Add concurrency interleaving tests for cancel_bid versus accept_bid_and_fund on the same invoice

[Baskarayelu]

📋 Description

BidStorage::cancel_bid in quicklendx-contracts/src/bid.rs transitions a bid Placed -> Cancelled, while accept_bid_and_fund in escrow.rs transitions a winning bid Placed -> Accepted and funds the invoice. The repo already added a concurrent-acceptance race regression for two acceptances, but there is no test for the cancel-versus-accept interleaving: what happens when an investor cancels the exact bid an operator is accepting in the same logical window.

Why this matters: a bid that is both cancelled and accepted would corrupt invoice/escrow/investment state — funding a cancelled bid or cancelling an already-funding bid. The status-machine guard must reject the second transition deterministically. This interleaving is a distinct race from double-acceptance and deserves its own regression.

🎯 Requirements & Context

Functional requirements

• Add quicklendx-contracts/src/test_bid_cancel_accept_race.rs modeling: (a) cancel succeeds then accept of the same bid must fail; (b) accept succeeds then cancel of the same (now Accepted) bid must fail.
• Assert the losing operation fails with the appropriate QuickLendXError (InvalidStatus / OperationNotAllowed) and leaves no partial state.
• After each interleaving, assert cross-module consistency: invoice status, escrow existence, and investment status agree (reuse assertions from test_cross_module_consistency.rs).
• Assert that a cancelled bid can never later be selected by get_best_bid or funded.

Context & constraints

• Soroban tests are single-threaded; model the race as ordered sequential invocations that simulate the interleaving, consistent with test_accept_bid_race.rs.
• Test-only; document any genuine guard gap rather than patching silently.
• Soroban SDK 25.1.1.

🛠️ Suggested Execution

1. Fork & branch

git checkout -b test/bid-cancel-accept-race

2. Implement changes

• Build the two ordered interleavings and the cross-module consistency assertions.
• Doc-comment the threat model for each ordering.

3. Test & commit

cargo test -p quicklendx-contracts test_bid_cancel_accept_race -- --nocapture

• Edge cases: cancel by non-owner rejected first; accept of an already-expired bid; cancel after the bid expired but before cleanup.

Example commit message

test(bidding): add cancel-vs-accept interleaving race regression

Verifies the bid status machine rejects the second transition and that
invoice/escrow/investment state stays consistent under both orderings.

✅ Acceptance Criteria & Guidelines

Requirement	Target
Both interleavings covered with correct rejection	Required
Cross-module consistency asserted	Required
Cancelled bid never fundable	Required
Coverage of cancel/accept branches	≥ 95%
Docs + doc comments	Required
cargo test + cargo clippy clean	Required
Timeframe	96 hours from assignment

💬 Community & Support

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[Kuhai9801]

I’d like to work on this.

I have experience with smart-contract security testing and bug bounty-style invariant/regression work. I’ll add focused coverage for the cancel_bid vs accept_bid_and_fund interleaving on the same invoice, checking both transition orders and asserting the second transition is rejected without corrupting bid, invoice, escrow, or investment state.

[grantfox-oss]

🦊 GrantFox — @Kuhai9801 has been assigned to this issue!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #1302)
2. Your PR will be reviewed by the QuickLendX maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox!

@Kuhai9801's PR #1380 was approved and merged by @Baskarayelu.

🏆 @Kuhai9801: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (258 total points). Track your full progress on GrantFox.

👏 Great work, @Kuhai9801! Keep contributing to QuickLendX.