[VM/Developer] Expose through the developer API a gc count for the isolate group that could potentially trigger execution of associated finalizers of objects that were collected.

TEST=reachability_test.dart

Change-Id: I27bdac49a4b7069ca428282d55d8ec572c9fc0e0
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/266424
Reviewed-by: Ben Konyi <bkonyi@google.com>
Commit-Queue: Siva Annamalai <asiva@google.com>

runtime/lib/developer.cc

@@ -9,6 +9,8 @@
 #include "vm/debugger.h"
 #include "vm/exceptions.h"
 #include "vm/flags.h"
+#include "vm/heap/heap.h"
+#include "vm/isolate.h"
 #include "vm/message.h"
 #include "vm/native_entry.h"
 #include "vm/object.h"
@@ -164,4 +166,16 @@ DEFINE_NATIVE_ENTRY(Developer_getIsolateIDFromSendPort, 0, 1) {
 #endif
 }
 
+DEFINE_NATIVE_ENTRY(Developer_reachability_barrier, 0, 0) {
+#if defined(PRODUCT)
+  return Smi::New(0);
+#else
+  IsolateGroup* isolate_group = thread->isolate_group();
+  ASSERT(isolate_group != nullptr);
+  Heap* heap = isolate_group->heap();
+  ASSERT(heap != nullptr);
+  return Integer::New(heap->ReachabilityBarrier());
+#endif
+}
+
 }  // namespace dart

runtime/tests/vm/dart/reachability_test.dart

@@ -0,0 +1,37 @@
+// Copyright (c) 2022, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+//
+// VMOptions=--new_gen_semi_max_size=1
+
+import 'dart:_internal' show VMInternalsForTesting;
+import 'dart:developer';
+import 'package:expect/expect.dart';
+
+var g_arr = List<List<int>?>.filled(1000, null);
+void main() {
+  var count = reachabilityBarrier;
+  var l_arr = List<List<int>?>.filled(1000, null);
+  for (int i = 0; i < 1000; i++) {
+    l_arr[i] = List<int>.filled(100, 0);
+  }
+  for (int j = 0; j < 1000; j++) {
+    for (int i = 0; i < 1000; i++) {
+      g_arr[i] = l_arr[i];
+    }
+    for (int i = 0; i < 1000; i++) {
+      l_arr[i] = List<int>.filled(100, 0);
+    }
+  }
+  VMInternalsForTesting.collectAllGarbage();
+  for (int j = 0; j < 1000; j++) {
+    for (int i = 0; i < 1000; i++) {
+      g_arr[i] = l_arr[i];
+    }
+    for (int i = 0; i < 1000; i++) {
+      l_arr[i] = List<int>.filled(100, 0);
+    }
+  }
+  VMInternalsForTesting.collectAllGarbage();
+  Expect.isTrue(reachabilityBarrier > count);
+}

runtime/vm/bootstrap_natives.h

@@ -80,6 +80,7 @@ namespace dart {
   V(Developer_log, 8)                                                          \
   V(Developer_postEvent, 2)                                                    \
   V(Developer_webServerControl, 3)                                             \
+  V(Developer_reachability_barrier, 0)                                         \
   V(Double_hashCode, 1)                                                        \
   V(Double_getIsNegative, 1)                                                   \
   V(Double_getIsInfinite, 1)                                                   \

runtime/vm/heap/heap.cc

@@ -58,6 +58,8 @@ Heap::Heap(IsolateGroup* isolate_group,
     old_weak_tables_[sel] = new WeakTable();
   }
   stats_.num_ = 0;
+  stats_.state_ = kInitial;
+  stats_.reachability_barrier_ = 0;
 }
 
 Heap::~Heap() {

runtime/vm/heap/heap.h

@@ -51,6 +51,20 @@ class Heap {
     kNumWeakSelectors
   };
 
+  // States for a state machine that represents the worst-case set of GCs
+  // that an unreachable object could survive before begin collected:
+  // a new-space object that is involved with a cycle with an old-space object
+  // is copied to survivor space, then promoted during concurrent marking,
+  // and finally proven unreachable in the next round of old-gen marking.
+  // We ignore the case of unreachable-but-not-yet-collected objects being
+  // made reachable again by allInstances.
+  enum LeakCountState {
+    kInitial = 0,
+    kFirstScavenge,
+    kSecondScavenge,
+    kMarkingStart,
+  };
+
   // Pattern for unused new space and swept old space.
   static const uint8_t kZapByte = 0xf3;
 
@@ -273,6 +287,8 @@ class Heap {
                                 JSONStream* stream) {
     old_space_.PrintHeapMapToJSONStream(isolate_group, stream);
   }
+
+  intptr_t ReachabilityBarrier() { return stats_.reachability_barrier_; }
 #endif  // PRODUCT
 
