h.query(req(params, "facet.field", "myfield_s"));
+    });
+    assertEquals(ErrorCode.BAD_REQUEST.code, ex.code());
+    assertTrue(ex.getMessage().contains("Specify the group.field as parameter or local parameter"));
+
+    // with facet.query
+    ex = expectThrows(SolrException.class, () -> {
+      h.query(req(params, "facet.query", "myfield_s:*"));
+    });
+    assertEquals(ErrorCode.BAD_REQUEST.code, ex.code());
+    assertTrue(ex.getMessage().contains("Specify the group.field as parameter or local parameter"));
+
+    // with facet.range
+    ex = expectThrows(SolrException.class, () -> h.query(req(params, "facet.range", "range_facet_l",
+        "facet.range.start", "43", "facet.range.end", "450", "facet.range.gap", "10"))
+    );
+    assertEquals(ErrorCode.BAD_REQUEST.code, ex.code());
+    assertTrue(ex.getMessage().contains("Specify the group.field as parameter or local parameter"));
+
+    // with facet.interval
+    ex = expectThrows(SolrException.class, () -> h.query(req(params, "facet.interval", "range_facet_l",
+        "f.range_facet_l.facet.interval.set", "(43,60]"))
+    );
+    assertEquals(ErrorCode.BAD_REQUEST.code, ex.code());
+    assertTrue(ex.getMessage().contains("Interval Faceting can't be used with group.facet"));
+  }
   
   public void testRangeFacetingBadRequest() {
     String field = "range_facet_l";