keep _parent field while updating child type mapping

[PnPie]

Be able to update child type mapping without specifying it's _parent field, because it should be considered as unchanged if it is not explicitly specified.
In this case, the merged mapper's parentType field should be null, so we can just merge it to keep the original one instead of throwing an exception of "trying to change it to 'null' "

Close #23381

[elasticmachine]

Since this is a community submitted pull request, a Jenkins build has not been kicked off automatically. Can an Elastic organization member please verify the contents of this patch and then kick off a build manually?

[clintongormley]

@colings86 please could somebody review this

[martijnvg]

Thanks @PnPie! This looks good. I left two comments regarding the test.

[martijnvg]

Replace this try-catch block with an expectThrows(...) call?

Exception e = expectThrows(IllegalArgumentException.class, () ->  initMapper.merge(modParentMapper.mapping(), false))
assertThat(e.getMessage(), containsString("The _parent field's type option can't be changed:"));

[martijnvg]

Maybe also add a merge with current parent field and a new field?

String updatedMapping = XContentFactory.jsonBuilder().startObject().startObject("child")
   .startObject("_parent").field("type", "parent").endObject()
   .startObject("properties")
   .startObject("field2").field("type", "text").endObject()
   .endObject().endObject().endObject().string();
 DocumentMapper updatedMapper = parser.parse("child", new CompressedXContent(updatedMapping));
 DocumentMapper mergedMapper = initMapper.merge(updatedMapper.mapping(), false);

[PnPie]

Thanks for your comments @martijnvg, I added a merge case with current parent field while I found a new problem: I cannot find the _parent field with this assertion didn't pass
assertThat(mergedMapper2.mappers().getMapper("_parent"), notNullValue());

And then I found that the parent field name of a document is in format: _parent#[parent type], so in the test it should be _parent#parent, if a new mapper without parent field, it's name is just _parent, and when we do the merge we firstly call the super.doMerge https://github.com/PnPie/elasticsearch/blob/346889a93bb18cda93736a8076aef198b8f86fe1/core/src/main/java/org/elasticsearch/index/mapper/ParentFieldMapper.java#L298 method of ParentFieldMapper and it always replaces the fieldType with the new one's, so in consequence the parent field name is changed to the new one's.

So previously after this we can find the _parent field with
assertThat(mergedMapper.mappers().getMapper("_parent"), notNullValue());
but in fact the parent field shouldn't be changed and it should be _parent#parent, so I modified in ParentFieldMapper's doMerge method and make it keep the original parent field name.

And I also add a test for trying to add parent field to an exiting mapper without parent.

[martijnvg]

Cool, looks good @PnPie. I'll merge it in soon and backport to 5.x branch.