syscall/js: garbage collect references to JavaScript values

The js.Value struct now contains a pointer, so a finalizer can
determine if the value is not referenced by Go any more.

Unfortunately this breaks Go's == operator with js.Value. This change
adds a new Equal method to check for the equality of two Values.
This is a breaking change. The == operator is now disallowed to
not silently break code.

Additionally the helper methods IsUndefined, IsNull and IsNaN got added.

Fixes #35111

Change-Id: I58a50ca18f477bf51a259c668a8ba15bfa76c955
Reviewed-on: https://go-review.googlesource.com/c/go/+/203600
Run-TryBot: Richard Musiol <neelance@gmail.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

misc/wasm/wasm_exec.js

@@ -205,26 +205,31 @@
 						return;
 				}
 
-				let ref = this._refs.get(v);
-				if (ref === undefined) {
-					ref = this._values.length;
-					this._values.push(v);
-					this._refs.set(v, ref);
+				let id = this._ids.get(v);
+				if (id === undefined) {
+					id = this._idPool.pop();
+					if (id === undefined) {
+						id = this._values.length;
+					}
+					this._values[id] = v;
+					this._goRefCounts[id] = 0;
+					this._ids.set(v, id);
 				}
-				let typeFlag = 0;
+				this._goRefCounts[id]++;
+				let typeFlag = 1;
 				switch (typeof v) {
 					case "string":
-						typeFlag = 1;
+						typeFlag = 2;
 						break;
 					case "symbol":
-						typeFlag = 2;
+						typeFlag = 3;
 						break;
 					case "function":
-						typeFlag = 3;
+						typeFlag = 4;
 						break;
 				}
 				this.mem.setUint32(addr + 4, nanHead | typeFlag, true);
-				this.mem.setUint32(addr, ref, true);
+				this.mem.setUint32(addr, id, true);
 			}
 
 			const loadSlice = (addr) => {
@@ -263,7 +268,9 @@
 						this.exited = true;
 						delete this._inst;
 						delete this._values;
-						delete this._refs;
+						delete this._goRefCounts;
+						delete this._ids;
+						delete this._idPool;
 						this.exit(code);
 					},
 
@@ -323,6 +330,18 @@
 						crypto.getRandomValues(loadSlice(sp + 8));
 					},
 
+					// func finalizeRef(v ref)
+					"syscall/js.finalizeRef": (sp) => {
+						const id = this.mem.getUint32(sp + 8, true);
+						this._goRefCounts[id]--;
+						if (this._goRefCounts[id] === 0) {
+							const v = this._values[id];
+							this._values[id] = null;
+							this._ids.delete(v);
+							this._idPool.push(id);
+						}
+					},
+
 					// func stringVal(value string) ref
 					"syscall/js.stringVal": (sp) => {
 						storeValue(sp + 24, loadString(sp + 8));
@@ -462,7 +481,7 @@
 		async run(instance) {
 			this._inst = instance;
 			this.mem = new DataView(this._inst.exports.mem.buffer);
-			this._values = [ // TODO: garbage collection
+			this._values = [ // JS values that Go currently has references to, indexed by reference id
 				NaN,
 				0,
 				null,
@@ -471,8 +490,10 @@
 				global,
 				this,
 			];
-			this._refs = new Map();
-			this.exited = false;
+			this._goRefCounts = []; // number of references that Go has to a JS value, indexed by reference id
+			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
 
 			// Pass command line arguments and environment variables to WebAssembly by writing them to the linear memory.
 			let offset = 4096;

src/net/http/roundtrip_js.go

@@ -41,7 +41,7 @@ const jsFetchCreds = "js.fetch:credentials"
 // Reference: https://developer.mozilla.org/en-US/docs/Web/API/WindowOrWorkerGlobalScope/fetch#Parameters
 const jsFetchRedirect = "js.fetch:redirect"
 
-var useFakeNetwork = js.Global().Get("fetch") == js.Undefined()
+var useFakeNetwork = js.Global().Get("fetch").IsUndefined()
 
 // RoundTrip implements the RoundTripper interface using the WHATWG Fetch API.
 func (t *Transport) RoundTrip(req *Request) (*Response, error) {
@@ -50,7 +50,7 @@ func (t *Transport) RoundTrip(req *Request) (*Response, error) {
 	}
 