   IsolateGroup* isolate_group() const { return isolate_group_; }
@@ -296,6 +312,8 @@ class Heap {
     intptr_t num_;
     GCType type_;
     GCReason reason_;
+    LeakCountState state_;  // State to track finalization of GCed object.
+    intptr_t reachability_barrier_;  // Tracks reachability of GCed objects.
 
     class Data : public ValueObject {
      public:

runtime/vm/heap/marker.cc

@@ -479,9 +479,17 @@ class MarkingWeakVisitor : public HandleVisitor {
 
 void GCMarker::Prologue() {
   isolate_group_->ReleaseStoreBuffers();
+  if (heap_->stats_.state_ == Heap::kSecondScavenge) {
+    heap_->stats_.state_ = Heap::kMarkingStart;
+  }
 }
 
-void GCMarker::Epilogue() {}
+void GCMarker::Epilogue() {
+  if (heap_->stats_.state_ == Heap::kMarkingStart) {
+    heap_->stats_.state_ = Heap::kInitial;
+    heap_->stats_.reachability_barrier_ += 1;
+  }
+}
 
 enum RootSlices {
   kIsolate = 0,

runtime/vm/heap/scavenger.cc

@@ -1696,10 +1696,17 @@ void Scavenger::Scavenge(Thread* thread, GCType type, GCReason reason) {
   if (abort_) {
     ReverseScavenge(&from);
     bytes_promoted = 0;
-  } else if ((CapacityInWords() - UsedInWords()) < KBInWords) {
-    // Don't scavenge again until the next old-space GC has occurred. Prevents
-    // performing one scavenge per allocation as the heap limit is approached.
-    heap_->assume_scavenge_will_fail_ = true;
+  } else {
+    if (heap_->stats_.state_ == Heap::kInitial) {
+      heap_->stats_.state_ = Heap::kFirstScavenge;
+    } else if (heap_->stats_.state_ == Heap::kFirstScavenge) {
+      heap_->stats_.state_ = Heap::kSecondScavenge;
+    }
+    if ((CapacityInWords() - UsedInWords()) < KBInWords) {
+      // Don't scavenge again until the next old-space GC has occurred. Prevents
+      // performing one scavenge per allocation as the heap limit is approached.
+      heap_->assume_scavenge_will_fail_ = true;
+    }
   }
   ASSERT(promotion_stack_.IsEmpty());
   MournWeakHandles();

sdk/lib/_internal/js_dev_runtime/patch/developer_patch.dart

@@ -60,6 +60,9 @@ void log(String message,
   JS('', 'console.debug("dart.developer.log", #)', items);
 }
 
+@patch
+int get reachabilityBarrier => 0;
+
 final _extensions = <String, ServiceExtensionHandler>{};
 
 @patch

sdk/lib/_internal/js_runtime/lib/developer_patch.dart

@@ -35,6 +35,9 @@ void log(String message,
   // TODO.
 }
 
+@patch
+int get reachabilityBarrier => 0;
+
 final _extensions = <String, ServiceExtensionHandler>{};
 
 @patch

sdk/lib/_internal/vm/lib/developer.dart

@@ -50,6 +50,10 @@ void log(String message,
       error, stackTrace);
 }
 
+@patch
+@pragma("vm:external-name", "Developer_reachability_barrier")
+external int get reachabilityBarrier;
+
 int _nextSequenceNumber = 0;
 
 @pragma("vm:external-name", "Developer_log")

sdk/lib/_internal/wasm/lib/developer.dart

@@ -26,6 +26,9 @@ void log(String message,
     Object? error,
     StackTrace? stackTrace}) {}
 
+@patch
+int get reachabilityBarrier => 0;
+
 @patch
 bool get extensionStreamHasListener => false;
 

sdk/lib/developer/developer.dart

@@ -67,3 +67,18 @@ external void log(
   Object? error,
   StackTrace? stackTrace,
 });
+
+/// Current reachability barrier state.
+///
+/// A reachability barrier state that provides a way to synchronize on
+/// reachability. At value 'x', any object that became unreachable during
+/// 'value' < 'x' has been collected and any associated finalizers have been
+/// scheduled for execution, i.e. the non-execution of a finalizer reliably
+/// indicates the object is still reachable in the previous barrier state.
+///
+/// Objects that became unreachable in the current barrier state may have not
+/// yet been collected or finalized.
+///
+/// NOTE: There are no guarantees of forward progress. An implementation may
+/// return the same value forever for this barrier state.
+external int get reachabilityBarrier;