[d8] Make counters fully atomic

Counter updates were already atomic, but reading the counter values was
not. This lead to data races if one isolate called `quit` while other
isolates were still running.
This makes counters fully atomic, and reflects that by making the fields
{std::atomic<int>}.

R=mlippautz@chromium.org

Bug: v8:12481, v8:12482
Change-Id: I6fc78ad6461b93c4b3e87bec052b0a67694539e3
Cq-Include-Trybots: luci.v8.try:v8_linux64_tsan_rel_ng
Cq-Include-Trybots: luci.v8.try:v8_linux64_ubsan_rel_ng
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3320428Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78279}

src/d8/d8.cc

@@ -2721,19 +2721,16 @@ void Shell::ReportException(v8::Isolate* isolate, v8::TryCatch* try_catch) {
   ReportException(isolate, try_catch->Message(), try_catch->Exception());
 }
 
-int32_t* Counter::Bind(const char* name, bool is_histogram) {
-  int i;
-  for (i = 0; i < kMaxNameSize - 1 && name[i]; i++)
-    name_[i] = static_cast<char>(name[i]);
-  name_[i] = '\0';
+void Counter::Bind(const char* name, bool is_histogram) {
+  base::OS::StrNCpy(name_, kMaxNameSize, name, kMaxNameSize);
+  // Explicitly null-terminate, in case {name} is longer than {kMaxNameSize}.
+  name_[kMaxNameSize - 1] = '\0';
   is_histogram_ = is_histogram;
-  return ptr();
 }
 
-void Counter::AddSample(int32_t sample) {
-  base::Relaxed_AtomicIncrement(reinterpret_cast<base::Atomic32*>(&count_), 1);
-  base::Relaxed_AtomicIncrement(
-      reinterpret_cast<base::Atomic32*>(&sample_total_), sample);
+void Counter::AddSample(int sample) {
+  count_.fetch_add(1, std::memory_order_relaxed);
+  sample_total_.fetch_add(sample, std::memory_order_relaxed);
 }
 
 CounterCollection::CounterCollection() {

src/d8/d8.h

@@ -45,18 +45,24 @@ struct DynamicImportData;
 class Counter {
  public:
   static const int kMaxNameSize = 64;
-  int32_t* Bind(const char* name, bool histogram);
-  int32_t* ptr() { return &count_; }
-  int32_t count() { return count_; }
-  int32_t sample_total() { return sample_total_; }
-  bool is_histogram() { return is_histogram_; }
+  void Bind(const char* name, bool histogram);
+  // TODO(12482): Return pointer to an atomic.
+  int* ptr() {
+    STATIC_ASSERT(sizeof(int) == sizeof(count_));
+    return reinterpret_cast<int*>(&count_);
+  }
+  int count() const { return count_.load(std::memory_order_relaxed); }
+  int sample_total() const {
+    return sample_total_.load(std::memory_order_relaxed);
+  }
+  bool is_histogram() const { return is_histogram_; }
   void AddSample(int32_t sample);
 
  private:
-  int32_t count_;
-  int32_t sample_total_;
+  std::atomic<int> count_;
+  std::atomic<int> sample_total_;
   bool is_histogram_;
-  uint8_t name_[kMaxNameSize];
+  char name_[kMaxNameSize];
 };
 
 // A set of counters and associated information.  An instance of this