feat(std/wasi): add return on exit option (#8605)

This adds an exitOnReturn option to context making it 
possible to unwind the stack on the exit(2) syscall 
instead of delegating to it directly.

Use case is being able to treat WASI execution contexts 
as children that don't kill the parent on exit.

std/wasi/snapshot_preview1.ts

@@ -207,6 +207,10 @@ function syscall<T extends CallableFunction>(target: T) {
     try {
       return target(...args);
     } catch (err) {
+      if (err instanceof ExitStatus) {
+        throw err;
+      }
+
       switch (err.name) {
         case "NotFound":
           return ERRNO_NOENT;
@@ -266,15 +270,25 @@ interface FileDescriptor {
   entries?: Deno.DirEntry[];
 }
 
+export class ExitStatus {
+  code: number;
+
+  constructor(code: number) {
+    this.code = code;
+  }
+}
+
 export interface ContextOptions {
   args?: string[];
   env?: { [key: string]: string | undefined };
   preopens?: { [key: string]: string };
+  exitOnReturn?: boolean;
 }
 
 export default class Context {
   args: string[];
   env: { [key: string]: string | undefined };
+  exitOnReturn: boolean;
   memory: WebAssembly.Memory;
 
   fds: FileDescriptor[];
@@ -284,6 +298,7 @@ export default class Context {
   constructor(options: ContextOptions) {
     this.args = options.args ? options.args : [];
     this.env = options.env ? options.env : {};
+    this.exitOnReturn = options.exitOnReturn ?? true;
     this.memory = null!;
 
     this.fds = [
@@ -1497,7 +1512,11 @@ export default class Context {
       "proc_exit": syscall((
         rval: number,
       ): never => {
-        Deno.exit(rval);
+        if (this.exitOnReturn) {
+          Deno.exit(rval);
+        }
+
+        throw new ExitStatus(rval);
       }),
 
       "proc_raise": syscall((

std/wasi/snapshot_preview1_test.ts

@@ -1,6 +1,6 @@
 // Copyright 2018-2020 the Deno authors. All rights reserved. MIT license.
-import Context from "./snapshot_preview1.ts";
-import { assertEquals, assertThrows } from "../testing/asserts.ts";
+import Context, { ExitStatus } from "./snapshot_preview1.ts";
+import { assert, assertEquals, assertThrows } from "../testing/asserts.ts";
 import { copy } from "../fs/mod.ts";
 import * as path from "../path/mod.ts";
 
@@ -179,6 +179,24 @@ Deno.test("context_start", function () {
     TypeError,
     "export _start must be a function",
   );
+
+  try {
+    const context = new Context({
+      exitOnReturn: false,
+    });
+    context.start({
+      exports: {
+        _start() {
+          const exit = context.exports["proc_exit"] as CallableFunction;
+          exit(0);
+        },
+        memory: new WebAssembly.Memory({ initial: 1 }),
+      },
+    });
+  } catch (err) {
+    assert(err instanceof ExitStatus);
+    assertEquals(err.code, 0);
+  }
 });
 
 Deno.test("context_initialize", function () {