Fix dynamic mapping update generation. (#27467)

When a field is not mapped, Elasticsearch tries to generate a mapping update
from the parsed document. Some documents can introduce corner-cases, for
instance in the event of a multi-valued field whose values would be mapped to
different field types if they were supplied on their own, see for instance:

```
PUT index/doc/1
{
  "foo": ["2017-11-10T02:00:01.247Z","bar"]
}
```

In that case, dynamic mappings want to map the first value as a `date` field
and the second one as a `text` field. This currently throws an exception,
which is expected, but the wrong one since it throws a `class_cast_exception`
(which triggers a HTTP 5xx code) when it should throw an
`illegal_argument_exception` (HTTP 4xx).

Author: jpountz

core/src/main/java/org/elasticsearch/index/mapper/DateFieldMapper.java

@@ -488,8 +488,8 @@ protected void parseCreateField(ParseContext context, List<IndexableField> field
 
     @Override
     protected void doMerge(Mapper mergeWith, boolean updateAllTypes) {
-        final DateFieldMapper other = (DateFieldMapper) mergeWith;
         super.doMerge(mergeWith, updateAllTypes);
+        final DateFieldMapper other = (DateFieldMapper) mergeWith;
         this.includeInAll = other.includeInAll;
         if (other.ignoreMalformed.explicit()) {
             this.ignoreMalformed = other.ignoreMalformed;

core/src/test/java/org/elasticsearch/index/mapper/DateFieldMapperTests.java

@@ -410,4 +410,20 @@ public void testMergeDate() throws IOException {
                 MapperService.MergeReason.MAPPING_UPDATE, randomBoolean()));
         assertThat(e.getMessage(), containsString("[mapper [release_date] has different [format] values]"));
     }
+
+    public void testMergeText() throws Exception {
+        String mapping = XContentFactory.jsonBuilder().startObject().startObject("doc")
+                .startObject("properties").startObject("date").field("type", "date").endObject()
+                .endObject().endObject().endObject().string();
+        DocumentMapper mapper = indexService.mapperService().parse("doc", new CompressedXContent(mapping), false);
+
+        String mappingUpdate = XContentFactory.jsonBuilder().startObject().startObject("doc")
+                .startObject("properties").startObject("date").field("type", "text").endObject()
+                .endObject().endObject().endObject().string();
+        DocumentMapper update = indexService.mapperService().parse("doc", new CompressedXContent(mappingUpdate), false);
+
+        IllegalArgumentException e = expectThrows(IllegalArgumentException.class,
+                () -> mapper.merge(update.mapping(), randomBoolean()));
+        assertEquals("mapper [date] of different type, current_type [date], merged_type [text]", e.getMessage());
+    }
 }