Disabling fingerprinting

[Erick Erickson <er...@gmail.com>]

re: SOLR-8690. I get that this is a super-expert option, and I get
that it's a safety valve for unexpected behavior, mostly having to do
with expensive fingerprint calculations.

My question is "What are the dangers of disabling fingerprinting"?

I have two use cases:
- I'm indexing and committing while restarting a replica.
- I'm issuing delete-by-id and/or DBQ (perhaps while restarting a node).

The situation I'm seeing is that indexing is occurring while a node is
restarted. There should be _plenty_ of updates in the tlogs to do a
peer sync, but it's falling back to full sync due to fingerprint
mismatch. SOLR-11216 should address this, but won't be out for a
while.

What I'm wondering is if, as a stop-gap, turning off fingerprinting is
an option. Fingerprinting wasn't added just because someone was bored,
but I don't fully understand the situation that can lead to
fingerprinting catching a problem that the rest of peer sync doesn't.

If it's an edge case we can guarantee won't be exercised (e.g. DBQ)
then we can be more confident about turning it off.

Thanks!
Erick

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