🛑 Interrupting Execution

WebAssembly is currently always run in the same process synchronously. Thus, once WebAssembly starts executing, you have to wait for the execution to complete to continue running code on the host (your application).

However, there are cases where you may want to interrupt this synchronous execution while the guest Wasm module is calling a host function. This can be useful for saving resources, and not returning back to the guest Wasm for execution, when you already know the Wasm execution will fail, or no longer be needed.

In this example, we will run a Wasm module that calls the imported host function interrupt_execution. This host function will immediately stop executing the WebAssembly module.

First we are going to want to initialize a new project. To do this we can navigate to our project folder, or create one. In this example, we will create a new project. Lets create it and navigate to it:

Rust
Go
Python
PHP
Ruby
Rust

The final Rust code for this example can be found on Github: early_exit.rs.

Please take a look at the setup steps for Rust.

cargo new early-exit
cd early-exit

We have to modify Cargo.toml to add the Wasmer dependencies as shown below:

[dependencies]
# The Wasmer API
wasmer = "2.0"
Go

The final Go code for this example can be found on Github: early_exit.go.

Please take a look at the setup steps for Go.

mkdir wasmer-example-early-exit
cd wasmer-example-early-exit
go mod init github.com/$USER/wasmer-example-early-exit
Python

The final Python code for this example can be found on Github: imports_function_early_exit.py.

Please take a look at the setup steps for Python.

mkdir wasmer-example-early-exit
cd wasmer-example-early-exit
pip install wasmer wasmer_compiler_cranelift
PHP

The final PHP code for this example can be found on Github: imports-function-early-exit.php.

Please take a look at the setup steps for PHP.

mkdir wasmer-example-early-exit
cd wasmer-example-early-exit
composer init --name=wasmer-example-early-exit
composer require wasm/wasm
Ruby

The final Ruby code for this example can be found on Github: imports_function_early_exit.rb.

Please take a look at the setup steps for Ruby.

gem install wasmer

Now that we have everything set up, let's go ahead and try it out!

Setting up

Before we start with the Wasm part we'll have to declare the error we'll use to terminate the execution of the guest module:

Rust
Go
Python
PHP
Rust
#[derive(Debug, Clone, Copy)]
struct ExitCode(u32);
​
impl fmt::Display for ExitCode {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
write!(f, "{}", self.0)
}
}
​
impl std::error::Error for ExitCode {}
Go
type exitCode struct {
code int32
}
​
func (self *exitCode) Error() string {
return fmt.Sprintf("exit code: %d", self.code)
}
Python
class ExitCode(Exception):
pass
PHP
class ExitCode extends Exception {}

There is nothing special or Wasmer specific here but it will be required later in the example.

Defining and importing the host function

To terminate the execution of the Wasm module we'll have to define a function on the host which will then be imported in the guest and called whenever execution is not required to continue. Let's do that:

Rust
Go
Python
PHP
Ruby
Rust
fn early_exit() {
RuntimeError::raise(Box::new(ExitCode(1)));
}
​
let import_object = imports! {
"env" => {
"early_exit" => Function::new_native(&store, early_exit),
}
};
Go
func earlyExit(args []wasmer.Value) ([]wasmer.Value, error) {
return nil, &exitCode{1}
}
​
importObject := wasmer.NewImportObject()
importObject.Register(
"env",
map[string]wasmer.IntoExtern{
"early_exit": wasmer.NewFunction(
store,
wasmer.NewFunctionType(wasmer.NewValueTypes(), wasmer.NewValueTypes()),
earlyExit,
),
},
)
Python
def early_exit():
raise ExitCode("oops")
​
import_object = ImportObject()
import_object.register(
"env",
{
"early_exit": Function(store, early_exit),
}
)
PHP
function earlyExit() {
throw new ExitCode(1);
}
​
$funcType = Wasm\Type\FuncType::new(new Wasm\Vec\ValType(), new Wasm\Vec\ValType());
$func = Wasm\Func::new($store, $funcType, 'earlyExit');
$extern = $func->asExtern();
$externs = new Wasm\Vec\Extern([$extern->inner()]);
Ruby
def early_exit
raise "oops"
end
​
func_type = Wasmer::FunctionType.new([], [])
func = Wasmer::Function.new(store, method(:early_exit), func_type)
import_object = Wasmer::ImportObject.new
import_object.register("env", { :early_exit => func })

As we saw in previous examples we defined a Rust function, wrap it in a native function definition and import it in the guest module, in the env namespace, using the ImportObject.

