Fix class cast issue in bwc

Author: jordan-powers

modules/mapper-extras/src/main/java/org/elasticsearch/index/mapper/extras/MatchOnlyTextFieldMapper.java

@@ -365,7 +365,7 @@ public Query phrasePrefixQuery(TokenStream stream, int slop, int maxExpansions,
         @Override
         public BlockLoader blockLoader(BlockLoaderContext blContext) {
             if (textFieldType.isSyntheticSource()) {
-                return new BlockStoredFieldsReader.BytesFromBytesRefsBlockLoader(storedFieldNameForSyntheticSource());
+                return new BlockStoredFieldsReader.BytesFromMixedStringsBytesRefBlockLoader(storedFieldNameForSyntheticSource());
             }
             SourceValueFetcher fetcher = SourceValueFetcher.toString(blContext.sourcePaths(name()));
             // MatchOnlyText never has norms, so we have to use the field names field
@@ -386,7 +386,12 @@ public IndexFieldData.Builder fielddataBuilder(FieldDataContext fieldDataContext
                 ) {
                     @Override
                     protected BytesRef storedToBytesRef(Object stored) {
-                        return (BytesRef) stored;
+                        if (stored instanceof BytesRef) {
+                            return (BytesRef) stored;
+                        } else {
+                            assert stored instanceof String;
+                            return new BytesRef((String) stored);
+                        }
                     }
                 };
             }
@@ -477,7 +482,12 @@ protected SyntheticSourceSupport syntheticSourceSupport() {
             () -> new StringStoredFieldFieldLoader(fieldType().storedFieldNameForSyntheticSource(), fieldType().name(), leafName()) {
                 @Override
                 protected void write(XContentBuilder b, Object value) throws IOException {
-                    b.value(((BytesRef) value).utf8ToString());
+                    if (value instanceof BytesRef) {
+                        b.value(((BytesRef) value).utf8ToString());
+                    } else {
+                        assert value instanceof String;
+                        b.value((String) value);
+                    }
                 }
             }
         );

server/src/main/java/org/elasticsearch/index/mapper/BlockStoredFieldsReader.java

@@ -63,6 +63,34 @@ public final SortedSetDocValues ordinals(LeafReaderContext context) {
         }
     }
 
+    public static class BytesFromMixedStringsBytesRefBlockLoader extends StoredFieldsBlockLoader {
+        public BytesFromMixedStringsBytesRefBlockLoader(String field) {
+            super(field);
+        }
+
+        @Override
+        public Builder builder(BlockFactory factory, int expectedCount) {
+            return factory.bytesRefs(expectedCount);
+        }
+
+        @Override
+        public RowStrideReader rowStrideReader(LeafReaderContext context) throws IOException {
+            return new Bytes(field) {
+                private final BytesRef scratch = new BytesRef();
+
+                @Override
+                protected BytesRef toBytesRef(Object v) {
+                    if (v instanceof BytesRef) {
+                        return (BytesRef) v;
+                    } else {
+                        assert v instanceof String;
+                        return BlockSourceReader.toBytesRef(scratch, (String) v);
+                    }
+                }
+            };
+        }
+    }
+
     /**
      * Load {@link BytesRef} blocks from stored {@link BytesRef}s.
      */