Add example of execution limits using fuel consumption (#2869)

* Add example of execution limits using fuel consumption

* run rustfmt

* Use a more naive WAT implementation

* Catch error and return cleanly

* Add a C example to demonstrate fuel consumption

* Add error handling for add_fuel

examples/fuel.c

@@ -0,0 +1,132 @@
+/*
+Example of instantiating of the WebAssembly module and invoking its exported
+function.
+
+You can compile and run this example on Linux with:
+
+   cargo build --release -p wasmtime-c-api
+   cc examples/fuel.c \
+       -I crates/c-api/include \
+       -I crates/c-api/wasm-c-api/include \
+       target/release/libwasmtime.a \
+       -lpthread -ldl -lm \
+       -o fuel
+   ./fuel
+
+Note that on Windows and macOS the command will be similar, but you'll need
+to tweak the `-lpthread` and such annotations.
+*/
+
+#include <assert.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <wasm.h>
+#include <wasmtime.h>
+
+static void exit_with_error(const char *message, wasmtime_error_t *error, wasm_trap_t *trap);
+
+int main() {
+  wasmtime_error_t *error = NULL;
+
+  wasm_config_t *config = wasm_config_new();
+  assert(config != NULL);
+  wasmtime_config_consume_fuel_set(config, true);
+
+  // Create an *engine*, which is a compilation context, with our configured options.
+  wasm_engine_t *engine = wasm_engine_new_with_config(config);
+  assert(engine != NULL);
+  wasm_store_t *store = wasm_store_new(engine);
+  assert(store != NULL);
+  error = wasmtime_store_add_fuel(store, 10000);
+  if (error != NULL)
+    exit_with_error("failed to add fuel", error, NULL);
+
+  // Load our input file to parse it next
+  FILE* file = fopen("examples/fuel.wat", "r");
+  if (!file) {
+    printf("> Error loading file!\n");
+    return 1;
+  }
+  fseek(file, 0L, SEEK_END);
+  size_t file_size = ftell(file);
+  fseek(file, 0L, SEEK_SET);
+  wasm_byte_vec_t wat;
+  wasm_byte_vec_new_uninitialized(&wat, file_size);
+  if (fread(wat.data, file_size, 1, file) != 1) {
+    printf("> Error loading module!\n");
+    return 1;
+  }
+  fclose(file);
+
+  // Parse the wat into the binary wasm format
+  wasm_byte_vec_t wasm;
+  error = wasmtime_wat2wasm(&wat, &wasm);
+  if (error != NULL)
+    exit_with_error("failed to parse wat", error, NULL);
+  wasm_byte_vec_delete(&wat);
+
+  // Compile and instantiate our module
+  wasm_module_t *module = NULL;
+  error = wasmtime_module_new(engine, &wasm, &module);
+  if (module == NULL)
+    exit_with_error("failed to compile module", error, NULL);
+  wasm_byte_vec_delete(&wasm);
+  wasm_trap_t *trap = NULL;
+  wasm_instance_t *instance = NULL;
+  wasm_extern_vec_t imports = WASM_EMPTY_VEC;
+  error = wasmtime_instance_new(store, module, &imports, &instance, &trap);
+  if (instance == NULL)
+    exit_with_error("failed to instantiate", error, trap);
+
+  // Lookup our `fibonacci` export function
+  wasm_extern_vec_t externs;
+  wasm_instance_exports(instance, &externs);
+  assert(externs.size == 1);
+  wasm_func_t *fibonacci = wasm_extern_as_func(externs.data[0]);
+  assert(fibonacci != NULL);
+
+  // Call it repeatedly until it fails
+  for (int n = 1; ; n++) {
+    uint64_t fuel_before;
+    wasmtime_store_fuel_consumed(store, &fuel_before);
+    wasm_val_t params[1] = { WASM_I32_VAL(n) };
+    wasm_val_t results[1];
+    wasm_val_vec_t params_vec = WASM_ARRAY_VEC(params);
+    wasm_val_vec_t results_vec = WASM_ARRAY_VEC(results);
+    error = wasmtime_func_call(fibonacci, &params_vec, &results_vec, &trap);
+    if (error != NULL || trap != NULL) {
+      printf("Exhausted fuel computing fib(%d)\n", n);
+      break;
+    }
+
+    uint64_t fuel_after;
+    wasmtime_store_fuel_consumed(store, &fuel_after);
+    assert(results[0].kind == WASM_I32);
+    printf("fib(%d) = %d [consumed %lld fuel]\n", n, results[0].of.i32, fuel_after - fuel_before);
+
+    error = wasmtime_store_add_fuel(store, fuel_after - fuel_before);
+    if (error != NULL)
+      exit_with_error("failed to add fuel", error, NULL);
+  }
+
+  // Clean up after ourselves at this point
+  wasm_extern_vec_delete(&externs);
+  wasm_instance_delete(instance);
+  wasm_module_delete(module);
+  wasm_store_delete(store);
+  wasm_engine_delete(engine);
+  return 0;
+}
+
+static void exit_with_error(const char *message, wasmtime_error_t *error, wasm_trap_t *trap) {
+  fprintf(stderr, "error: %s\n", message);
+  wasm_byte_vec_t error_message;
+  if (error != NULL) {
+    wasmtime_error_message(error, &error_message);
+  } else {
+    wasm_trap_message(trap, &error_message);
+  }
+  fprintf(stderr, "%.*s\n", (int) error_message.size, error_message.data);
+  wasm_byte_vec_delete(&error_message);
+  exit(1);
+}

examples/fuel.rs

@@ -0,0 +1,33 @@
+//! Example of limiting a WebAssembly function's runtime using "fuel consumption".
+
+// You can execute this example with `cargo run --example fuel`
+
+use anyhow::Result;
+use wasmtime::*;
+
+fn main() -> Result<()> {
+    let mut config = Config::new();
+    config.consume_fuel(true);
+    let engine = Engine::new(&config)?;
+    let store = Store::new(&engine);
+    store.add_fuel(10_000)?;
+    let module = Module::from_file(store.engine(), "examples/fuel.wat")?;
+    let instance = Instance::new(&store, &module, &[])?;
+
+    // Invoke `fibonacci` export with higher and higher numbers until we exhaust our fuel.
+    let fibonacci = instance.get_typed_func::<i32, i32>("fibonacci")?;
+    for n in 1.. {
+        let fuel_before = store.fuel_consumed().unwrap();
+        let output = match fibonacci.call(n) {
+            Ok(v) => v,
+            Err(_) => {
+                println!("Exhausted fuel computing fib({})", n);
+                break;
+            }
+        };
+        let fuel_consumed = store.fuel_consumed().unwrap() - fuel_before;
+        println!("fib({}) = {} [consumed {} fuel]", n, output, fuel_consumed);
+        store.add_fuel(fuel_consumed)?;
+    }
+    Ok(())
+}

examples/fuel.wat

@@ -0,0 +1,13 @@
+(module
+  (func $fibonacci (param $n i32) (result i32)
+    (if
+      (i32.lt_s (local.get $n) (i32.const 2))
+      (return (local.get $n))
+    )
+    (i32.add
+      (call $fibonacci (i32.sub (local.get $n) (i32.const 1)))
+      (call $fibonacci (i32.sub (local.get $n) (i32.const 2)))
+    )
+  )
+  (export "fibonacci" (func $fibonacci))
+)