SOLR-10613: add flag to FunctionQParser to use FieldNameValueSource place holder when encountering a field name

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

@@ -25,6 +25,7 @@ import org.apache.solr.common.params.SolrParams;
 import org.apache.solr.request.SolrQueryRequest;
 import org.apache.solr.schema.SchemaField;
 import org.apache.solr.search.facet.AggValueSource;
+import org.apache.solr.search.function.FieldNameValueSource;
 
 import java.util.ArrayList;
 import java.util.List;
@@ -33,6 +34,7 @@ public class FunctionQParser extends QParser {
 
   public static final int FLAG_CONSUME_DELIMITER = 0x01;  // consume delimiter after parsing arg
   public static final int FLAG_IS_AGG = 0x02;
+  public static final int FLAG_USE_FIELDNAME_SOURCE = 0x04; // When a field name is encountered, use the placeholder FieldNameValueSource instead of resolving to a real ValueSource
   public static final int FLAG_DEFAULT = FLAG_CONSUME_DELIMITER;
 
   /** @lucene.internal */
@@ -374,8 +376,13 @@ public class FunctionQParser extends QParser {
         } else if ("false".equals(id)) {
           valueSource = new BoolConstValueSource(false);
         } else {
-          SchemaField f = req.getSchema().getField(id);
-          valueSource = f.getType().getValueSource(f, this);
+          if ((flags & FLAG_USE_FIELDNAME_SOURCE) != 0) {
+            // Don't try to create a ValueSource for the field, just use a placeholder.
+            valueSource = new FieldNameValueSource(id);
+          } else {
+            SchemaField f = req.getSchema().getField(id);
+            valueSource = f.getType().getValueSource(f, this);
+          }
         }
       }
 

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

@@ -1017,14 +1017,14 @@ public abstract class ValueSourceParser implements NamedListInitializedPlugin {
     addParser("agg_min", new ValueSourceParser() {
       @Override
       public ValueSource parse(FunctionQParser fp) throws SyntaxError {
-        return new MinMaxAgg("min", fp.parseValueSource());
+        return new MinMaxAgg("min", fp.parseValueSource(FunctionQParser.FLAG_DEFAULT | FunctionQParser.FLAG_USE_FIELDNAME_SOURCE));
       }
     });
 
     addParser("agg_max", new ValueSourceParser() {
       @Override
       public ValueSource parse(FunctionQParser fp) throws SyntaxError {
-        return new MinMaxAgg("max", fp.parseValueSource());
+        return new MinMaxAgg("max", fp.parseValueSource(FunctionQParser.FLAG_DEFAULT | FunctionQParser.FLAG_USE_FIELDNAME_SOURCE));
       }
     });
 

solr/core/src/java/org/apache/solr/search/facet/MinMaxAgg.java

@@ -28,6 +28,7 @@ import org.apache.lucene.util.BytesRef;
 import org.apache.lucene.util.LongValues;
 import org.apache.solr.schema.SchemaField;
 import org.apache.solr.schema.StrFieldSource;
+import org.apache.solr.search.function.FieldNameValueSource;
 
 public class MinMaxAgg extends SimpleAggValueSource {
   final int minmax; // a multiplier to reverse the normal order of compare if this is max instead of min (i.e. max will be -1)
@@ -41,9 +42,16 @@ public class MinMaxAgg extends SimpleAggValueSource {
   public SlotAcc createSlotAcc(FacetContext fcontext, int numDocs, int numSlots) throws IOException {
     ValueSource vs = getArg();
 
+    SchemaField sf = null;
+
+    if (vs instanceof FieldNameValueSource) {
+      String field = ((FieldNameValueSource)vs).getFieldName();
+      sf = fcontext.qcontext.searcher().getSchema().getField(field);
+
+      vs = sf.getType().getValueSource(sf, null); // temporary implementation to make existing code work
+    }
+
     if (vs instanceof StrFieldSource) {
-      String field = ((StrFieldSource) vs).getField();
-      SchemaField sf = fcontext.qcontext.searcher().getSchema().getField(field);
       if (sf.multiValued() || sf.getType().multiValuedFieldCache()) {
         if (sf.hasDocValues()) {
           // dv

solr/core/src/java/org/apache/solr/search/function/FieldNameValueSource.java

@@ -0,0 +1,60 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.solr.search.function;
+
+
+import java.io.IOException;
+import java.util.Map;
+
+import org.apache.lucene.index.LeafReaderContext;
+import org.apache.lucene.queries.function.FunctionValues;
+import org.apache.lucene.queries.function.ValueSource;
+
+/** Placeholder value source.
+ * @lucene.internal */
+public class FieldNameValueSource extends ValueSource {
+  private String fieldName;
+
+  public FieldNameValueSource(String fieldName) {
+    this.fieldName = fieldName;
+  }
+
+  public String getFieldName() {
+    return fieldName;
+  }
+
+  @Override
+  public FunctionValues getValues(Map context, LeafReaderContext readerContext) throws IOException {
+    throw new UnsupportedOperationException("FieldNameValueSource should not be directly used: " + this);
+  }
+
+  @Override
+  public boolean equals(Object o) {
+    return o instanceof FieldNameValueSource && fieldName.equals(((FieldNameValueSource)o).getFieldName());
+  }
+
+  @Override
+  public int hashCode() {
+    return fieldName.hashCode();
+  }
+
+  @Override
+  public String description() {
+    return "FIELDNAME(" + fieldName + ")";
+  }
+}