fix: validate CbTx chainlock height diff

[thepastaclaw]

Issue being fixed or feature implemented

CheckCbTxBestChainlock derived an ancestor height from bestCLHeightDiff
without first ensuring the derived height was in range. This PR adds an
explicit consensus rejection for out-of-range CbTx chainlock height diffs before
ancestor lookup.

What was done?

• Validate bestCLHeightDiff before deriving and using the referenced chainlock
  ancestor height.
• Return bad-cbtx-cldiff for out-of-range values.
• Add a defensive null check around the ancestor lookup.
• Add focused unit coverage for boundary values in a new evo_cbtx_tests suite.

How Has This Been Tested?

Tested locally on macOS arm64:

cd src && make -j6 test/test_dash
./test/test_dash --run_test=evo_cbtx_tests
./test/test_dash '--run_test=evo_*'

Results:

make -j6 test/test_dash: pass
./test/test_dash --run_test=evo_cbtx_tests: 1/1 pass
./test/test_dash '--run_test=evo_*': 38/38 pass

Pre-PR review gate:

code-review dashpay/dash upstream/develop review/fix-cbtx-chainlock-height-bound "Pre-PR review: validate CbTx bestCLHeightDiff range before ancestor lookup and add focused regression tests"

Result: ship.

Breaking Changes

None expected. This only rejects malformed CbTx chainlock metadata that
references an invalid ancestor height.

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

[thepastaclaw]

@coderabbitai review

[coderabbitai]

✅ Action performed

Review finished.

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[github-actions]

⚠️ Potential Merge Conflicts Detected

This PR has potential conflicts with the following open PRs:

• feat: add masternode reward shares #7340 - feat: add masternode reward shares

Please coordinate with the authors of these PRs to avoid merge conflicts.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: 32f988ad-9a7b-49e9-8907-18f2bfd0f6a4

📥 Commits

Reviewing files that changed from the base of the PR and between 7312fff and dc82f29.

📒 Files selected for processing (2)

• src/evo/specialtxman.cpp
• src/test/evo_cbtx_tests.cpp

🚧 Files skipped from review as they are similar to previous changes (2)

• src/evo/specialtxman.cpp
• src/test/evo_cbtx_tests.cpp

---

Walkthrough

This PR introduces stricter validation for the bestCLHeightDiff field in Dash coinbase transactions. The changes export a validation function that checks both the height difference bounds (rejecting values that would imply pre-genesis ancestors) and safely handles missing block indices during ancestor lookups. A unit test verifies rejection at boundary conditions, and the test is integrated into the build system.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

Suggested reviewers

• UdjinM6
• PastaPastaPasta

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 33.33% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically describes the main change: validating CbTx chainlock height diff, which is the core issue being fixed across all modified files.
Description check	✅ Passed	The description is directly related to the changeset, explaining the validation gap being fixed, what was done, testing results, and impact, with clear alignment to the code changes.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

---

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[thepastaclaw]

CI note: the only red item is predict_conflicts; the full build/test matrix is green. The predictor is flagging overlapping test/build files and Evo specialtx files against other open PRs (including #7340 and nearby queued work). I’m leaving this as a merge-sequencing/conflict-prediction blocker rather than an application/test failure for now.

[thepastaclaw]

✅ Review complete (commit dc82f29)

[thepastaclaw]

Code Review

Small, well-scoped defensive fix that closes a remote crash vector in CheckCbTxBestChainlock by rejecting out-of-range bestCLHeightDiff before calling GetAncestor(), plus a defensive null check on the returned ancestor. The boundary math is correct and the change is strictly more-rejecting, so it cannot reject any historically valid block. Two minor non-blocking observations about test coverage and reject-reason granularity.

🟡 1 suggestion(s) | 💬 1 nitpick(s)

🤖 Prompt for all review comments with AI agents

These findings are from an automated code review. Verify each finding against the current code and only fix it if needed.

In `src/test/evo_cbtx_tests.cpp`:
- [SUGGESTION] src/test/evo_cbtx_tests.cpp:56-62: Defensive GetAncestor() null-check branch is not exercised
  Both test cases set bestCLHeightDiff to nHeight (5) and uint32_max, so both trip the range check at specialtxman.cpp:77 and return before the new `if (pAncestor == nullptr)` branch at specialtxman.cpp:89 is reached. The commit message highlights the null check as an intentional defense, but from the unit test's perspective that branch is currently unexercised. A case with `bestCLHeightDiff = nHeight - 1` on a standalone CBlockIndex (pprev == nullptr) would pass the range check, compute curBlockCoinbaseCLHeight = 0, and drive GetAncestor() down a chain it cannot walk, covering the defensive path.