syscall/js: show goroutine stack traces on deadlock

When using callbacks, it is not necessarily a deadlock if there is no
runnable goroutine, since a callback might still be pending. If there
is no callback pending, Node.js simply exits with exit code zero,
which is not desired if the Go program is still considered running.
This is why an explicit check on exit is used to trigger the "deadlock"
error. This CL makes it so this is Go's normal "deadlock" error, which
includes the stack traces of all goroutines.

Updates #26382

Change-Id: If88486684d0517a64f570009a5ea0ad082679a54
Reviewed-on: https://go-review.googlesource.com/123936
Run-TryBot: Richard Musiol <neelance@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

misc/wasm/wasm_exec.js

@@ -333,14 +333,10 @@
 				false,
 				global,
 				this._inst.exports.mem,
-				() => { // resolveCallbackPromise
-					if (this.exited) {
-						throw new Error("bad callback: Go program has already exited");
-					}
-					setTimeout(this._resolveCallbackPromise, 0); // make sure it is asynchronous
-				},
+				this,
 			];
 			this._refs = new Map();
+			this._callbackShutdown = false;
 			this.exited = false;
 
 			const mem = new DataView(this._inst.exports.mem.buffer)
@@ -377,7 +373,12 @@
 
 			while (true) {
 				const callbackPromise = new Promise((resolve) => {
-					this._resolveCallbackPromise = resolve;
+					this._resolveCallbackPromise = () => {
+						if (this.exited) {
+							throw new Error("bad callback: Go program has already exited");
+						}
+						setTimeout(resolve, 0); // make sure it is asynchronous
+					};
 				});
 				this._inst.exports.run(argc, argv);
 				if (this.exited) {
@@ -399,17 +400,16 @@
 		go.env = process.env;
 		go.exit = process.exit;
 		WebAssembly.instantiate(fs.readFileSync(process.argv[2]), go.importObject).then((result) => {
-			process.on("exit", () => { // Node.js exits if no callback is pending
-				if (!go.exited) {
-					console.error("error: all goroutines asleep and no JavaScript callback pending - deadlock!");
-					process.exit(1);
+			process.on("exit", (code) => { // Node.js exits if no callback is pending
+				if (code === 0 && !go.exited) {
+					// deadlock, make Go print error and stack traces
+					go._callbackShutdown = true;
+					go._inst.exports.run();
 				}
 			});
 			return go.run(result.instance);
 		}).catch((err) => {
-			console.error(err);
-			go.exited = true;
-			process.exit(1);
+			throw err;
 		});
 	}
 })();

src/syscall/js/callback.go

@@ -11,10 +11,10 @@ import "sync"
 var pendingCallbacks = Global().Get("Array").New()
 
 var makeCallbackHelper = Global().Call("eval", `
-	(function(id, pendingCallbacks, resolveCallbackPromise) {
+	(function(id, pendingCallbacks, go) {
 		return function() {
 			pendingCallbacks.push({ id: id, args: arguments });
-			resolveCallbackPromise();
+			go._resolveCallbackPromise();
 		};
 	})
 `)
@@ -71,7 +71,7 @@ func NewCallback(fn func(args []Value)) Callback {
 	callbacks[id] = fn
 	callbacksMu.Unlock()
 	return Callback{
-		Value: makeCallbackHelper.Invoke(id, pendingCallbacks, resolveCallbackPromise),
+		Value: makeCallbackHelper.Invoke(id, pendingCallbacks, jsGo),
 		id:    id,
 	}
 }
@@ -116,7 +116,7 @@ func (c Callback) Release() {
 var callbackLoopOnce sync.Once
 
 func callbackLoop() {
-	for {
+	for !jsGo.Get("_callbackShutdown").Bool() {
 		sleepUntilCallback()
 		for {
 			cb := pendingCallbacks.Call("shift")

src/syscall/js/js.go

@@ -63,7 +63,7 @@ var (
 	valueFalse     = predefValue(4)
 	valueGlobal    = predefValue(5)
 	memory         = predefValue(6) // WebAssembly linear memory
-	resolveCallbackPromise = predefValue(7) // function that the callback helper uses to resume the execution of Go's WebAssembly code
+	jsGo           = predefValue(7) // instance of the Go class in JavaScript
 )
 
 // Undefined returns the JavaScript value "undefined".