Invoice Status Transition Enforcement

[Kingsman-99]

Description

The contract lacks explicit validation of which status transitions are permissible, allowing operations to succeed in logically invalid states. A state-machine guard should enforce that transitions only follow the defined flow: Open → PartiallyFunded → Funded → Finalised (with Cancelled and Disputed as side exits).

Technical Context

Add a valid_transition(from: InvoiceStatus, to: InvoiceStatus) -> bool function in contracts/split/src/lib.rs. Call it before every status write using the stored current status. Transition table lives as a match block returning bool. Return InvalidStateTransition { from, to } error (defined in types.rs) on invalid attempts.

Acceptance Criteria

• Transitioning from Finalised to any other state is rejected
• Transitioning from Cancelled to any state is rejected
• Open → Funded without passing through PartiallyFunded is permitted (edge: single contribution)
• InvalidStateTransition error carries both from and to status values
• A test exhaustively checks all invalid transitions in a table-driven format
• All CI checks (cargo build --target wasm32-unknown-unknown, cargo test, cargo clippy) pass and the branch has no merge conflicts

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[Venrable18]

@Venrable18 has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Good day maintainer..

I’m a full-stack and test integration developer…. 

Please assign this task to me…


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