SOLR-13970: disallow using collapse/expand with grouping

* Using collapse with grouping would cause inconsistent behavior.
  This is because grouping calls the same postfilter twice without
  resetting the internal state of the DocValues cache
* Using expand with grouping would cause NPE

solr/CHANGES.txt

@@ -124,6 +124,9 @@ Upgrade Notes
 
 * SOLR-13904: timeAllowed parameter is allowed to have 0 value (Houston Putman, Mikhail Khludnev)
 
+* SOLR-13970: Using Collapse filter or expand component with grouping is explicitly disallowed as the combination
+  would cause inconsistent behavior and NPEs.
+
 New Features
 ---------------------
 * SOLR-13821: A Package store to store and load package artifacts (noble, Ishan Chattopadhyaya)
@@ -190,6 +193,8 @@ Improvements
 * SOLR-13904: Analytic component abandon requests exceedig limit passed via timeAllowed parameter 
   (Houston Putman, Mikhail Khludnev).
 
+* SOLR-13970: Fail the request when collapsing or expand is used with Grouping. (Erick Erickson, Joel Bernstein, Munendra S N)
+
 Optimizations
 ---------------------
 (No changes)

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

@@ -63,6 +63,7 @@ import org.apache.lucene.util.LongValues;
 import org.apache.solr.common.SolrDocumentList;
 import org.apache.solr.common.SolrException;
 import org.apache.solr.common.params.ExpandParams;
+import org.apache.solr.common.params.GroupParams;
 import org.apache.solr.common.params.SolrParams;
 import org.apache.solr.common.util.NamedList;
 import org.apache.solr.common.util.SimpleOrderedMap;
@@ -113,6 +114,9 @@ public class ExpandComponent extends SearchComponent implements PluginInfoInitia
   @Override
   public void prepare(ResponseBuilder rb) throws IOException {
     if (rb.req.getParams().getBool(ExpandParams.EXPAND, false)) {
+      if (rb.req.getParams().getBool(GroupParams.GROUP, false)) {
+        throw new SolrException(SolrException.ErrorCode.BAD_REQUEST, "Can not use expand with Grouping enabled");
+      }
       rb.doExpand = true;
     }
   }

solr/core/src/java/org/apache/solr/search/CollapsingQParserPlugin.java

@@ -65,6 +65,7 @@ import org.apache.lucene.util.BytesRef;
 import org.apache.lucene.util.FixedBitSet;
 import org.apache.lucene.util.LongValues;
 import org.apache.solr.common.SolrException;
+import org.apache.solr.common.params.GroupParams;
 import org.apache.solr.common.params.ModifiableSolrParams;
 import org.apache.solr.common.params.SolrParams;
 import org.apache.solr.handler.component.QueryElevationComponent;
@@ -279,6 +280,11 @@ public class CollapsingQParserPlugin extends QParserPlugin {
     }
 
     public CollapsingPostFilter(SolrParams localParams, SolrParams params, SolrQueryRequest request) {
+      // Don't allow collapsing if grouping is being used.
+      if (request.getParams().getBool(GroupParams.GROUP, false)) {
+        throw new SolrException(SolrException.ErrorCode.BAD_REQUEST, "Can not use collapse with Grouping enabled");
+      }
+
       this.collapseField = localParams.get("field");
       if (this.collapseField == null) {
         throw new SolrException(SolrException.ErrorCode.BAD_REQUEST, "Required 'field' param is missing.");

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

@@ -392,8 +392,15 @@ public class TestExpandComponent extends SolrTestCaseJ4 {
     ignoreException("Can't determine a Sort Order");
     ignoreException("Expand not supported for fieldType:'text'");
 
