fix: use correct hash of quorum as cycle hash for regtest

[knst]

Issue being fixed or feature implemented

feature_llmq_singlenode.py works unstable, roughly 10% failure rate. It fails with this error:

2025-05-17T15:03:08.586000Z TestFramework (INFO): InstantSend lock on tx: 202b6698cf0da8adfeff8430daff3d1cfe767d1120751bb7765f6df15d09a3ee is expecting
2025-05-17T15:04:09.033000Z TestFramework.utils (ERROR): wait_until() failed. Predicate: ''''
        def check_instantlock():
            self.bump_mocktime(1)
            try:
                return node.getrawtransaction(txid, True)["instantlock"]
            except:
                return False
'''
2025-05-17T15:04:09.033000Z TestFramework (ERROR): Assertion failed

Deep in logs the failure look like this:

 node0 2025-05-17T14:40:45.628148Z (mocktime: 2014-12-12T05:56:24Z) [     isman] [llmq/instantsend.cpp:985] [ProcessPendingInstantSendLocks] [instantsend] CInstantSendManager::ProcessPendingInstantSendLocks -- txid=202b6698cf0da8adfeff8430daff3d1cfe767d1120751bb7765f6df15d09a3ee, islock=89b6a8bb50c2a6b07ebd2bbde0727a72ed4fbe4f78247aab509189b50fc7293f: invalid sig in islock, peer=1

Further debug shown that somehow sign-hash sometimes is different; while msgHash is same; signature is valid and bytes-per-bytes matched; just validation code does not work as expected.

What was done?

For non-rotating quorums on RegTest should be used directly quorum hash instead cycleQuorum; no quorum choosing procedure is needed.

How Has This Been Tested?

Run multiple times functional tests.

Breaking Changes

N/A; affect only RegTests for special case of non-rotating quorum used for InstantSend.

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have made corresponding changes to the documentation
• I have assigned this pull request to a milestone

[coderabbitai]

Walkthrough

The changes update the logic in the InstantSend module to handle quorum cycle hash computation and quorum selection differently based on whether quorum rotation is enabled. When rotation is active, the cycle hash is derived from the ancestor block at the calculated cycle height; otherwise, it uses the quorum base block index. Similarly, quorum selection now conditionally uses either the signing quorum (with rotation) or directly retrieves the quorum by type and cycle hash (without rotation). No changes were made to function signatures or public API declarations. No other logic or control flow was altered.

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Reviewing files that changed from the base of the PR and between 9cb43c2 and 4b66448.

📒 Files selected for processing (1)

• src/llmq/instantsend.cpp (2 hunks)

⏰ Context from checks skipped due to timeout of 90000ms (3)

• GitHub Check: x86_64-apple-darwin / Build depends
• GitHub Check: x86_64-w64-mingw32 / Build depends
• GitHub Check: x86_64-pc-linux-gnu_nowallet / Build depends

🔇 Additional comments (2)

src/llmq/instantsend.cpp (2)

693-695: Appropriate fix for non-rotating quorums in RegTest mode.

This change correctly addresses the cycleHash calculation for non-rotating quorums. The conditional logic ensures that:

• For rotating quorums (llmq_params_opt->useRotation == true): The code continues to use the ancestor block at the calculated cycle height
• For non-rotating quorums (RegTest): It directly uses the quorum base block index

This fixes the instability issue in feature_llmq_singlenode.py test by ensuring the correct hash is used as cycle hash when validating InstantSend locks.

---

939-941: Consistent handling of quorum selection for non-rotating quorums.

This change maintains consistency with the cycleHash calculation fix:

• For rotating quorums: Continue using SelectQuorumForSigning which factors in signing height and offset
• For non-rotating quorums: Directly get the quorum by type and cycle hash

This eliminates the unnecessary quorum choosing procedure for RegTest non-rotating quorums, ensuring the signing process uses the correct quorum hash during verification.

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• 📝 Generate Docstrings

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