Xian Smart Contract Development - Cursor Rules

# Xian Smart Contract Development - Cursor Rules

XIAN is the currency of the Xian blockchain.
Never mention TAU or Lamden.

## Contract Structure

### Basic Structure
- Smart contracts are written in native Python without transpilation
- Contract names must follow the pattern: `^con_[a-z][a-z0-9_]*$`
- Contract names must start with 'con_' prefix (except system contracts like 'currency')
- Contract names must be lowercase, only contain letters, numbers and underscores after prefix
- Contract names must be max 64 characters

### Naming Conventions
- You cannot use '_' as a prefix for variables or functions (e.g., `_private_var` is not allowed)
- Follow standard Python naming conventions otherwise
- Use descriptive names for clarity
- A contract can not be deployed by another contract

### Function Types
- `@export` decorator defines public functions callable by any user or contract
- `@construct` decorator defines initialization function executed once at contract submission (optional)
- Functions without decorators are private and can only be called by the contract itself
- Functions with `@export` can call private functions internally

### Constructor Arguments
- Optional arguments can be provided to the `@construct` function
- Initial state can be setup using these arguments

## State Management

### Variable
- `Variable` is a way to define a singular state variable in the contract
- Use `variable.set(value)` to modify
- Use `variable.get()` to retrieve

```python
my_var = Variable()

@construct
def seed():
    my_var.set(0)  # Initialize variable

@export
def increment():
    my_var.set(my_var.get() + 1)
```

### Hash
- `Hash` is a key-value store for the contract
- Default value can be specified with `Hash(default_value=0)`
- Access through dictionary-like syntax: `hash[key] = value` and `hash[key]`
- Supports nested keys with tuple: `hash[key1, key2] = value`

```python
my_hash = Hash(default_value=0)

@export
def set_value(key: str, value: int):
    my_hash[key] = value

@export
def get_value(key: str):
    return my_hash[key]
```

#### Illegal Delimiters 
":" and "." cannot be used in Variable or Hash keys.

### Foreign State Access
- `ForeignHash` provides read-only access to a Hash from another contract
- `ForeignVariable` provides read-only access to a Variable from another contract

```python
token_balances = ForeignHash(foreign_contract='con_my_token', foreign_name='balances')
foundation_owner = ForeignVariable(foreign_contract='foundation', foreign_name='owner')
```

## Context Variables

### ctx.caller
- The identity of the person or contract calling the function
- Changes when a contract calls another contract's function
- Used for permission checks in token contracts

### ctx.signer
- The top-level user who signed the transaction
- Remains constant throughout transaction execution
- Only used for security guards/blacklisting, not for account authorization

### ctx.this
- The identity/name of the current contract
- Never changes
- Useful when the contract needs to refer to itself

### ctx.owner
- Owner of the contract, optional field set at time of submission
- Only the owner can call exported functions if set
- Can be changed with `ctx.owner = new_owner`

### ctx.entry
- Returns tuple of (contract_name, function_name) of the original entry point
- Helps identify what contract and function initiated the call chain

## Built-in Variables

### Time and Blockchain Information
- `now` - Returns the current datetime
- `block_num` - Returns the current block number, useful for block-dependent logic
- `block_hash` - Returns the current block hash, can be used as a source of randomness

Example usage:
```python
@construct
def seed():
    submission_time = Variable()
    submission_block_num = Variable()
    submission_block_hash = Variable()
    
    # Store blockchain state at contract creation
    submission_time.set(now)
    submission_block_num.set(block_num)
    submission_block_hash.set(block_hash)
```

## Imports and Contract Interaction

### Importing Contracts
- Use `importlib.import_module(contract_name)` for dynamic contract imports
- Static contract imports can be done with `import <contract_name>`
- Only use 'import' syntax for contracts, not for libraries or Python modules
- Trying to import standard libraries will not work within a contract (they're automatically available)
- Dynamic imports are preferred when the contract name is determined at runtime
- Can enforce interface with `importlib.enforce_interface()`
- NEVER import anything other than a contract.
- ALL contracting libraries are available globally
- NEVER IMPORT importlib. It is already available globally.

```python
@export
def interact_with_token(token_contract: str, recipient: str, amount: float):
    token = importlib.import_module(token_contract)
    
    # Define expected interface
    interface = [
        importlib.Func('transfer', args=('amount', 'to')),
        importlib.Var('balances', Hash)
    ]
    
    # Enforce interface
    assert importlib.enforce_interface(token, interface)
    
    # Call function on other contract
    token.transfer(amount=amount, to=recipient)
```

