Many duplicate dynamic mappers are created when indexing arrays of unknown fields. #117593
Description
In org.elasticsearch.index.mapper.DocumentParserContext#addDynamicMapper we have a TODO that asks to optimize the way we dynamically map object arrays:
// TODO we may want to stop adding object mappers to the dynamic mappers list: most times they will be mapped when parsing their
// sub-fields (see ObjectMapper.Builder#addDynamic), which causes extra work as the two variants of the same object field
// will be merged together when creating the final dynamic update. The only cases where object fields need extra treatment are
// dynamically mapped objects when the incoming document defines no sub-fields in them:
// 1) by default, they would be empty containers in the mappings, is it then important to map them?
// 2) they can be the result of applying a dynamic template which may define sub-fields or set dynamic, enabled or subobjects.
dynamicMappers.computeIfAbsent(mapper.fullPath(), k -> new ArrayList<>()).add(mapper);
We should address this TODO. The current behavior of potentially creating a bunch of duplicate mapper instances causes a massive spike in heap use for duplicate mapper instances if the first document containing a new (sub-)field via an array of objects is mapped.