How can you design smart contracts to handle exceptions and non-deterministic events?

- Use Require Statements: Use require to validate conditions. - Implement Assert and Revert: Employ assert for internal checks, revert to cancel on violations. - Explicitly Check Inputs: Verify input validity by checking data types, ranges, and other critical requirements. - Handle Out-of-Gas Scenarios: Manage gas consumption and incorporate fallbacks. - Utilize Circuit Breakers: Suspend contracts during unexpected scenarios. - Log Events for Debugging: Log events for debugging and critical data. - Implement Timeouts: Set timeouts for interactions with external systems or oracles. - Regularly Update Contracts - External Audits - Rigorous testing: including unit, integration, and stress tests.

In the context of EVM + Solidity, exceptions would pretty much fall into three main categories: Validations: Things like function input and return values. You'll check these with a require (or custom revert statement until require supports custom errors). Externals: These occur during calls to external contracts. If required, you can handle such cases with a try-catch statement using a catch clause for: - catching a custom error - catching a string reason - catching returned bytes (catch-almost-all) Internals: Panics and errors from internal execution such as integer overflows outside unchecked blocks. Also using assert to validate fundamental invariants that should never fail falls into this category (in my opinion).

- Diversify Data Sources: Use at least 3 or more reputable oracles. - Cross-Check and Validate: Employ multiple oracles to cross-check and validate data. - Decentralized Oracles: Opt for decentralized oracles to avoid single points of failure. e.g. Tellor, Chainlink, Witnet. - Avoid Direct Use of Spot Prices: 1. Never use a decentralized exchange's spot price directly for price discovery. 2. Implement secure price calculation methods, such as Time-Weighted Average Prices (TWAPs) over longer intervals. - Attack Prevention: Implement commit-reveal schemes to prevent freeloading attacks. - Mirroring Attack Mitigation: Use solutions that consider private data transfers between Sybil nodes to detect mirroring attacks.

A lot of years went by where I used to see (and defaulted to myself) a lot of verbose imperative testing in Solidity. A lot of that was due to lack of tools where developers would have either truffle or hardhat setup where it's tedious to setup complex testing. Nowadays you'll probably want to use forge (foundry) for tests: - Implement and test using the same language. - Better debugging and tracing capability. - Apply test input fuzzing instead of imperatively calling some functions with predefined values. - Still a lot of hassle for complex setups - but at least everything is closer to reality. Other things worth mentioning is to run slither on your codebase and for further fuzzing needs, use echidna.

Here's a comprehensive guide to preparing for a smart contract security. - Audit Scope: 1.Define Scope Clearly 2.Identify Key Areas 3.Communicate Concerns - Time Allocation: 1.Immediate Action 2.Retesting Phase 3.Open Dialogue - Code Freeze: 1.Pause Active Development 2.Fixed Code Revision - Clean Solidity Code: 1.Solidity Styleguide 2.NatSpec and Inline Comments 3.Compiler and Linter Warnings 4.Comprehensive Documentation

1. For Composable Design: - Maintaining Existing Code Base: Identify functionalities. Map logic and data flows. Incrementally detach functionalities. Thoroughly test isolated modules. - Starting From Scratch: Define system functionalities. Break down into independent modules. Encapsulate functionalities. Plan architecture with a module map. 2. For Data Separation Patterns: - Striking the Right Balance: Focus on concise data structures. Strive for domain separation. Avoid pervasive separation. - Considerations for Diamond Pattern: Fragment business logic into facets. Ensure proper initialization. Handle new state variables carefully. Avoid nested structs. Protect the initialize() function.

In the context of EVM, Solidity and Exceptions: Simplest pattern to apply is to first define all rules/invariants that should always hold true in the system according to the execution context. After performing some core logic in external state modifying functions you'll be checking that these invariants hold true and handle the case where they don't - by reverting or something more complex. This enables confidence when adding more functionality or quality of life improvements with things like multicalls in your system. You do not really care what the execution does as long as the invariants hold true.

Define all exceptions within a deterministic state machine using formal specifications, and test your system with a formal verification tool such as Lean 4. For non-deterministic events, you can employ game theory to establish a system of incentives and penalties, or alternatively, entrust trust to an oracle.