wasm: backport "garbage collect references to JavaScript values"

See commit:
54e6ba6724

Warning: this will drop support for Go 1.13 for WebAssembly targets!
I have modified the integration tests to specifically blacklist Go 1.13
instead of whitelisting any other version, to avoid accidentally not
testing WebAssembly.

main_test.go

@@ -13,12 +13,14 @@ import (
 	"path/filepath"
 	"runtime"
 	"sort"
+	"strings"
 	"sync"
 	"testing"
 	"time"
 
 	"github.com/tinygo-org/tinygo/builder"
 	"github.com/tinygo-org/tinygo/compileopts"
+	"github.com/tinygo-org/tinygo/goenv"
 )
 
 const TESTDATA = "testdata"
@@ -71,10 +73,21 @@ func TestCompiler(t *testing.T) {
 		t.Run("ARM64Linux", func(t *testing.T) {
 			runPlatTests("aarch64--linux-gnu", matches, t)
 		})
+		goVersion, err := builder.GorootVersionString(goenv.Get("GOROOT"))
+		if err != nil {
+			t.Error("could not get Go version:", err)
+			return
+		}
+		minorVersion := strings.Split(goVersion, ".")[1]
+		if minorVersion != "13" {
+			// WebAssembly tests fail on Go 1.13, so skip them there. Versions
+			// below that are also not supported but still seem to pass, so
+			// include them in the tests for now.
 			t.Run("WebAssembly", func(t *testing.T) {
 				runPlatTests("wasm", matches, t)
 			})
 		}
+	}
 }
 
 func runPlatTests(target string, matches []string, t *testing.T) {

targets/wasm_exec.js

@@ -202,26 +202,31 @@
 						return;
 				}
 
-				let ref = this._refs.get(v);
-				if (ref === undefined) {
-					ref = this._values.length;
-					this._values.push(v);
-					this._refs.set(v, ref);
-				}
-				let typeFlag = 0;
+				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);
+				}
+				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;
 				}
 				mem().setUint32(addr + 4, nanHead | typeFlag, true);
-				mem().setUint32(addr, ref, true);
+				mem().setUint32(addr, id, true);
 			}
 
 			const loadSlice = (array, len, cap) => {
@@ -284,6 +289,13 @@
 						setTimeout(this._inst.exports.go_scheduler, timeout);
 					},
 
+					// func finalizeRef(v ref)
+					"syscall/js.finalizeRef": (sp) => {
+						// Note: TinyGo does not support finalizers so this should never be
+						// called.
+						console.error('syscall/js.finalizeRef not implemented');
+					},
+
 					// func stringVal(value string) ref
 					"syscall/js.stringVal": (ret_ptr, value_ptr, value_len) => {
 						const s = loadString(value_ptr, value_len);
@@ -405,7 +417,7 @@
 
 		async run(instance) {
 			this._inst = instance;
-			this._values = [ // TODO: garbage collection
+			this._values = [ // JS values that Go currently has references to, indexed by reference id
 				NaN,
 				0,
 				null,
@@ -414,8 +426,10 @@
 				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
 
 			const mem = new DataView(this._inst.exports.memory.buffer)