## Error Handling

### Assertions
- Use `assert` statements for validation and error checking
- Include error messages: `assert condition, "Error message"`

### No Try/Except
- Exception handling with try/except is not allowed
- Use conditional logic with if/else statements instead

```python
# DO NOT USE:
try:
    result = 100 / value
except:
    result = 0

# CORRECT APPROACH:
assert value != 0, "Cannot divide by zero"
result = 100 / value

# OR
if value == 0:
    result = 0
else:
    result = 100 / value
```

### Prohibited Built-ins
- `getattr` is an illegal built-in function and must not be used
- Other Python built-ins may also be restricted for security reasons

## Modules

### Random
- Seed RNG with `random.seed()`
- Generate random integers with `random.randint(min, max)`

### Datetime
- Available by default without importing
- Compare timestamps with standard comparison operators
- Use the built-in `now` variable for current time

### Crypto
- Provides cryptographic functionality using the PyNaCl library under the hood
- Employs the Ed25519 signature scheme for digital signatures
- Main function is `verify` for signature validation

```python
# Verify a signature
is_valid = crypto.verify(vk, msg, signature)
# Returns True if the signature is valid for the given message and verification key
```

Example usage in a contract:
```python
@export
def verify_signature(vk: str, msg: str, signature: str):
    # Use the verify function to check if the signature is valid 
    is_valid = crypto.verify(vk, msg, signature)
    
    # Return the result of the verification
    return is_valid
```

### Hashlib
- Xian provides a simplified version of hashlib with a different API than Python's standard library
- Does not require setting up an object and updating it with bytes
- Functions directly accept and return hexadecimal strings

```python
# Hash a hex string with SHA3 (256 bit)
hash_result = hashlib.sha3("68656c6c6f20776f726c64")  # hex for "hello world"

# If not a valid hex string, it will encode the string to bytes first
text_hash = hashlib.sha3("hello world")

# SHA256 works the same way (SHA2 256-bit, used in Bitcoin)
sha256_result = hashlib.sha256("68656c6c6f20776f726c64")
```

## Testing

### Setting Up Tests
- Use Python's unittest framework
- Client available via `from contracting.client import ContractingClient`
- Flush client before and after each test

### Setting Test Environment
- Pass environment variables like `now` (datetime) in a dictionary

```python
from contracting.stdlib.bridge.time import Datetime

env = {"now": Datetime(year=2021, month=1, day=1, hour=0)}
result = self.some_contract.some_fn(some_arg=some_value, environment=env)
```

### Specifying Signer
- Specify the signer when calling contract functions in tests

```python
result = self.some_contract.some_fn(some_arg=some_value, signer="some_signer")
```

## Events

### Defining Events
- Use `LogEvent` to define events at the top level of a contract
- Each event has a name and a schema of parameters with their types
- Set `idx: True` for parameters that should be indexed for querying

```python
TransferEvent = LogEvent(
    event="Transfer",
    params={
        "from": {'type': str, 'idx': True},
        "to": {'type': str, 'idx': True},
        "amount": {'type': (int, float, decimal)}
    }
)

ApprovalEvent = LogEvent(
    event="Approval",
    params={
        "owner": {'type': str, 'idx': True},
        "spender": {'type': str, 'idx': True},
        "amount": {'type': (int, float, decimal)}
    }
)
```

### Emitting Events
- Call the event variable as a function and pass a dictionary of parameter values
- All parameters defined in the event schema must be provided
- Event parameters must match the specified types

```python
@export
def transfer(amount: float, to: str):
    sender = ctx.caller
    
    # ... perform transfer logic ...
    
    # Emit the transfer event
    TransferEvent({
        "from": sender,
        "to": to,
        "amount": amount
    })
```

