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Errors

The errors example demonstrates how to define and generate errors in a contract that invokers of the contract can understand and handle. This example is an extension of the storing data example.

Open in Gitpod

Run the Example​

First go through the Setup process to get your development environment configured, then clone the v21.6.0 tag of soroban-examples repository:

git clone -b v21.6.0 https://github.com/stellar/soroban-examples

Or, skip the development environment setup and open this example in Gitpod.

To run the tests for the example, navigate to the errors directory, and use cargo test.

cd errors
cargo test

You should see output that begins like this:

running 2 tests

count: U32(0)
count: U32(1)
count: U32(2)
count: U32(3)
count: U32(4)
count: U32(5)
Status(ContractError(1))
contract call invocation resulted in error Status(ContractError(1))
test test::test ... ok

thread 'test::test_panic' panicked at 'called `Result::unwrap()` on an `Err` value: HostError
Value: Status(ContractError(1))

Debug events (newest first):
   0: "Status(ContractError(1))"
   1: "count: U32(5)"
   2: "count: U32(4)"
   3: "count: U32(3)"
...
test test::test_panic - should panic ... ok

test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.33s

Code​

errors/src/lib.rs
#[contracterror]
#[derive(Copy, Clone, Debug, Eq, PartialEq, PartialOrd, Ord)]
#[repr(u32)]
pub enum Error {
    LimitReached = 1,
}

const COUNTER: Symbol = symbol_short!("COUNTER");
const MAX: u32 = 5;

#[contract]
pub struct IncrementContract;

#[contractimpl]
impl IncrementContract {
    /// Increment increments an internal counter, and returns the value. Errors
    /// if the value is attempted to be incremented past 5.
    pub fn increment(env: Env) -> Result<u32, Error> {
        // Get the current count.
        let mut count: u32 = env.storage().instance().get(&COUNTER).unwrap_or(0); // If no value set, assume 0.
        log!(&env, "count: {}", count);

        // Increment the count.
        count += 1;

        // Check if the count exceeds the max.
        if count <= MAX {
            // Save the count.
            env.storage().instance().set(&COUNTER, &count);

            // Return the count to the caller.
            Ok(count)
        } else {
            // Return an error if the max is exceeded.
            Err(Error::LimitReached)
        }
    }
}

Ref: https://github.com/stellar/soroban-examples/tree/v21.6.0/errors

How it Works​

Open the errors/src/lib.rs file to follow along.

Defining an Error​

Contract errors are Rust u32 enums where every variant of the enum is assigned an integer. The #[contracterror] attribute is used to set the error up so it can be used in the return value of contract functions.

The enum has some constraints:

  • It must have the #[repr(u32)] attribute.
  • It must have the #[derive(Copy)] attribute.
  • Every variant must have an explicit integer value assigned.
#[contracterror]
#[derive(Copy, Clone, Debug, Eq, PartialEq, PartialOrd, Ord)]
#[repr(u32)]
pub enum Error {
    LimitReached = 1,
}

Contract errors cannot be stored as contract data, and therefore cannot be used as types on fields of contract types.

tip

If an error is returned from a function anything the function has done is rolled back. If ledger entries have been altered, or contract data stored, all those changes are reverted and will not be persisted.

Returning an Error​

Errors can be returned from contract functions by returning Result<_, E>.

The increment function returns a Result<u32, Error>, which means it returns Ok(u32) in the successful case, and Err(Error) in the error case.

pub fn increment(env: Env) -> Result<u32, Error> {
    // ...
    if count <= MAX {
        // ...
        Ok(count)
    } else {
        // ...
        Err(Error::LimitReached)
    }
}

Panicking with an Error​

Errors can also be panicked instead of being returned from the function.

The increment function could also be written as follows with a u32 return value. The error can be passed to the environment using the panic_with_error! macro.

pub fn increment(env: Env) -> u32 {
    // ...
    if count <= MAX {
        // ...
        count
    } else {
        // ...
        panic_with_error!(&env, Error::LimitReached)
    }
}
caution

Functions that do not return a Result<_, E> type do not include in their specification what the possible error values are. This makes it more difficult for other contracts and clients to integrate with the contract. However, this might be ideal if the errors are diagnostic and debugging, and not intended to be handled.

