SERVER-45168 Create add-only syncronized list type

3 files changed, 354 insertions, 0 deletions

diff --git a/src/mongo/util/SConscript b/src/mongo/util/SConscript
index 5b9ced06500..06ae54e98cc 100644
--- a/src/mongo/util/SConscript
+++ b/src/mongo/util/SConscript
@@ -578,6 +578,7 @@ icuEnv.CppUnitTest(
         'procparser_test.cpp' if env.TargetOSIs('linux') else [],
         'producer_consumer_queue_test.cpp',
         'progress_meter_test.cpp',
+        'registry_list_test.cpp',
         'represent_as_test.cpp',
         'safe_num_test.cpp',
         'secure_zero_memory_test.cpp',
diff --git a/src/mongo/util/registry_list.h b/src/mongo/util/registry_list.h
new file mode 100644
index 00000000000..f451cb78a46
--- /dev/null
+++ b/src/mongo/util/registry_list.h
@@ -0,0 +1,194 @@
+/**
+ *    Copyright (C) 2018-present MongoDB, Inc.
+ *
+ *    This program is free software: you can redistribute it and/or modify
+ *    it under the terms of the Server Side Public License, version 1,
+ *    as published by MongoDB, Inc.
+ *
+ *    This program is distributed in the hope that it will be useful,
+ *    but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *    Server Side Public License for more details.
+ *
+ *    You should have received a copy of the Server Side Public License
+ *    along with this program. If not, see
+ *    <http://www.mongodb.com/licensing/server-side-public-license>.
+ *
+ *    As a special exception, the copyright holders give permission to link the
+ *    code of portions of this program with the OpenSSL library under certain
+ *    conditions as described in each individual source file and distribute
+ *    linked combinations including the program with the OpenSSL library. You
+ *    must comply with the Server Side Public License in all respects for
+ *    all of the code used other than as permitted herein. If you modify file(s)
+ *    with this exception, you may extend this exception to your version of the
+ *    file(s), but you are not obligated to do so. If you do not wish to do so,
+ *    delete this exception statement from your version. If you delete this
+ *    exception statement from all source files in the program, then also delete
+ *    it in the license file.
+ */
+
+#pragma once
+
+#include <deque>
+#include <memory>
+
+#include "mongo/stdx/mutex.h"
+#include "mongo/util/concepts.h"
+
+namespace mongo {
+
+/**
+ * Simple generator-type iterator
+ *
+ * This Iterator caches/obscures the underlying data type to allow subclasses to wrap its more()
+ * and next() functions.
+ */
+template <typename DataT>
+class RegistryListIterator {
+public:
+    using DataType = DataT;
+    using ElementType = typename DataT::value_type;
+
+    explicit RegistryListIterator(DataT data) : _data{std::move(data)} {}
+
+    virtual ~RegistryListIterator() = default;
+
+    /**
+     * Return the next element and advance the iterator
+     */
+    auto next() {
+        if (!more()) {
+            return ElementType();
+        }
+
+        return _data[_index++];
+    }
+
+    /**
+     * Return true if there are more elements available
+     */
+    virtual bool more() const {
+        return _index < _data.size();
+    }
+
+private:
+    DataType _data;
+
+    size_t _index = 0;
+};
+
+/**
+ * A synchronized add-only list of elements
+ *
+ * A RegistryList is intended to allow concurrent iteration, insertion, and access with minimal
+ * amounts of resource contention. If each element in the list is a pointer or index, the overhead
+ * of "deactivated" elements is minimal.
+ *
+ * This class does no lifetime management for its elements besides construction and destruction. If
+ * you use it to store pointers, the pointed-to memory should be immortal.
+ */
+TEMPLATE(typename ElementT)
+REQUIRES(std::is_default_constructible_v<ElementT>)
+class RegistryList {
+public:
+    using ElementType = ElementT;
+    using DataType = std::deque<ElementT>;
+    using Iterator = RegistryListIterator<DataType>;
+
+    virtual ~RegistryList() = default;
+
+    /**
+     * Add an element to the list
+     *
+     * @returns  The index of the new pointer element
+     */
+    size_t add(ElementType ptr) {
+        stdx::lock_guard lk(_m);
+
+        _data.emplace_back(std::move(ptr));
+        return _data.size() - 1;
+    }
+
+    /**
+     * Returns an element at the given index within the list
+     */
+    ElementType at(size_t index) const {
+        stdx::lock_guard lk(_m);
+
+        if (index >= _data.size()) {
+            // If index is past our synchronized end on the deque, then indexing it will be UB.
+            return ElementType();
+        }
+
+        return _data[index];
+    }
+
+    /**
+     * Return the total number of elements in this list
+     */
+    size_t size() const {
+        stdx::lock_guard lk(_m);
+
+        return _data.size();
+    }
+
+    /**
+     * Return a copy of the underlying data structure
+     */
+    DataType data() const {
+        stdx::lock_guard lk(_m);
+
+        return _data;
+    }
+
+    /**
+     * Return an iterator for this list
+     *
+     * This iterator copies the state of the list at the time of capture. If additional elements are
+     * added after this function is invoked, they will not be visible via the Iterator.