### Testing Events
- Use `return_full_output=True` when calling contract functions in tests to capture events
- Access events in the result dictionary's 'events' key
- Assert on event types and parameters in tests

```python
# In your test function
result = self.contract.transfer(
    amount=100,
    to="recipient",
    signer="sender",
    return_full_output=True
)

# Verify events
events = result['events']
assert len(events) == 1
assert events[0]['event'] == 'Transfer'
assert events[0]['from'] == 'sender'
assert events[0]['to'] == 'recipient'
assert events[0]['amount'] == 100
```

### Common Event Types
- Transfer: When value moves between accounts
- Approval: When spending permissions are granted
- Mint/Burn: When tokens are created or destroyed
- StateChange: When significant contract state changes
- ActionPerformed: When important contract actions execute

## Smart Contract Testing Best Practices

### Test Structure
- Use Python's unittest framework for structured testing
- Create a proper test class that inherits from `unittest.TestCase`
- Implement `setUp` and `tearDown` methods to isolate tests
- Define the environment and chain ID in setUp for consistent testing

```python
class TestMyContract(unittest.TestCase):
    def setUp(self):
        # Bootstrap the environment
        self.chain_id = "test-chain"
        self.environment = {"chain_id": self.chain_id}
        self.deployer_vk = "test-deployer"
        
        # Initialize the client
        self.client = ContractingClient(environment=self.environment)
        self.client.flush()
        
        # Load and submit the contract
        with open('path/to/my_contract.py') as f:
            code = f.read()
            self.client.submit(code, name="my_contract", constructor_args={"owner": self.deployer_vk})
            
        # Get contract instance
        self.contract = self.client.get_contract("my_contract")
        
    def tearDown(self):
        # Clean up after each test
        self.client.flush()
```

### Test Organization
- Group tests by functionality using descriptive method names
- Follow the Given-When-Then pattern for clear test cases
- Test both positive paths and error cases
- Define all variables within the test, not in setUp
- Define all variables and parameters used by a test WITHIN THE TEST, not within setUp
- This ensures test isolation and prevents unexpected side effects between tests

```python
def test_transfer_success(self):
    # GIVEN a sender with balance
    sender = "alice"
    self.contract.balances[sender] = 1000
    
    # WHEN a transfer is executed
    result = self.contract.transfer(amount=100, to="bob", signer=sender)
    
    # THEN the balances should be updated correctly
    self.assertEqual(self.contract.balances["bob"], 100)
    self.assertEqual(self.contract.balances[sender], 900)
```

### Testing for Security Vulnerabilities

#### 1. Authorization and Access Control
- Test that only authorized users can perform restricted actions
- Verify that contract functions check `ctx.caller` or `ctx.signer` appropriately

```python
def test_change_metadata_unauthorized(self):
    # GIVEN a non-operator trying to change metadata
    with self.assertRaises(Exception):
        self.contract.change_metadata(key="name", value="NEW", signer="attacker")
```

#### 2. Replay Attack Protection
- Test that transaction signatures cannot be reused
- Verify nonce mechanisms or one-time-use permits

```python
def test_permit_double_spending(self):
    # GIVEN a permit already used once
    self.contract.permit(owner="alice", spender="bob", value=100, deadline=deadline, 
                        signature=signature)
    
    # WHEN the permit is used again
    # THEN it should fail
    with self.assertRaises(Exception):
        self.contract.permit(owner="alice", spender="bob", value=100, 
                            deadline=deadline, signature=signature)
```

#### 3. Time-Based Vulnerabilities
- Test behavior around time boundaries (begin/end dates)
- Test with different timestamps using the environment parameter

```python
def test_time_sensitive_function(self):
    # Test with time before deadline
    env = {"now": Datetime(year=2023, month=1, day=1)}
    result = self.contract.some_function(signer="alice", environment=env)
    self.assertTrue(result)
    
    # Test with time after deadline
    env = {"now": Datetime(year=2024, month=1, day=1)}
    with self.assertRaises(Exception):
        self.contract.some_function(signer="alice", environment=env)
```

#### 4. Balance and State Checks
- Verify state changes after operations
- Test for correct balance updates after transfers
- Ensure state consistency through complex operations

```python
def test_transfer_balances(self):
    # Set initial balances
    self.contract.balances["alice"] = 1000
    self.contract.balances["bob"] = 500
    
    # Perform transfer
    self.contract.transfer(amount=300, to="bob", signer="alice")
    
    # Verify final balances
    self.assertEqual(self.contract.balances["alice"], 700)
    self.assertEqual(self.contract.balances["bob"], 800)
```