+    // expand with grouping
+    SolrException e = expectThrows(SolrException.class, () -> {
+      h.query(req("q", "*:*", "expand", "true", "expand.field", "id", "group", "true", "group.field", "id"));
+    });
+    assertEquals(SolrException.ErrorCode.BAD_REQUEST.code, e.code());
+    assertEquals("Can not use expand with Grouping enabled", e.getMessage());
+
     // no expand field
-    SolrException e = expectThrows(SolrException.class,  () -> h.query(req("q", "*:*", "expand", "true")));
+    e = expectThrows(SolrException.class,  () -> h.query(req("q", "*:*", "expand", "true")));
     assertEquals(SolrException.ErrorCode.BAD_REQUEST.code, e.code());
     assertEquals("missing expand field", e.getMessage());
 

solr/core/src/test/org/apache/solr/search/TestCollapseQParserPlugin.java

@@ -33,6 +33,8 @@ import org.junit.Before;
 import org.junit.BeforeClass;
 import org.junit.Test;
 
+import static org.hamcrest.core.StringContains.containsString;
+
 public class TestCollapseQParserPlugin extends SolrTestCaseJ4 {
   @BeforeClass
   public static void beforeClass() throws Exception {
@@ -206,7 +208,6 @@ public class TestCollapseQParserPlugin extends SolrTestCaseJ4 {
   }
 
   @Test
-  @AwaitsFix(bugUrl="https://issues.apache.org/jira/browse/SOLR-11974")
   public void testStringCollapse() throws Exception {
     for (final String hint : new String[] {"", " hint="+CollapsingQParserPlugin.HINT_TOP_FC}) {
       testCollapseQueries("group_s", hint, false);
@@ -759,14 +760,12 @@ public class TestCollapseQParserPlugin extends SolrTestCaseJ4 {
     params.add("facet.mincount", "1");
     assertQ(req(params), "*[count(//doc)=1]", "*[count(//lst[@name='facet_fields']/lst[@name='test_i']/int)=2]");
 
-    // SOLR-5230 - ensure CollapsingFieldValueCollector.finish() is called
-    params = new ModifiableSolrParams();
-    params.add("q", "*:*");
-    params.add("fq", "{!collapse field="+group+hint+"}");
-    params.add("group", "true");
-    params.add("group.field", "id");
-    assertQ(req(params), "*[count(//doc)=2]");
-
+    // SOLR-13970
+    SolrException ex = expectThrows(SolrException.class, () -> {
+      h.query(req(params("q", "*:*", "fq", "{!collapse field="+group+hint+"}", "group", "true", "group.field", "id")));
+    });
+    assertEquals(SolrException.ErrorCode.BAD_REQUEST.code, ex.code());
+    assertThat(ex.getMessage(), containsString("Can not use collapse with Grouping enabled"));
 
     // delete the elevated docs, confirm collapsing still works
     assertU(delI("1"));
@@ -784,7 +783,6 @@ public class TestCollapseQParserPlugin extends SolrTestCaseJ4 {
                          "//result/doc[2]/str[@name='id'][.='6']",
                          "//result/doc[3]/str[@name='id'][.='7']");
 
-
   }
 
   @Test

solr/solr-ref-guide/src/collapse-and-expand-results.adoc

@@ -18,7 +18,7 @@
 
 The Collapsing query parser and the Expand component combine to form an approach to grouping documents for field collapsing in search results.
 
-The Collapsing query parser groups documents (collapsing the result set) according to your parameters, while the Expand component provides access to documents in the collapsed group for use in results display or other processing by a client application. Collapse & Expand can together do what the older <<result-grouping.adoc#result-grouping,Result Grouping>> (`group=true`) does for _most_ use-cases but not all. Generally, you should prefer Collapse & Expand.
+The Collapsing query parser groups documents (collapsing the result set) according to your parameters, while the Expand component provides access to documents in the collapsed group for use in results display or other processing by a client application. Collapse & Expand can together do what the older <<result-grouping.adoc#result-grouping,Result Grouping>> (`group=true`) does for _most_ use-cases but not all. Collapse and Expand are not supported when Result Grouping is enabled. Generally, you should prefer Collapse & Expand.
 
 [IMPORTANT]
 ====