🛑
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
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 = "3.0"
mkdir wasmer-example-early-exit
cd wasmer-example-early-exit
go mod init github.com/$USER/wasmer-example-early-exit
mkdir wasmer-example-early-exit
cd wasmer-example-early-exit
pip install wasmer wasmer_compiler_cranelift
mkdir wasmer-example-early-exit
cd wasmer-example-early-exit
composer init --name=wasmer-example-early-exit
composer require wasm/wasm
gem install wasmer
Now that we have everything set up, let's go ahead and try it out!
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
#[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 {}
type exitCode struct {
code int32
}
func (self *exitCode) Error() string {
return fmt.Sprintf("exit code: %d", self.code)
}
class ExitCode(Exception):
pass
class ExitCode extends Exception {}
There is nothing special or Wasmer specific here but it will be required later in the example.
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
fn early_exit() -> Result<(), ExitCode> {
Err(ExitCode(1))
}
let import_object = imports! {
"env" => {
"early_exit" => Function::new_typed(&mut store, early_exit),
}
};
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,
),
},
)
def early_exit():
raise ExitCode("oops")
import_object = ImportObject()
import_object.register(
"env",
{
"early_exit": Function(store, early_exit),
}
)
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()]);
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 Imports
.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
let run_func: TypedFunction<(i32, i32), i32> = instance
.exports.get_typed_function(&mut store, "run")?;
match run_func.call(&mut store, 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.
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)
try:
instance.exports.run(1, 2)
except RuntimeError as err:
assert "oops" in str(err)
else:
assert False
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;
}
begin
instance.exports.run.(1, 2)
rescue RuntimeError => e
puts e.message
else
raise "There was no error"
end
We now have everything we need to run the Wasm module, let's do it!
Rust
Go
Python
PHP
Ruby
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
git clone https://github.com/wasmerio/wasmer.git
cd wasmer
cargo run --example early-exit --release --features "cranelift"
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
git clone https://github.com/wasmerio/wasmer-go.git
cd wasmer-go
go test examples/example_memory_test.go
You should be able to run it using the
python imports_function_early_exit.py
command.git clone https://github.com/wasmerio/wasmer-python.git
cd wasmer-python
python examples/imports_function_early_exit.py
You should be able to run it using the
php imports-function-early-exit.php
command.git clone https://github.com/wasmerio/wasmer-php.git
cd wasmer-php
make EXAMPLE=imports-function-early-exit test-doc-examples
You should be able to run it using the
ruby imports_function_early_exit.rb
command.git clone https://github.com/wasmerio/wasmer-ruby.git
cd wasmer-ruby
ruby examples/imports_function_early_exit.rb
Last modified 7mo ago