cppgc: young-gen: Unmark the heap on terminate()

When the heap is gracefully terminated, all the destructors are called.
CrossThreadPersistents must clear the back references (references from
CrossThreadPersistentRegion) so that further GCs on other threads will
not access freed CTPs.

To force destruction with young-gen enabled, the CL unmarks the heap on
termination.

Bug: chromium:1029379
Change-Id: I7f4a34a914ca20b50fe6d2ad493d56e0ba525ecc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3568473Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Anton Bikineev <bikineev@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79780}

src/heap/cppgc/heap-base.cc

@@ -19,6 +19,7 @@
 #include "src/heap/cppgc/platform.h"
 #include "src/heap/cppgc/prefinalizer-handler.h"
 #include "src/heap/cppgc/stats-collector.h"
+#include "src/heap/cppgc/unmarker.h"
 
 namespace cppgc {
 namespace internal {
@@ -153,6 +154,7 @@ void HeapBase::Terminate() {
   constexpr size_t kMaxTerminationGCs = 20;
   size_t gc_count = 0;
   bool more_termination_gcs_needed = false;
+
   do {
     CHECK_LT(gc_count++, kMaxTerminationGCs);
 
@@ -165,6 +167,13 @@ void HeapBase::Terminate() {
       weak_cross_thread_persistent_region_.ClearAllUsedNodes();
     }
 
+#if defined(CPPGC_YOUNG_GENERATION)
+    // Unmark the heap so that the sweeper destructs all objects.
+    // TODO(chromium:1029379): Merge two heap iterations (unmarking + sweeping)
+    // into forced finalization.
+    SequentialUnmarker unmarker(raw_heap());
+#endif  // defined(CPPGC_YOUNG_GENERATION)
+
     in_atomic_pause_ = true;
     stats_collector()->NotifyMarkingStarted(
         GarbageCollector::Config::CollectionType::kMajor,
@@ -172,6 +181,8 @@ void HeapBase::Terminate() {
     object_allocator().ResetLinearAllocationBuffers();
     stats_collector()->NotifyMarkingCompleted(0);
     ExecutePreFinalizers();
+    // TODO(chromium:1029379): Prefinalizers may black-allocate objects (under a
+    // compile-time option). Run sweeping with forced finalization here.
     sweeper().Start(
         {Sweeper::SweepingConfig::SweepingType::kAtomic,
          Sweeper::SweepingConfig::CompactableSpaceHandling::kSweep});