Implement a Flash Loan module with strict post-execution balance invariant assertions

[Damola-Sodiq]

Description

Develop a feature to allow intra-transaction, zero-collateral borrowing of the vault's fractional liquidity, secured entirely by deterministic mathematical proofs.

Architecture & Context

To facilitate efficient market arbitrage across fractional token pools, traders need access to flash liquidity. The contract will release the requested funds, execute the borrower's custom callback contract, and strictly verify that balance_after >= balance_before + fee before resolving.

Technical Requirements

• Utilize env.invoke_contract to execute the untrusted external callback securely.
• Cache the pre-execution vault balances in protected memory variables.
• Force an uncatchable panic if the post-execution balance delta is negative.

Acceptance Criteria

• Flash loans execute successfully for compliant arbitrage contracts.
• Non-compliant or malicious callbacks result in complete state reversion.
• Flash loan fees are securely credited to the fractional asset holders.

[DevSolex]

Hi Maintainer,

I’m a smart contract and full-stack developer and would love to contribute to this issue. I’m an experienced contributor with hands-on expertise in Rust, Soroban, Cairo, and TypeScript.

I’m passionate about building secure, scalable blockchain solutions, with a strong emphasis on writing clean and maintainable code. Based on the scope of this issue, I’m confident I can deliver a complete solution within 24 hours of assignment.

I’d greatly appreciate the opportunity to contribute and help move this forward.

[grantfox-oss]

🦊 GrantFox — @DevSolex has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #480)
2. Your PR will be reviewed by the Sahara-Pay maintainers

Good luck! Track your progress on GrantFox.