LibJS: Add Cell::must_survive_garbage_collection() mechanism

This allows cells to prevent themselves from being garbage collected,
even when there are no references to them.

Userland/Libraries/LibJS/Heap/Cell.h

@@ -1,11 +1,12 @@
 /*
- * Copyright (c) 2020, Andreas Kling <kling@serenityos.org>
+ * Copyright (c) 2020-2022, Andreas Kling <kling@serenityos.org>
  *
  * SPDX-License-Identifier: BSD-2-Clause
  */
 
 #pragma once
 
+#include <AK/Badge.h>
 #include <AK/Format.h>
 #include <AK/Forward.h>
 #include <AK/Noncopyable.h>
@@ -80,15 +81,25 @@ public:
     // This will be called on unmarked objects by the garbage collector in a separate pass before destruction.
     virtual void finalize() { }
 
+    // This allows cells to survive GC by choice, even if nothing points to them.
+    // It's used to implement special rules in the web platform.
+    // NOTE: Cells must call set_overrides_must_survive_garbage_collection() for this to be honored.
+    virtual bool must_survive_garbage_collection() const { return false; }
+
+    bool overrides_must_survive_garbage_collection(Badge<Heap>) const { return m_overrides_must_survive_garbage_collection; }
+
     Heap& heap() const;
     VM& vm() const;
 
 protected:
     Cell() = default;
 
+    void set_overrides_must_survive_garbage_collection(bool b) { m_overrides_must_survive_garbage_collection = b; }
+
 private:
     bool m_mark : 1 { false };
-    State m_state : 7 { State::Live };
+    bool m_overrides_must_survive_garbage_collection { false };
+    State m_state : 1 { State::Live };
 };
 
 }

Userland/Libraries/LibJS/Heap/Heap.cpp

@@ -232,11 +232,18 @@ void Heap::mark_live_cells(HashTable<Cell*> const& roots)
     m_uprooted_cells.clear();
 }
 
+bool Heap::cell_must_survive_garbage_collection(Cell const& cell)
+{
+    if (!cell.overrides_must_survive_garbage_collection({}))
+        return false;
+    return cell.must_survive_garbage_collection();
+}
+
 void Heap::finalize_unmarked_cells()
 {
     for_each_block([&](auto& block) {
         block.template for_each_cell_in_state<Cell::State::Live>([](Cell* cell) {
-            if (!cell->is_marked())
+            if (!cell->is_marked() && !cell_must_survive_garbage_collection(*cell))
                 cell->finalize();
         });
         return IterationDecision::Continue;
@@ -258,7 +265,7 @@ void Heap::sweep_dead_cells(bool print_report, Core::ElapsedTimer const& measure
         bool block_has_live_cells = false;
         bool block_was_full = block.is_full();
         block.template for_each_cell_in_state<Cell::State::Live>([&](Cell* cell) {
-            if (!cell->is_marked()) {
+            if (!cell->is_marked() && !cell_must_survive_garbage_collection(*cell)) {
                 dbgln_if(HEAP_DEBUG, "  ~ {}", cell);
                 block.deallocate(cell);
                 ++collected_cells;

Userland/Libraries/LibJS/Heap/Heap.h

@@ -79,6 +79,8 @@ public:
     void uproot_cell(Cell* cell);
 
 private:
+    static bool cell_must_survive_garbage_collection(Cell const&);
+
     Cell* allocate_cell(size_t);
 
     void gather_roots(HashTable<Cell*>&);