Avoid constructing the payload until we need it for reflection.

This saves memory and CPU when the map fields are only used as maps.

PiperOrigin-RevId: 695449397

csharp/src/Google.Protobuf/Reflection/FeatureSetDescriptor.g.cs

@@ -1,17 +0,0 @@
-#region Copyright notice and license
-// Protocol Buffers - Google's data interchange format
-// Copyright 2008 Google Inc.  All rights reserved.
-//
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file or at
-// https://developers.google.com/open-source/licenses/bsd
-#endregion
-
-namespace Google.Protobuf.Reflection;
-
-internal sealed partial class FeatureSetDescriptor
-{
-    // Canonical serialized form of the edition defaults, generated by embed_edition_defaults.
-    private const string DefaultsBase64 =
-        "ChMYhAciACoMCAEQAhgCIAMoATACChMY5wciACoMCAIQARgBIAIoATABChMY6AciDAgBEAEYASACKAEwASoAIOYHKOgH";
-}

src/google/protobuf/map_field.cc

@@ -158,10 +158,15 @@ size_t MapFieldBase::SpaceUsedExcludingSelfLong() const {
   ConstAccess();
   size_t size = 0;
   if (auto* p = maybe_payload()) {
-    {
-      absl::MutexLock lock(&p->mutex);
-      size = SpaceUsedExcludingSelfNoLock();
-    }
+    absl::MutexLock lock(&p->mutex);
+    // Measure the map under the lock, because there could be some repeated
+    // field data that might be sync'd back into the map.
+    size = SpaceUsedExcludingSelfNoLock();
+    size += p->repeated_field.SpaceUsedExcludingSelfLong();
+    ConstAccess();
+  } else {
+    // Only measure the map without the repeated field, because it is not there.
+    size = SpaceUsedExcludingSelfNoLock();
     ConstAccess();
   }
   return size;
@@ -179,13 +184,6 @@ bool MapFieldBase::IsRepeatedFieldValid() const {
   return state() != STATE_MODIFIED_MAP;
 }
 
-void MapFieldBase::SetMapDirty() {
-  MutableAccess();
-  // These are called by (non-const) mutator functions. So by our API it's the
-  // callers responsibility to have these calls properly ordered.
-  payload().state.store(STATE_MODIFIED_MAP, std::memory_order_relaxed);
-}
-
 void MapFieldBase::SetRepeatedDirty() {
   MutableAccess();
   // These are called by (non-const) mutator functions. So by our API it's the

src/google/protobuf/map_field.h

@@ -410,7 +410,15 @@ class PROTOBUF_EXPORT MapFieldBase : public MapFieldBaseForParse {
   static void UnsafeShallowSwapImpl(MapFieldBase& lhs, MapFieldBase& rhs);
 
   // Tells MapFieldBase that there is new change to Map.
-  void SetMapDirty();
+  void SetMapDirty() {
+    MutableAccess();
+    // These are called by (non-const) mutator functions. So by our API it's the
+    // callers responsibility to have these calls properly ordered.
+    if (auto* p = maybe_payload()) {
+      // If we don't have a payload, it is already assumed `STATE_MODIFIED_MAP`.
+      p->state.store(STATE_MODIFIED_MAP, std::memory_order_relaxed);
+    }
+  }
 
   // Tells MapFieldBase that there is new change to RepeatedPtrField.
   void SetRepeatedDirty();

src/google/protobuf/map_field_inl.h

@@ -155,18 +155,13 @@ void TypeDefinedMapFieldBase<Key, T>::MergeFromImpl(MapFieldBase& base,
 template <typename Key, typename T>
 size_t TypeDefinedMapFieldBase<Key, T>::SpaceUsedExcludingSelfNoLockImpl(
     const MapFieldBase& map) {
-  auto& self = static_cast<const TypeDefinedMapFieldBase&>(map);
-  size_t size = 0;
-  if (auto* p = self.maybe_payload()) {
-    size += p->repeated_field.SpaceUsedExcludingSelfLong();
-  }
   // We can't compile this expression for DynamicMapField even though it is
   // never used at runtime, so disable it at compile time.
-  std::get<std::is_same<Map<Key, T>, Map<MapKey, MapValueRef>>::value>(
-      std::make_tuple(
-          [&](const auto& map) { size += map.SpaceUsedExcludingSelfLong(); },
-          [](const auto&) {}))(self.map_);
-  return size;
+  if constexpr (!std::is_same<Map<Key, T>, Map<MapKey, MapValueRef>>::value) {
+    return static_cast<const TypeDefinedMapFieldBase&>(map)
+        .map_.SpaceUsedExcludingSelfLong();
+  }
+  return 0;
 }
 
 template <typename Key, typename T>

src/google/protobuf/map_field_test.cc

@@ -452,6 +452,18 @@ TEST(MapFieldTest, ConstInit) {
   EXPECT_EQ(field.size(), 0);
 }
 
+TEST(MapFieldTest, MutableMapDoesNotAllocatePayload) {
+  struct MaybePayload : MapFieldBase {
+    // Use a derived type to get access to the protected method.
+    // We steal the function pointer here to use below to inspect the instance.
+    static constexpr auto getter() { return &MaybePayload::maybe_payload; }
+  };
+  MyMapField field;
+  EXPECT_FALSE((field.*MaybePayload::getter())());
+  field.MutableMap();
+  EXPECT_FALSE((field.*MaybePayload::getter())());
+}
+
 
 }  // namespace internal
 }  // namespace protobuf