adding non existent field verification to copy to

Author: henriquepaes1

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

@@ -341,6 +341,9 @@ public final void validate(MappingLookup mappers) {
                 if (mappers.isObjectField(copyTo)) {
                     throw new IllegalArgumentException("Cannot copy to field [" + copyTo + "] since it is mapped as an object");
                 }
+                if (mappers.getMapper(copyTo) == null){
+                    throw new IllegalArgumentException("Cannot copy to field [" + copyTo + "] since it is a non existent field");
+                }
 
                 final String targetScope = mappers.nestedLookup().getNestedParent(copyTo);
                 checkNestedScopeCompatibility(sourceScope, targetScope);

server/src/test/java/org/elasticsearch/index/mapper/CopyToMapperTests.java

@@ -645,6 +645,18 @@ public void testCopyFromMultiField() {
         assertThat(e.getMessage(), Matchers.containsString("[copy_to] may not be used to copy from a multi-field: [field.bar]"));
     }
 
+    public void testCopyToNonExistentField() {
+        Exception e = expectThrows(IllegalArgumentException.class, () -> createDocumentMapper(fieldMapping(b -> {
+            b.startObject("test_field");
+            {
+                b.field("type", "text");
+                b.array("copy_to", "missing_field");
+            }
+            b.endObject();
+        })));
+        assertThat(e.getMessage(), Matchers.containsString("Cannot copy to field [missing_field] since it is a non existent field"));
+    }
+
     public void testCopyToDateRangeFailure() throws Exception {
         DocumentMapper docMapper = createDocumentMapper(topMapping(b -> {
             b.startObject("properties");