Github user dsmiley commented on the issue:
https://github.com/apache/lucene-solr/pull/443
Oh right; we'd get an infinite loop as the ChildDocTransformerFactory is
already being called by SolrReturnFields! Ouch!
Sorry but I don't like your solution of trying to parse/strip the child
transformer "fl"; it's very error-prone to do such things.
Lets define a ThreadLocal<Boolean> that allows us to know if we're being
asked to create this transformer from within a recursive call to
SolrReturnFields. When we see this happening, we can return a no-op
transformer (see RawValueTransformerFactory.NoopFieldTransformer which could be
made public and moved to an inner class of DocTransformer). This thread local
would be set & cleared in a try-finally, and this logic would be in some
utility method to keep it from being distracting to the caller. In general I
avoid ThreadLocals as it's a singleton design pattern which is usually bad, but
it can be used sparingly/judiciously.
BTW it appears that doc transformers will not be executed on the child
docs. The transformer is not invoking them, and I suspect they won't be
invoked on the way out to the client. Can you check/test if this is true? If
I'm correct, this is a pre-existing bug/limitation that would be good to mark
as a TODO; or you can try to address here since it's related (since you before
couldn't specify what 'fl' was therefore it wasn't possible to specify a
transformer until now).
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