wasi: add returnOnExit option

doc/api/wasi.md

@@ -58,6 +58,10 @@ added: v13.3.0
     sandbox directory structure. The string keys of `preopens` are treated as
     directories within the sandbox. The corresponding values in `preopens` are
     the real paths to those directories on the host machine.
+  * `returnOnExit` {boolean} By default, WASI applications terminate the Node.js
+    process via the `__wasi_proc_exit()` function. Setting this option to `true`
+    causes `wasi.start()` to return the exit code rather than terminate the
+    process. **Default:** `false`.
 
 ### `wasi.start(instance)`
 <!-- YAML

lib/wasi.js

@@ -15,6 +15,7 @@ const {
 } = require('internal/errors').codes;
 const { emitExperimentalWarning } = require('internal/util');
 const { WASI: _WASI } = internalBinding('wasi');
+const kExitCode = Symbol('exitCode');
 const kSetMemory = Symbol('setMemory');
 const kStarted = Symbol('started');
 
@@ -26,7 +27,7 @@ class WASI {
     if (options === null || typeof options !== 'object')
       throw new ERR_INVALID_ARG_TYPE('options', 'object', options);
 
-    const { env, preopens } = options;
+    const { env, preopens, returnOnExit = false } = options;
     let { args = [] } = options;
 
     if (ArrayIsArray(args))
@@ -56,16 +57,26 @@ class WASI {
       throw new ERR_INVALID_ARG_TYPE('options.preopens', 'Object', preopens);
     }
 
+    if (typeof returnOnExit !== 'boolean') {
+      throw new ERR_INVALID_ARG_TYPE(
+        'options.returnOnExit', 'boolean', returnOnExit);
+    }
+
     const wrap = new _WASI(args, envPairs, preopenArray);
 
     for (const prop in wrap) {
       wrap[prop] = FunctionPrototypeBind(wrap[prop], wrap);
     }
 
+    if (returnOnExit) {
+      wrap.proc_exit = FunctionPrototypeBind(wasiReturnOnProcExit, this);
+    }
+
     this[kSetMemory] = wrap._setMemory;
     delete wrap._setMemory;
     this.wasiImport = wrap;
     this[kStarted] = false;
+    this[kExitCode] = 0;
   }
 
   start(instance) {
@@ -93,12 +104,30 @@ class WASI {
     this[kStarted] = true;
     this[kSetMemory](memory);
 
-    if (exports._start)
-      exports._start();
-    else if (exports.__wasi_unstable_reactor_start)
-      exports.__wasi_unstable_reactor_start();
+    try {
+      if (exports._start)
+        exports._start();
+      else if (exports.__wasi_unstable_reactor_start)
+        exports.__wasi_unstable_reactor_start();
+    } catch (err) {
+      if (err !== kExitCode) {
+        throw err;
+      }
+    }
+
+    return this[kExitCode];
   }
 }
 
 
 module.exports = { WASI };
+
+
+function wasiReturnOnProcExit(rval) {
+  // If __wasi_proc_exit() does not terminate the process, an assertion is
+  // triggered in the wasm runtime. Node can sidestep the assertion and return
+  // an exit code by recording the exit code, and throwing a JavaScript
+  // exception that WebAssembly cannot catch.
+  this[kExitCode] = rval;
+  throw kExitCode;
+}

test/wasi/test-return-on-exit.js

@@ -0,0 +1,18 @@
+// Flags: --experimental-wasi-unstable-preview1 --experimental-wasm-bigint
+'use strict';
+const common = require('../common');
+const assert = require('assert');
+const fs = require('fs');
+const path = require('path');
+const { WASI } = require('wasi');
+const wasi = new WASI({ returnOnExit: true });
+const importObject = { wasi_snapshot_preview1: wasi.wasiImport };
+const wasmDir = path.join(__dirname, 'wasm');
+const modulePath = path.join(wasmDir, 'exitcode.wasm');
+const buffer = fs.readFileSync(modulePath);
+
+(async () => {
+  const { instance } = await WebAssembly.instantiate(buffer, importObject);
+
+  assert.strictEqual(wasi.start(instance), 120);
+})().then(common.mustCall());

test/wasi/test-wasi-options-validation.js

@@ -21,6 +21,10 @@ assert.throws(() => { new WASI({ env: 'fhqwhgads' }); },
 assert.throws(() => { new WASI({ preopens: 'fhqwhgads' }); },
               { code: 'ERR_INVALID_ARG_TYPE', message: /\bpreopens\b/ });
 
+// If returnOnExit is not a boolean and not undefined, it should throw.
+assert.throws(() => { new WASI({ returnOnExit: 'fhqwhgads' }); },
+              { code: 'ERR_INVALID_ARG_TYPE', message: /\breturnOnExit\b/ });
+
 // If options is provided, but not an object, the constructor should throw.
 [null, 'foo', '', 0, NaN, Symbol(), true, false, () => {}].forEach((value) => {
   assert.throws(() => { new WASI(value); },