Return the _source of inner hit nested as is without wrapping it into its full path context

[martijnvg]

Due to a change happened via #26102 to make the nested source consistent
with or without source filtering, the _source of a nested inner hit was
always wrapped in the parent path. This turned out to be not ideal for
users relying on the nested source, as it would require additional parsing
on the client side. This change fixes this, the _source of nested inner hits
is now no longer wrapped by parent json objects, irregardless of whether
the _source is included as is or source filtering is used.

Internally source filtering and highlighting relies on the fact that the
_source of nested inner hits are accessible by its full field path, so
in order to now break this, the conversion of the _source into its binary
form is performed in FetchSourceSubPhase, after any potential source filtering
is performed to make sure the structure of _source of the nested inner hit
is consistent irregardless if source filtering is performed.

PR for #26944

[colings86]

I left a few comments

[colings86]

Do we need to check source.internalSourceRef() == null before returning to ensure that for non-root hits its actually been set by the end of this method?

[martijnvg]

Yes, I think so. In case of regular hits the source has already been set in the FetchPhase and I guess this check exists to ensure that the binary representation of the _source has really been set.

[colings86]

source.internalSourceRef() could be null here now?

[martijnvg]

It is null in the case rootHit is false, which can't be the case here.

[tlrx]

I'd call this nestedHit and make it final

[tlrx]

It looks to me like it duplicates the logic of creating a XContentBuilder in a given type and then write the filtered source as map. Could it be something like this?

...
Object value = source.filter(fetchSourceContext);
try {
    if (nestedHit) {
        value = getNestedSource((Map<String, Object>) value, hitContext);
    }
    final int initialCapacity = Math.min(1024, source.internalSourceRef().length()); // deal with null here
    try (BytesStreamOutput streamOutput = new BytesStreamOutput(initialCapacity)) {
        XContentBuilder builder = new XContentBuilder(source.sourceContentType().xContent(), streamOutput);
        builder.value(value);
        hitContext.hit().sourceRef(builder.bytes());
    }
...

[tlrx]

LGTM, thanks for the changes