thread safe memoization

Firestore/core/src/core/composite_filter.cc

@@ -154,14 +154,14 @@ const model::FieldPath* CompositeFilter::Rep::GetFirstInequalityField() const {
 
 const std::vector<FieldFilter>& CompositeFilter::Rep::GetFlattenedFilters()
     const {
-  if (Filter::Rep::memoized_flattened_filters_.empty() && !filters().empty()) {
-    for (const auto& filter : filters()) {
+  return memoized_flattened_filters_->memoize([&]() {
+    std::vector<FieldFilter> flattened_filters;
+    for (const auto& filter : filters())
       std::copy(filter.GetFlattenedFilters().begin(),
                 filter.GetFlattenedFilters().end(),
-                std::back_inserter(Filter::Rep::memoized_flattened_filters_));
-    }
-  }
-  return Filter::Rep::memoized_flattened_filters_;
+                std::back_inserter(flattened_filters));
+    return flattened_filters;
+  });
 }
 
 }  // namespace core

Firestore/core/src/core/field_filter.cc

@@ -124,11 +124,10 @@ FieldFilter::FieldFilter(std::shared_ptr<const Filter::Rep> rep)
 
 const std::vector<FieldFilter>& FieldFilter::Rep::GetFlattenedFilters() const {
   // This is already a field filter, so we return a vector of size one.
-  if (Filter::Rep::memoized_flattened_filters_.empty()) {
-    Filter::Rep::memoized_flattened_filters_ = std::vector<FieldFilter>{
+  return memoized_flattened_filters_->memoize([&]() {
+    return std::vector<FieldFilter>{
         FieldFilter(std::make_shared<const Rep>(*this))};
-  }
-  return Filter::Rep::memoized_flattened_filters_;
+  });
 }
 
 std::vector<Filter> FieldFilter::Rep::GetFilters() const {

Firestore/core/src/core/filter.cc

@@ -18,6 +18,9 @@
 
 #include <ostream>
 
+#include "Firestore/core/src/core/field_filter.h"
+#include "Firestore/core/src/util/thread_safe_memoizer.h"
+
 namespace firebase {
 namespace firestore {
 namespace core {
@@ -32,6 +35,11 @@ std::ostream& operator<<(std::ostream& os, const Filter& filter) {
   return os << filter.ToString();
 }
 
+Filter::Rep::Rep()
+    : memoized_flattened_filters_(
+          std::make_shared<ThreadSafeMemoizer<FieldFilter>>()) {
+}
+
 }  // namespace core
 }  // namespace firestore
 }  // namespace firebase

Firestore/core/src/core/filter.h

@@ -23,10 +23,12 @@
 #include <vector>
 
 #include "Firestore/core/src/model/model_fwd.h"
+#include "Firestore/core/src/util/thread_safe_memoizer.h"
+
+using firebase::firestore::util::ThreadSafeMemoizer;
 
 namespace firebase {
 namespace firestore {
-
 namespace core {
 
 class FieldFilter;
@@ -122,6 +124,8 @@ class Filter {
  protected:
   class Rep {
    public:
+    Rep();
+
     virtual ~Rep() = default;
 
     virtual Type type() const {
@@ -162,8 +166,14 @@ class Filter {
     /**
      * Memoized list of all field filters that can be found by
      * traversing the tree of filters contained in this composite filter.
+     *
+     * Use a `std::shared_ptr<ThreadSafeMemoizer>` rather than using
+     * `ThreadSafeMemoizer` directly so that this class is copyable
+     * (`ThreadSafeMemoizer` is not copyable because of its `std::once_flag`
+     * member variable, which is not copyable).
      */
-    mutable std::vector<FieldFilter> memoized_flattened_filters_;
+    mutable std::shared_ptr<ThreadSafeMemoizer<FieldFilter>>
+        memoized_flattened_filters_;
   };
 
