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Use Symbols to store JS object references when serialising and delete ObjectIDCache

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When serialising JS objects, we keep track of any encountered object,
and serialize it only once. Any further serialisation instead stores an
ID referring to the original object (essentially an opaque pointer).
The trouble of course is to have a unique, persistent identifier for
such an object.
svn uses an ObjectIDCache, essentially a "JS Object -> ID" map (which
internally is essentially a "JS heap pointer -> ID" map).

JS, since ES15, includes a "Symbol" primitive type, which is a unique,
immutable identifier. They are also not iterable by for..in or
GetOwnPropertyName or related.
This means they can be used to store the tag directly on the object
(since it's impossible overwrite a user property).
Thanks to this, we can forgo ObjectIDCache in the serializers, and since
following D2897 it becomes unused, we can delete it, along with the
Finalization code it used.


Part of SM52 migration, stage: SM45-compatible changes.

Patch by: Itms
Tested By: Freagarach
Refs #4893

Differential Revision: https://code.wildfiregames.com/D3085
This was SVN commit r24167.
This commit is contained in:
wraitii
2020-11-12 06:40:19 +00:00
parent d5bbf900bc
commit dad2857538
12 changed files with 52 additions and 278 deletions
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source/scriptinterface/tests/test_ObjectToIDMap.h
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/* Copyright (C) 2016 Wildfire Games.
* This file is part of 0 A.D.
*
* 0 A.D. is free software: you can redistribute it and/or modify
* it under the terms of the GNU General Public License as published by
* the Free Software Foundation, either version 2 of the License, or
* (at your option) any later version.
*
* 0 A.D. is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* GNU General Public License for more details.
*
* You should have received a copy of the GNU General Public License
* along with 0 A.D. If not, see <http://www.gnu.org/licenses/>.
*/
#include "lib/self_test.h"
#include "scriptinterface/ScriptInterface.h"
#include "scriptinterface/ScriptRuntime.h"
#include "scriptinterface/third_party/ObjectToIDMap.h"
class TestObjectToIDMap : public CxxTest::TestSuite
{
public:
void test_movinggc()
{
ScriptInterface script("Test", "Test", g_ScriptRuntime);
JSContext* cx = script.GetContext();
JSAutoRequest rq(cx);
JS::RootedObject obj(cx, JS_NewPlainObject(cx));
ObjectIdCache<u32> map(g_ScriptRuntime);
map.init();
TS_ASSERT(map.add(cx, obj, 1));
JSObject* plainObj = obj;
// The map should contain the object we've just added
TS_ASSERT(map.has(plainObj));
JS_GC(g_ScriptRuntime->m_rt);
// After a GC, the object should have been moved and plainObj should
// not be valid anymore and not be found in the map anymore.
// Obj should have an updated reference too, so it should still be found
// in the map.
//
// NOTE: It's observed behaviour that a full GC always moves an object.
// This might change in future SpiderMonkey versions. We only rely on
// that behaviour for this test.
//
// TODO: It might be a good idea to test the behaviour when only a minor
// GC runs, but there's no API for calling a minor GC yet.
TS_ASSERT(plainObj != obj);
TS_ASSERT(!map.has(plainObj));
TS_ASSERT(map.has(obj));
// Finding the ID associated with the object
u32 ret(0);
TS_ASSERT(map.find(obj, ret));
TS_ASSERT_EQUALS(ret, 1);
}
};