cppgc: std::move of base::optional doesn't reset

Chromium builds indicate that moving an optional doesn't reset the
source, and the source still indicates it has a value.
That may be a bug in base::optional, but we should fix it here first to
resolve current crashes.

Bug: chromium:1154636
Cq-Include-Trybots: luci.v8.try:v8_linux_blink_rel
Change-Id: Ibfb53b6d06d5f0310e68b200cc27ca318a5a57e5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3366235Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Omer Katz <omerkatz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78475}

src/heap/cppgc-js/cpp-heap.cc

@@ -304,12 +304,16 @@ bool CppHeap::MetricRecorderAdapter::MetricsReportPending() const {
 
 const base::Optional<cppgc::internal::MetricRecorder::FullCycle>
 CppHeap::MetricRecorderAdapter::ExtractLastFullGcEvent() {
-  return std::move(last_full_gc_event_);
+  auto res = std::move(last_full_gc_event_);
+  last_full_gc_event_.reset();
+  return res;
 }
 
 const base::Optional<cppgc::internal::MetricRecorder::MainThreadIncrementalMark>
 CppHeap::MetricRecorderAdapter::ExtractLastIncrementalMarkEvent() {
-  return std::move(last_incremental_mark_event_);
+  auto res = std::move(last_incremental_mark_event_);
+  last_incremental_mark_event_.reset();
+  return res;
 }
 
 void CppHeap::MetricRecorderAdapter::ClearCachedEvents() {
@@ -427,6 +431,10 @@ bool ShouldReduceMemory(CppHeap::GarbageCollectionFlags flags) {
 void CppHeap::InitializeTracing(GarbageCollectionFlags gc_flags) {
   CHECK(!sweeper_.IsSweepingInProgress());
 
+  // Check that previous cycle metrics have been reported.
+  DCHECK_IMPLIES(GetMetricRecorder(),
+                 !GetMetricRecorder()->MetricsReportPending());
+
 #if defined(CPPGC_YOUNG_GENERATION)
   cppgc::internal::SequentialUnmarker unmarker(raw_heap());
 #endif  // defined(CPPGC_YOUNG_GENERATION)

src/heap/gc-tracer.cc

@@ -1403,6 +1403,7 @@ void GCTracer::ReportFullCycleToRecorder() {
         optional_cppgc_event =
             cpp_heap->GetMetricRecorder()->ExtractLastFullGcEvent();
     DCHECK(optional_cppgc_event.has_value());
+    DCHECK(!cpp_heap->GetMetricRecorder()->MetricsReportPending());
     const cppgc::internal::MetricRecorder::FullCycle& cppgc_event =
         optional_cppgc_event.value();
     CopyTimeMetrics(event.total_cpp, cppgc_event.total);