   explicit Filter(std::shared_ptr<const Rep>&& rep) : rep_(rep) {

Firestore/core/src/core/query.cc

@@ -90,19 +90,21 @@ absl::optional<Operator> Query::FindOpInsideFilters(
 }
 
 const std::vector<OrderBy>& Query::normalized_order_bys() const {
-  if (memoized_normalized_order_bys_.empty()) {
+  return memoized_normalized_order_bys_->memoize([&]() {
+    std::vector<OrderBy> result;
     const FieldPath* inequality_field = InequalityFilterField();
     const FieldPath* first_order_by_field = FirstOrderByField();
+
     if (inequality_field && !first_order_by_field) {
       // In order to implicitly add key ordering, we must also add the
       // inequality filter field for it to be a valid query. Note that the
       // default inequality field and key ordering is ascending.
       if (inequality_field->IsKeyFieldPath()) {
-        memoized_normalized_order_bys_ = {
+        result = {
             OrderBy(FieldPath::KeyFieldPath(), Direction::Ascending),
         };
       } else {
-        memoized_normalized_order_bys_ = {
+        result = {
             OrderBy(*inequality_field, Direction::Ascending),
             OrderBy(FieldPath::KeyFieldPath(), Direction::Ascending),
         };
@@ -114,8 +116,7 @@ const std::vector<OrderBy>& Query::normalized_order_bys() const {
           first_order_by_field->CanonicalString(),
           inequality_field->CanonicalString());
 
-      std::vector<OrderBy> result = explicit_order_bys_;
-
+      result = explicit_order_bys_;
       bool found_explicit_key_order = false;
       for (const OrderBy& order_by : explicit_order_bys_) {
         if (order_by.field().IsKeyFieldPath()) {
@@ -132,11 +133,10 @@ const std::vector<OrderBy>& Query::normalized_order_bys() const {
                                        : explicit_order_bys_.back().direction();
         result.emplace_back(FieldPath::KeyFieldPath(), last_direction);
       }
-
-      memoized_normalized_order_bys_ = std::move(result);
     }
-  }
-  return memoized_normalized_order_bys_;
+
+    return result;
+  });
 }
 
 const FieldPath* Query::FirstOrderByField() const {

Firestore/core/src/core/query.h

@@ -277,8 +277,17 @@ class Query {
   // sort at the end.
   std::vector<OrderBy> explicit_order_bys_;
 
-  // The memoized list of sort orders.
-  mutable std::vector<OrderBy> memoized_normalized_order_bys_;
+  /**
+   * The memoized list of sort orders.
+   *
+   * Use a `std::shared_ptr<ThreadSafeMemoizer>` rather than using
+   * `ThreadSafeMemoizer` directly so that this class is copyable
+   * (`ThreadSafeMemoizer` is not copyable because of its `std::once_flag`
+   * member variable, which is not copyable).
+   */
+  mutable std::shared_ptr<ThreadSafeMemoizer<OrderBy>>
+      memoized_normalized_order_bys_ =
+          std::make_shared<ThreadSafeMemoizer<OrderBy>>();
 
   int32_t limit_ = Target::kNoLimit;
   LimitType limit_type_ = LimitType::None;

Firestore/core/src/util/thread_safe_memoizer.h

@@ -0,0 +1,70 @@
+/*
+ * Copyright 2023 Google
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+#ifndef FIRESTORE_CORE_SRC_UTIL_THREAD_SAFE_MEMOIZER_H_
+#define FIRESTORE_CORE_SRC_UTIL_THREAD_SAFE_MEMOIZER_H_
+
+#include <mutex>  // NOLINT(build/c++11)
+#include <vector>
+
+namespace firebase {
+namespace firestore {
+namespace util {
+
+/**
+ * Stores a memoized value in a manner that is safe to be shared between
+ * multiple threads.
+ */
+template <typename T>
+class ThreadSafeMemoizer {
+ public:
+  ~ThreadSafeMemoizer() {
+    // Call `std::call_once` in order to synchronize with the "active"
+    // invocation of `memoize()`. Without this synchronization, there is a data
+    // race between this destructor, which "reads" `memoized_value_` to destroy
+    // it, and the write to `memoized_value_` done by the "active" invocation of
+    // `memoize()`.
+    std::call_once(once_, [&]() {});
+  }
+
+  /**
+   * Memoize a value.
+   *
+   * The std::function object specified by the first invocation of this
+   * function (the "active" invocation) will be invoked synchronously.
+   * None of the std::function objects specified by the subsequent
+   * invocations of this function (the "passive" invocations) will be
+   * invoked. All invocations, both "active" and "passive", will return a
+   * reference to the std::vector created by copying the return value from
+   * the std::function specified by the "active" invocation. It is,
+   * therefore, the "active" invocation's job to return the std::vector
+   * to memoize.
+   */
+  const std::vector<T>& memoize(std::function<std::vector<T>()> func) {
+    std::call_once(once_, [&]() { memoized_value_ = func(); });
+    return memoized_value_;
+  }
+
+ private:
+  std::once_flag once_;
+  std::vector<T> memoized_value_;
+};
+
+}  // namespace util
+}  // namespace firestore
+}  // namespace firebase
+
+#endif  // FIRESTORE_CORE_SRC_UTIL_THREAD_SAFE_MEMOIZER_H_