This is a partial* revival of #25968
It contains a list of most-unrelated simplifications and optimizations to the code merged in #25717:
- Make ProcessNextHeaders use the headers argument as in/out: This allows reusing the same vector storage for input and output headers to HeadersSync::ProcessNextHeaders. It’s also natural in the sense that this argument just represents the headers-to-be-processed, both in the caller and the callee, and both before and after the calls.
- Avoid an IsAncestorOfBestHeaderOrTip call: Just don’t call this function when it won’t have any effect.
- Compute work from headers without CBlockIndex: Avoid the need to construct a CBlockIndex object just to compute work for a header, when its nBits value suffices for that. Also use some Spans where possible.
- Remove useless CBlock::GetBlockHeader: There is no need for a function to convert a CBlock to a CBlockHeader, as it’s a child class of it.
It also contains the following code cleanups, which were suggested by reviewers in #25968:
- Remove redundant parameter from CheckHeadersPoW: No need to pass consensusParams, as CheckHeadersPow already has access to m_chainparams.GetConsensus()
- Remove unused parameter in ReportHeadersPresync
- Use initializer list in CompressedHeader, also make GetFullHeader const
- Use reference for chain_start in HeadersSyncState: chain_start can never be null, so it’s better to pass it as a reference rather than a raw pointer
*I decided to leave out two commits that were in #25968 (ab52fb4e95aa2732d1a1391331ea01362e035984, 7f1cf440ca1a9c86085716745ca64d3ac26957c0), since they’re a bit more involved, and I’m a new contributor. If this PR gets merged, I’ll comment under #25968 to note that these two commits are still up for grabs :)
