test Flow.flow nonReentrant guard via malicious token callback

[thedavidmeister]

Summary

Pins Flow.flow's nonReentrant guard. New test deploys a MaliciousReenteringToken whose transferFrom re-enters flow.flow(...) on the same flow contract and asserts the inner call reverts with the OZ ReentrancyGuardUpgradeable v4 string "ReentrancyGuard: reentrant call".

Mutation verified: dropping nonReentrant from Flow.flow makes the inner call succeed and the outer test fails (reverting with UnsupportedERC20Flow from a later branch instead of the reentrancy revert); reverting passes.

The malicious token contract lives in test/concrete/ per the project rule that test helper contracts are not inlined in test files.

This PR also resolves the empirical observation in #330 (Pass 5 mutation showed nonReentrant removal was uncaught by any test) by adding the test that catches it.

Closes #324 #330.

Test plan

• testFlowReentrancyGuardFiresOnTokenCallback — 100 fuzz runs
• mutation: drop nonReentrant from Flow.flow → test fails

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: 4f850acd-243e-4de7-9319-cf0597e99f23

📥 Commits

Reviewing files that changed from the base of the PR and between ceaea3c and 8a632c0.

📒 Files selected for processing (2)

• test/concrete/MaliciousReenteringToken.sol
• test/src/concrete/Flow.transfer.t.sol

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch 2026-05-05-issue-324-330-reentrancy-guard-test

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