refactor: Avoid UniValue copies

[maflcko]

UniValue copies have been a source of performance related bugs and regressions, so it would be good to remove the copy constructors.

This is the first change toward the goal. This change uses a const UniValue& where possible.

See also:

• various performance improvements bitcoin-core/univalue-subtree#27
• refactor: Avoid UniValue copy constructor #15974

[maflcko]

Review note: const will compile, but kill performance when std::move or RVO is intended.

[theStack]

Concept ACK

[fanquake]

cc @martinus

[DrahtBot]

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Reviews

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Type	Reviewers
Concept ACK	theStack, hebasto
Stale ACK	martinus, fanquake

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #26988 ([rpc]: Add test-only RPC addrmaninfo for new/tried table address count by stratospher)
• #26840 (refactor: importpubkey, importprivkey, importaddress, importmulti, and importdescriptors rpc by KolbyML)
• #26780 (rpc: simplify scan blocks by furszy)
• #26621 (refactor: Continue moving application data from CNode to Peer by dergoegge)
• #26612 (refactor: RPC: pass named argument value as string_view by stickies-v)
• #26426 (WIP: Fix coinstatsindex overflow issue by fjahr)
• #25974 (test, build: Separate read_json function into its own module by hebasto)
• #25390 (sync: introduce a thread-safe generic container and use it to remove a bunch of "GlobalMutex"es by vasild)
• #25344 (New outputs argument for bumpfee/psbtbumpfee by rodentrabies)
• #15294 (refactor: Extract RipeMd160 by Empact)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.

[martinus]

Nice! concept ACK. How about removing the copy constructor and copy assignment from the UniValue class, as done here? Fixing all copies would make the PR much bigger though.

[maflcko]

I am actually not sure how to fix all copies. Sometimes the std lib will copy if asked to:

std::vector<UniValue> a;
std::vector<UniValue> b;
a.insert(a.end(), b.begin(), b.end());

[laanwj]

I guess you could add an explicit 'clone' method (like rust) and use that when it's really necessary to make a copy. It will probably make some things much more verbose though.

[martinus]

a.insert(a.end(), b.begin(), b.end());

In that case you can use

a.insert(a.end(), std::make_move_iterator(b.begin()), std::make_move_iterator(b.end()));

But I there might still be case where copy constructor is necessary, I remember that I had difficulties in my old PR as well

[fanquake]

Concept ACK

[fanquake]

ACK bbbbcfd - as a first step.

[maflcko]

While the changes are correct, I'll try to make them easier to review and more complete. For now, please review actual UniValue-copy bugfixes, like #25464

[hebasto]

Concept ACK.

[DrahtBot]

🐙 This pull request conflicts with the target branch and needs rebase.