wasm: correctly return from run() in wasm_exec.js

Instead of hanging forever, it should return the exit code from os.Exit.

targets/wasm_exec.js

@@ -132,6 +132,7 @@
 	const decoder = new TextDecoder("utf-8");
 	let reinterpretBuf = new DataView(new ArrayBuffer(8));
 	var logLine = [];
+	const wasmExit = {}; // thrown to exit via proc_exit (not an error)
 
 	global.Go = class {
 		constructor() {
@@ -270,14 +271,11 @@
 					fd_close: () => 0,      // dummy
 					fd_fdstat_get: () => 0, // dummy
 					fd_seek: () => 0,       // dummy
-					"proc_exit": (code) => {
-						if (global.process) {
-							// Node.js
-							process.exit(code);
-						} else {
-							// Can't exit in a browser.
-							throw 'trying to exit with code ' + code;
-						}
+					proc_exit: (code) => {
+						this.exited = true;
+						this.exitCode = code;
+						this._resolveExitPromise();
+						throw wasmExit;
 					},
 					random_get: (bufPtr, bufLen) => {
 						crypto.getRandomValues(loadSlice(bufPtr, bufLen));
@@ -293,7 +291,14 @@
 					// func sleepTicks(timeout float64)
 					"runtime.sleepTicks": (timeout) => {
 						// Do not sleep, only reactivate scheduler after the given timeout.
-						setTimeout(this._inst.exports.go_scheduler, timeout);
+						setTimeout(() => {
+							if (this.exited) return;
+							try {
+								this._inst.exports.go_scheduler();
+							} catch (e) {
+								if (e !== wasmExit) throw e;
+							}
+						}, timeout);
 					},
 
 					// func finalizeRef(v ref)
@@ -465,12 +470,23 @@
 			this._ids = new Map();  // mapping from JS values to reference ids
 			this._idPool = [];      // unused ids that have been garbage collected
 			this.exited = false;    // whether the Go program has exited
+			this.exitCode = 0;
 
 			if (this._inst.exports._start) {
-				this._inst.exports._start();
+				let exitPromise = new Promise((resolve, reject) => {
+					this._resolveExitPromise = resolve;
+				});
 
-				// TODO: wait until the program exists.
-				await new Promise(() => {});
+				// Run program, but catch the wasmExit exception that's thrown
+				// to return back here.
+				try {
+					this._inst.exports._start();
+				} catch (e) {
+					if (e !== wasmExit) throw e;
+				}
+
+				await exitPromise;
+				return this.exitCode;
 			} else {
 				this._inst.exports._initialize();
 			}
@@ -480,7 +496,11 @@
 			if (this.exited) {
 				throw new Error("Go program has already exited");
 			}
-			this._inst.exports.resume();
+			try {
+				this._inst.exports.resume();
+			} catch (e) {
+				if (e !== wasmExit) throw e;
+			}
 			if (this.exited) {
 				this._resolveExitPromise();
 			}