Handling the error

Our module will call the early_exit function once we call its run function (which is an exported function). Let's get the function, call it and see how we can handle the error:

Rust
Go
Python
PHP
Ruby
Rust
let run_func: NativeFunc<(i32, i32), i32> = instance
.exports
.get_native_function("run")?;
​
match run_func.call(1, 7) {
Ok(result) => {
bail!("Expected early termination with `ExitCode`, found: {}", result);
}
Err(e) => match e.downcast::<ExitCode>() {
// We found the exit code used to terminate execution.
Ok(exit_code) => {
println!("Exited early with exit code: {}", exit_code);
Ok(())
}
Err(e) => {
bail!("Unknown error `{}` found. expected `ErrorCode`", e);
}
},
}

We expect to get an error when calling the run function so what we do here is look at the result and:

  • if we get a success, our test will fail;

  • if we get an error, we try to downcast to our ExitCode error.

If downcasting succeeds it means we actually got the expected error so we make the test pass. If it fails, it means the Wasm module reported an error but it wasn't the one we expected so we make the test fail.

Go
run, err := instance.Exports.GetFunction("run")
​
if err != nil {
panic(fmt.Sprintln("Failed to retrieve the `run` function:", err))
}
​
​
_, err = run(1, 7)
​
if err == nil {
panic(fmt.Sprintln("`run` did not error"))
}
​
fmt.Println("Exited early with:", err)
Python
try:
instance.exports.run(1, 2)
except RuntimeError as err:
assert "oops" in str(err)
else:
assert False
PHP
try {
$run($args);
​
echo '`run` did not error'.PHP_EOL;
​
exit(1);
} catch (ExitCode $exception) {
echo 'Exited early with: '.$exception->getMessage().' '.$exception->getCode().PHP_EOL;
}
Ruby
begin
instance.exports.run.(1, 2)
rescue RuntimeError => e
puts e.message
else
raise "There was no error"
end

Running

We now have everything we need to run the Wasm module, let's do it!

Rust
Go
Python
PHP
Ruby
Rust

You should be able to run it using the cargo run command. The output should look like this:

Compiling module...
Instantiating module...
Calling `run` function...
Exited early with exit code: 1

If you want to run the examples from the Wasmer repository codebase directly, you can also do:

git clone https://github.com/wasmerio/wasmer.git
cd wasmer
cargo run --example early-exit --release --features "cranelift"
Go

You should be able to run it using the go run main.go command. The output should look like this:

Compiling module...
Instantiating module...
Calling `run` function...
Exited early with: exit code: 1

If you want to run the examples from the Wasmer repository codebase directly, you can also do:

git clone https://github.com/wasmerio/wasmer-go.git
cd wasmer-go
go test examples/example_memory_test.go
Python

You should be able to run it using the python imports_function_early_exit.py command.

If you want to run the examples from the Wasmer repository codebase directly, you can also do:

git clone https://github.com/wasmerio/wasmer-python.git
cd wasmer-python
python examples/imports_function_early_exit.py
PHP

You should be able to run it using the php imports-function-early-exit.php command.

If you want to run the examples from the Wasmer PHP repository codebase directly, you can also do:

git clone https://github.com/wasmerio/wasmer-php.git
cd wasmer-php
make EXAMPLE=imports-function-early-exit test-doc-examples
Ruby

You should be able to run it using the ruby imports_function_early_exit.rb command.

If you want to run the examples from the Wasmer Ruby repository codebase directly, you can also do:

git clone https://github.com/wasmerio/wasmer-ruby.git
cd wasmer-ruby
ruby examples/imports_function_early_exit.rb