 	ac := js.Global().Get("AbortController")
-	if ac != js.Undefined() {
+	if !ac.IsUndefined() {
 		// Some browsers that support WASM don't necessarily support
 		// the AbortController. See
 		// https://developer.mozilla.org/en-US/docs/Web/API/AbortController#Browser_compatibility.
@@ -74,7 +74,7 @@ func (t *Transport) RoundTrip(req *Request) (*Response, error) {
 		opt.Set("redirect", h)
 		req.Header.Del(jsFetchRedirect)
 	}
-	if ac != js.Undefined() {
+	if !ac.IsUndefined() {
 		opt.Set("signal", ac.Get("signal"))
 	}
 	headers := js.Global().Get("Headers").New()
@@ -132,7 +132,7 @@ func (t *Transport) RoundTrip(req *Request) (*Response, error) {
 		var body io.ReadCloser
 		// The body is undefined when the browser does not support streaming response bodies (Firefox),
 		// and null in certain error cases, i.e. when the request is blocked because of CORS settings.
-		if b != js.Undefined() && b != js.Null() {
+		if !b.IsUndefined() && !b.IsNull() {
 			body = &streamReader{stream: b.Call("getReader")}
 		} else {
 			// Fall back to using ArrayBuffer
@@ -168,7 +168,7 @@ func (t *Transport) RoundTrip(req *Request) (*Response, error) {
 	respPromise.Call("then", success, failure)
 	select {
 	case <-req.Context().Done():
-		if ac != js.Undefined() {
+		if !ac.IsUndefined() {
 			// Abort the Fetch request
 			ac.Call("abort")
 		}

src/syscall/fs_js.go

@@ -259,7 +259,7 @@ func Lchown(path string, uid, gid int) error {
 	if err := checkPath(path); err != nil {
 		return err
 	}
-	if jsFS.Get("lchown") == js.Undefined() {
+	if jsFS.Get("lchown").IsUndefined() {
 		// fs.lchown is unavailable on Linux until Node.js 10.6.0
 		// TODO(neelance): remove when we require at least this Node.js version
 		return ENOSYS
@@ -497,7 +497,7 @@ func fsCall(name string, args ...interface{}) (js.Value, error) {
 		var res callResult
 
 		if len(args) >= 1 { // on Node.js 8, fs.utimes calls the callback without any arguments
-			if jsErr := args[0]; jsErr != js.Null() {
+			if jsErr := args[0]; !jsErr.IsNull() {
 				res.err = mapJSError(jsErr)
 			}
 		}

src/syscall/js/export_test.go

+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build js,wasm
+
+package js
+
+var JSGo = jsGo

src/syscall/js/func.go

@@ -64,7 +64,7 @@ func init() {
 
 func handleEvent() {
 	cb := jsGo.Get("_pendingEvent")
-	if cb == Null() {
+	if cb.IsNull() {
 		return
 	}
 	jsGo.Set("_pendingEvent", Null())

src/syscall/js/js_js.s

@@ -4,6 +4,10 @@
 
 #include "textflag.h"
 
+TEXT ·finalizeRef(SB), NOSPLIT, $0
+  CallImport
+  RET
+
 TEXT ·stringVal(SB), NOSPLIT, $0
   CallImport
   RET

src/syscall/js/js_test.go

@@ -18,6 +18,7 @@ package js_test
 import (
 	"fmt"
 	"math"
+	"runtime"
 	"syscall/js"
 	"testing"
 )
@@ -53,7 +54,7 @@ func TestBool(t *testing.T) {
 	if got := dummys.Get("otherBool").Bool(); got != want {
 		t.Errorf("got %#v, want %#v", got, want)
 	}
-	if dummys.Get("someBool") != dummys.Get("someBool") {
+	if !dummys.Get("someBool").Equal(dummys.Get("someBool")) {
 		t.Errorf("same value not equal")
 	}
 }
@@ -68,7 +69,7 @@ func TestString(t *testing.T) {
 	if got := dummys.Get("otherString").String(); got != want {
 		t.Errorf("got %#v, want %#v", got, want)
 	}
-	if dummys.Get("someString") != dummys.Get("someString") {
+	if !dummys.Get("someString").Equal(dummys.Get("someString")) {
 		t.Errorf("same value not equal")
 	}
 
@@ -105,7 +106,7 @@ func TestInt(t *testing.T) {
 	if got := dummys.Get("otherInt").Int(); got != want {
 		t.Errorf("got %#v, want %#v", got, want)
 	}
-	if dummys.Get("someInt") != dummys.Get("someInt") {
+	if !dummys.Get("someInt").Equal(dummys.Get("someInt")) {
 		t.Errorf("same value not equal")
 	}
 	if got := dummys.Get("zero").Int(); got != 0 {
@@ -141,20 +142,20 @@ func TestFloat(t *testing.T) {
 	if got := dummys.Get("otherFloat").Float(); got != want {
 		t.Errorf("got %#v, want %#v", got, want)
 	}
-	if dummys.Get("someFloat") != dummys.Get("someFloat") {
+	if !dummys.Get("someFloat").Equal(dummys.Get("someFloat")) {
 		t.Errorf("same value not equal")
 	}
 }
 
 func TestObject(t *testing.T) {
-	if dummys.Get("someArray") != dummys.Get("someArray") {
+	if !dummys.Get("someArray").Equal(dummys.Get("someArray")) {
 		t.Errorf("same value not equal")
 	}
 
