Filebeat copy_fields processor can recurse, leading to crash

[fearful-symmetry]

It's possible for the copy_fields processor to fall into infinite recursion if we point at two fields with the same root:

                    - copy_fields:
                        fields:
                          - from: message
                            to: message.original

The from and to fields will point to the same root, leading an eventual crash. It appears copy_fields doesn't do an actual copy and just moves references. @urso discovered the root cause here.

[elasticmachine]

Pinging @elastic/integrations (Team:Integrations)

[urso]

copy source: https://github.com/elastic/beats/blob/master/libbeat/processors/actions/copy_fields.go#L99

slices and maps are stored by reference. The code does not handle these types correctly, which leads to a child-document pointing to it's root. Full copy semantics are required.

Another bug we see due to these reference semantics copy_fields => delete field in the original document will also remove the deleted field from the copied sub-document. The copy_fields processor should provide copy semantics always.