From 5674c35e1477e21eaafab3a51241d84e47a679dd Mon Sep 17 00:00:00 2001
From: Ayke van Laethem <aykevanlaethem@gmail.com>
Date: Mon, 6 Apr 2020 23:12:41 +0200
Subject: wasm: backport "garbage collect references to JavaScript values"

See commit:
https://github.com/golang/go/commit/54e6ba6724dfde355070238f9abc16362cac2e3d

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.
---
 targets/wasm_exec.js | 40 +++++++++++++++++++++++++++-------------
 1 file changed, 27 insertions(+), 13 deletions(-)

(limited to 'targets/wasm_exec.js')

diff --git a/targets/wasm_exec.js b/targets/wasm_exec.js
index 85f147cd0..4c8790667 100644
--- a/targets/wasm_exec.js
+++ b/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 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;
 				}
 				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)
 
-- 
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