+     */
+    auto iter() const {
+        return Iterator(data());
+    }
+
+private:
+    mutable stdx::mutex _m;  // NOLINT
+    DataType _data;
+};
+
+/**
+ * Wrap the basic RegistryList concept to handle weak_ptrs
+ */
+TEMPLATE(typename T)
+REQUIRES(std::is_constructible_v<std::weak_ptr<T>>)
+class WeakPtrRegistryList : public RegistryList<std::weak_ptr<T>> {
+public:
+    using ElementType = std::weak_ptr<T>;
+    using BaseList = RegistryList<ElementType>;
+    using DataType = typename BaseList::DataType;
+
+    class Iterator final : public BaseList::Iterator {
+    public:
+        using BaseList::Iterator::Iterator;
+
+        auto next() {
+            return BaseList::Iterator::next().lock();
+        }
+    };
+
+    virtual ~WeakPtrRegistryList() = default;
+
+    auto add(const std::shared_ptr<T>& ptr) {
+        return BaseList::add(std::weak_ptr<T>(ptr));
+    }
+
+    auto at(size_t index) const {
+        return BaseList::at(index).lock();
+    }
+
+    auto iter() const {
+        return Iterator(BaseList::data());
+    }
+};
+
+}  // namespace mongo
diff --git a/src/mongo/util/registry_list_test.cpp b/src/mongo/util/registry_list_test.cpp
new file mode 100644
index 00000000000..cca1761b44d
--- /dev/null
+++ b/src/mongo/util/registry_list_test.cpp
@@ -0,0 +1,159 @@
+/**
+ *    Copyright (C) 2019-present MongoDB, Inc.
+ *
+ *    This program is free software: you can redistribute it and/or modify
+ *    it under the terms of the Server Side Public License, version 1,
+ *    as published by MongoDB, Inc.
+ *
+ *    This program is distributed in the hope that it will be useful,
+ *    but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *    Server Side Public License for more details.
+ *
+ *    You should have received a copy of the Server Side Public License
+ *    along with this program. If not, see
+ *    <http://www.mongodb.com/licensing/server-side-public-license>.
+ *
+ *    As a special exception, the copyright holders give permission to link the
+ *    code of portions of this program with the OpenSSL library under certain
+ *    conditions as described in each individual source file and distribute
+ *    linked combinations including the program with the OpenSSL library. You
+ *    must comply with the Server Side Public License in all respects for
+ *    all of the code used other than as permitted herein. If you modify file(s)
+ *    with this exception, you may extend this exception to your version of the
+ *    file(s), but you are not obligated to do so. If you do not wish to do so,
+ *    delete this exception statement from your version. If you delete this
+ *    exception statement from all source files in the program, then also delete
+ *    it in the license file.
+ */
+
+#include "mongo/util/registry_list.h"
+
+#include <vector>
+
+#include <boost/optional.hpp>
+
+#include "mongo/stdx/condition_variable.h"
+#include "mongo/stdx/mutex.h"
+#include "mongo/stdx/thread.h"
+#include "mongo/unittest/barrier.h"
+#include "mongo/unittest/unittest.h"
+
+namespace mongo {
+namespace {
+
+TEST(RegistryList, MixedOperationsSingleThread) {
+    /**
+     * Show that iterleaved add() and iter() calls function as expected
+     */
+
+    RegistryList<boost::optional<int>> list;
+
+    for (int i = 0; i < 10000; ++i) {
+        // Get our cached iterator first
+        auto iter = list.iter();
+
+        {
+            // Add a new value
+            auto index = list.add(i);
+
+            // Each index should be i since we are adding in sequence
+            ASSERT_EQ(index, i);
+        }
+
+        {
+            // Each index from before should be accessible
+            int j = 0;
+            for (; j <= i; ++j) {
+                // For each value that was already added, the value should also be the index
+                ASSERT_EQ(*list.at(j), j);
+            }
+
+            // Values outside the list should be default constructed
+            ASSERT_FALSE(list.at(j));
+        }
+
+        {
+            // The iterator we got out should have the state before we added the newest element
+            int count = 0;
+            while (iter.more()) {
+                auto val = iter.next();
+                ASSERT_EQ(*val, count);
+
+                ++count;
+            }
+
+            ASSERT_EQ(count, list.size() - 1);
+            ASSERT_FALSE(iter.next());
+        }
+    }
+}
+
+TEST(RegistryList, ConcurrentAdd) {
+    /**
+     * Show that multiple concurrent add() and iter() calls function to expectation
+     */
+    RegistryList<boost::optional<size_t>> list;
+
+    constexpr size_t kThreads = 4;
+    constexpr size_t kAdds = 100000;
+
+
+    struct State {
+        stdx::mutex m;  // NOLINT
+        size_t workersDone = 0;
+    } state;
+
+    unittest::Barrier barrier{kThreads + 1};
+
+    auto addWorkerFunc = [&] {
+        // Each worker adds a fixed number of elements
+        barrier.countDownAndWait();
+
+        for (size_t i = 0; i < kAdds; ++i) {
+            list.add(i);
+        }
+
+        stdx::lock_guard lk(state.m);
+        ++state.workersDone;
+    };
+
+    auto iteratorFunc = [&] {
+        // The iterator thread repeatedly tries to iterate over the list while add()s are happening.
+        barrier.countDownAndWait();
+
+        while ([&] {
+            stdx::lock_guard lk(state.m);
+            return state.workersDone != kThreads;
+        }()) {
+            // The list never shrinks over the course of iteration and probably grows, this proves
+            // that rule via iter() and size()
+
+            auto lastSize = list.size();
+            auto iter = list.iter();
+
+            auto count = 0;
+            while (auto e = iter.next()) {
+                ++count;
+            }
+
+            ASSERT_LTE(lastSize, count);
+            ASSERT_LTE(count, list.size());
+        }
+    };
+
+    std::vector<stdx::thread> threads;
+    threads.emplace_back(iteratorFunc);
+    for (size_t i = 0; i < kThreads; ++i) {
+        threads.emplace_back(addWorkerFunc);
+    }
+
+    for (auto& thread : threads) {
+        thread.join();
+    }
+
+    ASSERT_EQ(list.size(), kThreads * kAdds);
+}
+
+}  // namespace
+}  // namespace mongo