test: improve CoinJoin unit coverage

[thepastaclaw]

test: improve CoinJoin unit coverage

Issue being fixed or feature implemented

Improves Dash-specific CoinJoin unit test coverage with focused behavioral tests
for existing mixing support code. This follows up on the coverage review that
identified src/coinjoin as one of the weakest Dash-specific areas.

Tracker: https://github.com/thepastaclaw/tracker/issues/1592

What was done?

Added unit coverage for behavior used by CoinJoin server/client flows:

• CCoinJoinEntry::AddScriptSig now covers matching by prevout and sequence,
  duplicate rejection without overwrite, wrong prevout/wrong sequence rejection,
  and multi-input isolation.
• CoinJoinQueueManager now covers masternode queue lookup semantics, including
  readiness-aware lookups and duplicate detection for same masternode plus same
  readiness.
• CDSTXManager now covers clearing confirmed height when a tracked DSTX
  re-enters the mempool and ignoring mempool events for unrelated transactions.

This deliberately avoids superficial enum/string/status tests and does not add
production hooks or fake network harnesses.

Coverage impact

Historical baseline from the published Dash Core coverage snapshot for
develop at a59ad9e8b:
https://thepastaclaw.github.io/dash-coverage/2026-06-25-develop-a59ad9e8b/.

Old CoinJoin coverage from that snapshot:

• Unit: src/coinjoin was 464 / 2524 lines (18.4%) and 176 / 453 functions
  (38.9%).
• Unit + functional: src/coinjoin was 695 / 2524 lines (27.5%) and 237 /
  453 functions (52.3%).

Measured targeted lcov comparison for this PR:

• Base: 8c9f166a3da637ed4eb667731f5017a2cc8c33a6.
• Head: 873b14e9558ac8d9c11d031504083853b5d30678.
• Build: ./configure --enable-lcov --without-gui --disable-bench --with-daemon --with-cli --disable-stacktraces --without-libs.
• Test filter:
  ./src/test/test_dash --run_test=coinjoin_inouts_tests,coinjoin_queue_tests,coinjoin_dstxmanager_tests.
• Result: base ran 13 test cases with no errors; head ran 18 test cases with no
  errors.

Raw lcov totals from those targeted runs:

• src/coinjoin/coinjoin.cpp: 105 / 302 lines (34.8%) and 23 / 41 functions
  (56.1%) on base; 133 / 302 lines (44.0%) and 28 / 41 functions (68.3%) on
  head.
• Aggregate src/coinjoin: 214 / 2911 lines (7.4%) and 88 / 673 functions
  (13.1%) on base; 261 / 2930 lines (8.9%) and 105 / 685 functions (15.3%) on
  head.

The aggregate denominator differs from the published full-suite snapshot because
this measurement is a local targeted lcov run as emitted by llvm-cov gcov,
including the header records present in that trace. The coinjoin.cpp numbers
are the directly comparable source-file movement from the PR's focused tests.

How Has This Been Tested?

Ran locally on macOS arm64 after building src/test/test_dash in an isolated
worktree:

./src/test/test_dash --run_test=coinjoin_inouts_tests,coinjoin_queue_tests,coinjoin_dstxmanager_tests

Result: 18 test cases, no errors detected.

Pre-PR review gate:

code-review dashpay/dash upstream/develop test/coinjoin-unit-coverage

Result: ship.

Breaking Changes

None. Test-only change.

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 (for repository
  code-owners and collaborators only)

[thepastaclaw]

@coderabbitai review

[coderabbitai]

✅ Action performed

Review finished.

Note: CodeRabbit is an incremental review system and does not re-review already reviewed commits. This command is applicable only when automatic reviews are paused.

[github-actions]

✅ No Merge Conflicts Detected

This PR currently has no conflicts with other open PRs.

[github-actions]

This pull request has conflicts, please rebase.

[coderabbitai]

Walkthrough

The PR adds unit tests for CoinJoin-related behavior. It covers DSTX entries when transactions re-enter the mempool after confirmation, AddScriptSig matching and rejection rules for CoinJoin inputs, and CoinJoinQueueManager lookup and duplicate-detection behavior based on masternode identity, readiness, and queue timing.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~15 minutes

Suggested reviewers

• PastaPastaPasta
• UdjinM6

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% 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 concisely summarizes the main change: improved CoinJoin unit test coverage.
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.
Description check	✅ Passed	The pull request description clearly matches the test-only CoinJoin coverage changes in the diff.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[thepastaclaw]

CI note: all real Dash build/test jobs are green on 873b14e9558ac8d9c11d031504083853b5d30678. The only red check is check_merge; its log shows this PR fast-forwards cleanly onto develop, then fails at the existing develop -> master fast-forward step. predict_conflicts is green, and CodeRabbit had no actionable comments.

[thepastaclaw]

✅ Review complete (commit 873b14e)

[thepastaclaw]

Code Review

Test-only PR adding focused unit coverage for CCoinJoinEntry::AddScriptSig, CoinJoinQueueManager queue lookup/duplicate semantics, and CDSTXManager mempool re-entry. All assertions verified against the implementations in src/coinjoin/coinjoin.cpp. Both reviewers converged on no findings; spot-checks of AddScriptSig, TryHasQueueFromMasternode, TryCheckDuplicate, TransactionAddedToMempool, and BlockConnected confirm the tests assert real behavior. Clean single conventional commit.

Note: GitHub does not allow me to approve my own PR, so this clean review is posted as COMMENT.