Tests​

Open the errors/src/test.rs file to follow along.

errors/src/test.rs
#[test]
fn test() {
    let env = Env::default();
    let contract_id = env.register_contract(None, IncrementContract);
    let client = IncrementContractClient::new(&env, &contract_id);

    assert_eq!(client.try_increment(), Ok(Ok(1)));
    assert_eq!(client.try_increment(), Ok(Ok(2)));
    assert_eq!(client.try_increment(), Ok(Ok(3)));
    assert_eq!(client.try_increment(), Ok(Ok(4)));
    assert_eq!(client.try_increment(), Ok(Ok(5)));
    assert_eq!(client.try_increment(), Err(Ok(Error::LimitReached)));

    std::println!("{}", env.logs().all().join("\n"));
}

#[test]
#[should_panic(expected = "Status(ContractError(1))")]
#E3256B
fn test_panic() {
    let env = Env::default();
    let contract_id = env.register_contract(None, IncrementContract);
    let client = IncrementContractClient::new(&env, &contract_id);

    assert_eq!(client.increment(), 1);
    assert_eq!(client.increment(), 2);
    assert_eq!(client.increment(), 3);
    assert_eq!(client.increment(), 4);
    assert_eq!(client.increment(), 5);
    client.increment();
}

In any test the first thing that is always required is an Env, which is the Soroban environment that the contract will run in.

let env = Env::default();

The contract is registered with the environment using the contract type.

let contract_id = env.register_contract(None, IncrementContract);

All public functions within an impl block that is annotated with the #[contractimpl] attribute have a corresponding function generated in a generated client type. The client type will be named the same as the contract type with Client appended. For example, in our contract the contract type is IncrementContract, and the client is named IncrementContractClient.

let client = IncrementContractClient::new(&env, &contract_id);

Two functions are generated for every contract function, one that returns a Result<>, and the other that does not handle errors and panicks if an error occurs.

try_increment​

In the first test the try_increment function is called and returns Result<Result<u32, _>, Result<Error, Status>>.

assert_eq!(client.try_increment(), Ok(Ok(5)));
assert_eq!(client.try_increment(), Err(Ok(Error::LimitReached)));

  • If the function call is successful, Ok(Ok(u32)) is returned.

  • If the function call is successful but returns a value that is not a u32, Ok(Err(_)) is returned.

  • If the function call is unsuccessful, Err(Ok(Error)) is returned.

  • If the function call is unsuccessful but returns an error code not in the Error enum, or returns a system error code, Err(Err(Status)) is returned and the Status can be inspected.

increment​

In the second test the increment function is called and returns u32. When the last call is made the function panicks.

assert_eq!(client.increment(), 5);
client.increment();

  • If the function call is successful, u32 is returned.

  • If the function call is successful but returns a value that is not a u32, a panic occurs.

  • If the function call is unsuccessful, a panic occurs.

Build the Contract​

To build the contract, use the stellar contract build command.

stellar contract build

A .wasm file should be outputted in the target directory:

target/wasm32-unknown-unknown/release/soroban_errors_contract.wasm

Run the Contract​

Let's deploy the contract to Testnet so we can run it. The value provided as --source was set up in our Getting Started guide; please change accordingly if you created a different identity.

stellar contract deploy \
  --wasm target/wasm32-unknown-unknown/release/soroban_errors_contract.wasm \
  --source alice \
  --network testnet

The command above will output the contract id, which in our case is CC3UMHVTIEH6GGDBW7MM72Q545HBDCXGU3GMIXP23PQVSBFKNZRWT37X.

Now that we've deployed the contract, we can invoke it.

stellar contract invoke \
    --id CC3UMHVTIEH6GGDBW7MM72Q545HBDCXGU3GMIXP23PQVSBFKNZRWT37X \
    --network testnet \
    --source alice \
    -- \
    increment

Run the command a few times and on the 6th invocation you should see an error like this:

...
error: transaction simulation failed: host invocation failed

Caused by:
    HostError: Error(Contract, #1)

    Event log (newest first):
       0: [Diagnostic Event] contract:<your contract id>, topics:[error, Error(Contract, #1)], data:"escalating Ok(ScErrorType::Contract) frame-exit to Err"
       1: [Diagnostic Event] topics:[fn_call, Bytes(b7461eb3410fe31861b7d8cfea1de74e118ae6a6ccc45dfadbe15904aa6e6369), increment], data:Void
...

To retrieve the current counter value, use the command stellar contract read.

stellar contract read \
  --id CC3UMHVTIEH6GGDBW7MM72Q545HBDCXGU3GMIXP23PQVSBFKNZRWT37X \
  --network testnet \
  --source alice \
  --durability persistent \
  --output json