signet: fixing mining for OP_TRUE challenge

[Sjors]

BIP325 mentions the following rule:

In the special case where an empty solution is valid (ie scriptSig and scriptWitness are both empty) this additional commitment can optionally be left out. This special case is to allow non-signet-aware block generation code to be used to test a custom signet chain where the challenge is trivially true.

Such a signet can be created using e.g. -signetchallenge=51 (OP_TRUE). However contrib/signet/miner can't handle this, as it fails with PSBT signing failed.

This PR improves the miner by skipping the PSBT for known trivial scripts (just OP_TRUE for now). This also avoids appending the 4 byte signet header to the witness commitment, as allowed by the above rule.

[DrahtBot]

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #30203 (Enhance signet chain configuration in bitcoin.conf by BrandonOdiwuor)
• #30130 (contrib/signet/miner: increase miner search space by edilmedeiros)
• #29838 (Feature: Use different datadirs for different signets by BrandonOdiwuor)
• #28417 (contrib/signet/miner updates by ajtowns)

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.

[Sjors]

My own goal with this is to test Stratum v2 stuff, e.g. #28983. It's nice to have a test network with difficulty adjustment. Regular testnet difficulty is too high for my humble S9 (let alone a CPU miner) to win blocks.

Using real mining equipment as opposed to a CPU miner is useful, because of the many quirks real devices have (e.g. version bit grinding). Afaik there's no good emulator.

Although I won't actually need this mining script to test local pool software, I patched it anyway to serve as a reference to compare the pool software against.

cc @ajtowns, @kallewoof

[kallewoof]

This looks reasonable to me, although I'm not super familiar with all the code being changed.

[edilmedeiros]

Concept ACK

Code looks good, I'm still to test it.