[snapshot] Add serializer option to reconstruct the RO object cache

This option is used for --stress-snapshot when sharing the RO heap. The
RO heap is shared, so deserializing non-startup snapshots need to ensure
they use the same RO object cache mapping as the startup snapshot.

Cq-Include-Trybots: luci.v8.try:v8_linux64_gc_stress_custom_snapshot_dbg_ng
Bug: v8:11750
Change-Id: Ia2baa24d5b7d494ef5b7ff3c9cbcee846881182e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2897945
Reviewed-by: Dan Elphick <delphick@chromium.org>
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#74574}

src/heap/read-only-heap.cc

@@ -249,6 +249,10 @@ bool ReadOnlyHeap::read_only_object_cache_is_initialized() const {
   return read_only_object_cache_.size() > 0;
 }
 
+size_t ReadOnlyHeap::read_only_object_cache_size() const {
+  return read_only_object_cache_.size();
+}
+
 ReadOnlyHeapObjectIterator::ReadOnlyHeapObjectIterator(ReadOnlyHeap* ro_heap)
     : ReadOnlyHeapObjectIterator(ro_heap->read_only_space()) {}
 

src/heap/read-only-heap.h

@@ -81,6 +81,7 @@ class ReadOnlyHeap {
   // Returns a read-only cache entry at a particular index.
   Object cached_read_only_object(size_t i) const;
   bool read_only_object_cache_is_initialized() const;
+  size_t read_only_object_cache_size() const;
 
   ReadOnlySpace* read_only_space() const { return read_only_space_; }
 

src/snapshot/read-only-serializer.cc

@@ -74,6 +74,10 @@ void ReadOnlySerializer::SerializeReadOnlyRoots() {
                 isolate()->handle_scope_implementer()->blocks()->empty());
 
   ReadOnlyRoots(isolate()).Iterate(this);
+
+  if (reconstruct_read_only_object_cache_for_testing()) {
+    ReconstructReadOnlyObjectCacheForTesting();
+  }
 }
 
 void ReadOnlySerializer::FinalizeSerialization() {
@@ -129,5 +133,18 @@ bool ReadOnlySerializer::SerializeUsingReadOnlyObjectCache(
   return true;
 }
 
+void ReadOnlySerializer::ReconstructReadOnlyObjectCacheForTesting() {
+  ReadOnlyHeap* ro_heap = isolate()->read_only_heap();
+  DCHECK(ro_heap->read_only_object_cache_is_initialized());
+  for (size_t i = 0, size = ro_heap->read_only_object_cache_size(); i < size;
+       i++) {
+    Handle<HeapObject> obj(
+        HeapObject::cast(ro_heap->cached_read_only_object(i)), isolate());
+    int cache_index = SerializeInObjectCache(obj);
+    USE(cache_index);
+    DCHECK_EQ(cache_index, i);
+  }
+}
+
 }  // namespace internal
 }  // namespace v8

src/snapshot/read-only-serializer.h

@@ -37,6 +37,8 @@ class V8_EXPORT_PRIVATE ReadOnlySerializer : public RootsSerializer {
                                          Handle<HeapObject> obj);
 
  private:
+  void ReconstructReadOnlyObjectCacheForTesting();
+
   void SerializeObjectImpl(Handle<HeapObject> o) override;
   bool MustBeDeferred(HeapObject object) override;
 

src/snapshot/serializer.h

@@ -298,6 +298,10 @@ class Serializer : public SerializerDeserializer {
     return (flags_ & Snapshot::kAllowActiveIsolateForTesting) != 0;
   }
 
+  bool reconstruct_read_only_object_cache_for_testing() const {
+    return (flags_ & Snapshot::kReconstructReadOnlyObjectCacheForTesting) != 0;
+  }
+
  private:
   // A circular queue of hot objects. This is added to in the same order as in
   // Deserializer::HotObjectsList, but this stores the objects as an array of

src/snapshot/snapshot.cc

@@ -286,7 +286,10 @@ void Snapshot::SerializeDeserializeAndVerifyForTesting(
 
     Snapshot::SerializerFlags flags(
         Snapshot::kAllowUnknownExternalReferencesForTesting |
-        Snapshot::kAllowActiveIsolateForTesting);
+        Snapshot::kAllowActiveIsolateForTesting |
+        (ReadOnlyHeap::IsReadOnlySpaceShared()
+             ? Snapshot::kReconstructReadOnlyObjectCacheForTesting
+             : 0));
     serialized_data = Snapshot::Create(isolate, *default_context, no_gc, flags);
     auto_delete_serialized_data.reset(serialized_data.data);
   }

src/snapshot/snapshot.h

@@ -36,6 +36,15 @@ class Snapshot : public AllStatic {
     // after deserialization.
     // If unset, we assert that these previously mentioned areas are empty.
     kAllowActiveIsolateForTesting = 1 << 1,
+    // If set, the ReadOnlySerializer reconstructs the read-only object cache
+    // from the existing ReadOnlyHeap's read-only object cache so the same
+    // mapping is used.  This mode is used for testing deserialization of a
+    // snapshot from a live isolate that's using a shared
+    // ReadOnlyHeap. Otherwise during deserialization the indices will mismatch,
+    // causing deserialization crashes when e.g. types mismatch.
+    // If unset, the read-only object cache is populated as read-only objects
+    // are serialized.
+    kReconstructReadOnlyObjectCacheForTesting = 1 << 2,
   };
   using SerializerFlags = base::Flags<SerializerFlag>;
   V8_EXPORT_PRIVATE static constexpr SerializerFlags kDefaultSerializerFlags =