#### 5. Signature Validation
- Test with valid and invalid signatures
- Test with modified parameters to ensure signatures aren't transferable

```python
def test_signature_validation(self):
    # GIVEN a properly signed message
    signature = wallet.sign_msg(msg)
    
    # WHEN using the correct parameters
    result = self.contract.verify_signature(msg=msg, signature=signature, 
                                          public_key=wallet.public_key)
    
    # THEN verification should succeed
    self.assertTrue(result)
    
    # BUT when using modified parameters
    with self.assertRaises(Exception):
        self.contract.verify_signature(msg=msg+"tampered", signature=signature, 
                                     public_key=wallet.public_key)
```

#### 6. Edge Cases and Boundary Conditions
- Test with zero values, max values, empty strings
- Test operations at time boundaries (exactly at deadline)
- Test with invalid inputs and malformed data

```python
def test_edge_cases(self):
    # Test with zero amount
    with self.assertRaises(Exception):
        self.contract.transfer(amount=0, to="receiver", signer="sender")
    
    # Test with negative amount
    with self.assertRaises(Exception):
        self.contract.transfer(amount=-100, to="receiver", signer="sender")
```

#### 7. Capturing and Verifying Events
- Use `return_full_output=True` to capture events
- Verify event emissions and their parameters

```python
def test_event_emission(self):
    # GIVEN a setup for transfer
    sender = "alice"
    receiver = "bob"
    amount = 100
    self.contract.balances[sender] = amount
    
    # WHEN executing with return_full_output
    result = self.contract.transfer(
        amount=amount, 
        to=receiver, 
        signer=sender,
        return_full_output=True
    )
    
    # THEN verify the event was emitted with correct parameters
    self.assertIn('events', result)
    events = result['events']
    self.assertEqual(len(events), 1)
    event = events[0]
    self.assertEqual(event['event'], 'Transfer')
    self.assertEqual(event['data_indexed']['from'], sender)
    self.assertEqual(event['data_indexed']['to'], receiver)
    self.assertEqual(event['data']['amount'], amount)
```

### Common Exploits to Test For

#### Reentrancy
- Test that state is updated before external calls
- Verify operations complete atomically

```python
def test_no_reentrancy_vulnerability(self):
    # Set up the attack scenario (if possible with Xian)
    
    # Verify state is properly updated before any external calls
    # For example, check that balances are decreased before tokens are sent
    
    # Verify proper operation ordering in the contract
```

#### Integer Overflow/Underflow
- Test with extremely large numbers
- Test arithmetic operations at boundaries

```python
def test_integer_boundaries(self):
    # Set a large balance
    self.contract.balances["user"] = 10**20
    
    # Test with large transfers
    result = self.contract.transfer(amount=10**19, to="receiver", signer="user")
    
    # Verify results are as expected
    self.assertEqual(self.contract.balances["user"], 9*10**19)
    self.assertEqual(self.contract.balances["receiver"], 10**19)
```

#### Front-Running Protection
- Test mechanisms that prevent frontrunning (e.g., commit-reveal)
- Test deadline-based protections

```python
def test_front_running_protection(self):
    # Test with deadlines to ensure transactions expire
    deadline = Datetime(year=2023, month=1, day=1)
    current_time = Datetime(year=2023, month=1, day=2)  # After deadline
    
    with self.assertRaises(Exception):
        self.contract.time_sensitive_operation(
            param1="value",
            deadline=str(deadline),
            environment={"now": current_time}
        )
```

#### Authorization Bypass
- Test authorization for all privileged operations
- Try to access functions with different signers

```python
def test_authorization_checks(self):
    # Test admin functions with non-admin signers
    with self.assertRaises(Exception):
        self.contract.admin_function(param="value", signer="regular_user")
    
    # Test with proper authorization
    result = self.contract.admin_function(param="value", signer="admin")
    self.assertTrue(result)
```

### Best Practices Summary
- Test both positive and negative paths
- Test permissions and authorization thoroughly
- Use environment variables to test time-dependent behavior
- Verify event emissions using `return_full_output=True`
- Test against potential replay attacks and signature validation
- Check edge cases and boundary conditions
- Verify state consistency after operations
- Test for common security vulnerabilities