 	// An object and its prototype should not be equal.
 	proto := js.Global().Get("Object").Get("prototype")
 	o := js.Global().Call("eval", "new Object()")
-	if proto == o {
+	if proto.Equal(o) {
 		t.Errorf("object equals to its prototype")
 	}
 }
@@ -167,26 +168,66 @@ func TestFrozenObject(t *testing.T) {
 	}
 }
 
+func TestEqual(t *testing.T) {
+	if !dummys.Get("someFloat").Equal(dummys.Get("someFloat")) {
+		t.Errorf("same float is not equal")
+	}
+	if !dummys.Get("emptyObj").Equal(dummys.Get("emptyObj")) {
+		t.Errorf("same object is not equal")
+	}
+	if dummys.Get("someFloat").Equal(dummys.Get("someInt")) {
+		t.Errorf("different values are not unequal")
+	}
+}
+
 func TestNaN(t *testing.T) {
-	want := js.ValueOf(math.NaN())
-	got := dummys.Get("NaN")
-	if got != want {
-		t.Errorf("got %#v, want %#v", got, want)
+	if !dummys.Get("NaN").IsNaN() {
+		t.Errorf("JS NaN is not NaN")
+	}
+	if !js.ValueOf(math.NaN()).IsNaN() {
+		t.Errorf("Go NaN is not NaN")
+	}
+	if dummys.Get("NaN").Equal(dummys.Get("NaN")) {
+		t.Errorf("NaN is equal to NaN")
 	}
 }
 
 func TestUndefined(t *testing.T) {
-	dummys.Set("test", js.Undefined())
-	if dummys == js.Undefined() || dummys.Get("test") != js.Undefined() || dummys.Get("xyz") != js.Undefined() {
-		t.Errorf("js.Undefined expected")
+	if !js.Undefined().IsUndefined() {
+		t.Errorf("undefined is not undefined")
+	}
+	if !js.Undefined().Equal(js.Undefined()) {
+		t.Errorf("undefined is not equal to undefined")
+	}
+	if dummys.IsUndefined() {
+		t.Errorf("object is undefined")
+	}
+	if js.Undefined().IsNull() {
+		t.Errorf("undefined is null")
+	}
+	if dummys.Set("test", js.Undefined()); !dummys.Get("test").IsUndefined() {
+		t.Errorf("could not set undefined")
 	}
 }
 
 func TestNull(t *testing.T) {
-	dummys.Set("test1", nil)
-	dummys.Set("test2", js.Null())
-	if dummys == js.Null() || dummys.Get("test1") != js.Null() || dummys.Get("test2") != js.Null() {
-		t.Errorf("js.Null expected")
+	if !js.Null().IsNull() {
+		t.Errorf("null is not null")
+	}
+	if !js.Null().Equal(js.Null()) {
+		t.Errorf("null is not equal to null")
+	}
+	if dummys.IsNull() {
+		t.Errorf("object is null")
+	}
+	if js.Null().IsUndefined() {
+		t.Errorf("null is undefined")
+	}
+	if dummys.Set("test", js.Null()); !dummys.Get("test").IsNull() {
+		t.Errorf("could not set null")
+	}
+	if dummys.Set("test", nil); !dummys.Get("test").IsNull() {
+		t.Errorf("could not set nil")
 	}
 }
 
@@ -340,7 +381,7 @@ func TestValueOf(t *testing.T) {
 
 func TestZeroValue(t *testing.T) {
 	var v js.Value
-	if v != js.Undefined() {
+	if !v.IsUndefined() {
 		t.Error("zero js.Value is not js.Undefined()")
 	}
 }
@@ -497,12 +538,24 @@ func TestCopyBytesToJS(t *testing.T) {
 	}
 }
 
+func TestGarbageCollection(t *testing.T) {
+	before := js.JSGo.Get("_values").Length()
+	for i := 0; i < 1000; i++ {
+		_ = js.Global().Get("Object").New().Call("toString").String()
+		runtime.GC()
+	}
+	after := js.JSGo.Get("_values").Length()
+	if after-before > 500 {
+		t.Errorf("garbage collection ineffective")
+	}
+}
+
 // BenchmarkDOM is a simple benchmark which emulates a webapp making DOM operations.
 // It creates a div, and sets its id. Then searches by that id and sets some data.
 // Finally it removes that div.
 func BenchmarkDOM(b *testing.B) {
 	document := js.Global().Get("document")
-	if document == js.Undefined() {
+	if document.IsUndefined() {
 		b.Skip("Not a browser environment. Skipping.")
 	}
 	const data = "someString"