SERVER-114944: Add a templated atomic-based counter class. (#45058)

GitOrigin-RevId: 9b281196fc1e2b75a6884148a04a9d3177a9454d

src/mongo/otel/metrics/BUILD.bazel

@@ -18,6 +18,14 @@ idl_generator(
     src = "metrics_settings.idl",
 )
 
+mongo_cc_unit_test(
+    name = "metrics_counter_test",
+    srcs = [
+        "metrics_counter_test.cpp",
+    ],
+    tags = ["mongo_unittest_third_group"],
+)
+
 mongo_cc_library(
     name = "otel_metrics",
     srcs = [

src/mongo/otel/metrics/metrics_counter.h

@@ -0,0 +1,101 @@
+/**
+ *    Copyright (C) 2025-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 "mongo/platform/atomic.h"
+#include "mongo/util/assert_util.h"
+
+#include <string>
+
+#include "src/mongo/util/modules.h"
+
+namespace mongo::otel::metrics {
+
+template <typename T>
+class MONGO_MOD_PUBLIC Counter {
+public:
+    // T must be nonnegative.
+    virtual void add(T value) = 0;
+
+    virtual T value() const = 0;
+
+    virtual const std::string& name() const = 0;
+
+    virtual const std::string& description() const = 0;
+
+    virtual const std::string& unit() const = 0;
+};
+
+// A lock free (non-decreasing) counter and metadata about it.
+template <typename T>
+class CounterImpl : public Counter<T> {
+public:
+    CounterImpl(std::string name, std::string description, std::string unit);
+
+    void add(T value) override;
+
+    T value() const override {
+        return _value.load();
+    }
+
+    const std::string& name() const override {
+        return _name;
+    }
+
+    const std::string& description() const override {
+        return _description;
+    }
+
+    const std::string& unit() const override {
+        return _unit;
+    }
+
+private:
+    std::string _name;
+    std::string _description;
+    std::string _unit;
+    Atomic<T> _value;
+};
+
+///////////////////////////////////////////////////////////////////////////////
+// Implementation details
+///////////////////////////////////////////////////////////////////////////////
+
+template <typename T>
+CounterImpl<T>::CounterImpl(std::string name, std::string description, std::string unit)
+    : _name(std::move(name)), _description(std::move(description)), _unit(std::move(unit)) {}
+
+template <typename T>
+void CounterImpl<T>::add(T value) {
+    massert(ErrorCodes::BadValue, "Counter increment must be nonnegative", value >= 0);
+    _value.fetchAndAddRelaxed(value);
+}
+
+}  // namespace mongo::otel::metrics

src/mongo/otel/metrics/metrics_counter_test.cpp

@@ -0,0 +1,87 @@
+/**
+ *    Copyright (C) 2025-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/otel/metrics/metrics_counter.h"
+
+#include "mongo/stdx/thread.h"
+#include "mongo/unittest/unittest.h"
+
+#include <vector>
+
+namespace mongo::otel::metrics {
+
+TEST(Int64CounterImplTest, Adds) {
+    CounterImpl<int64_t> counter("name", "description", "unit");
+    ASSERT_EQ(counter.value(), 0);
+    counter.add(1);
+    ASSERT_EQ(counter.value(), 1);
+    counter.add(10);
+    ASSERT_EQ(counter.value(), 11);
+}
+
+TEST(Int64CounterImplTest, AddsZero) {
+    CounterImpl<int64_t> counter("name", "description", "unit");
+    ASSERT_EQ(counter.value(), 0);
+    counter.add(0);
+    ASSERT_EQ(counter.value(), 0);
+    counter.add(10);
+    counter.add(0);
+    ASSERT_EQ(counter.value(), 10);
+}
+
+TEST(Int64CounterImplTest, ExceptionOnNegativeAdd) {
+    CounterImpl<int64_t> counter("name", "description", "unit");
+    counter.add(1);
+    counter.add(3);
+    ASSERT_THROWS_CODE(counter.add(-1), DBException, ErrorCodes::BadValue);
+}
+
+// Any issues with thread safety should be caught by tsan on this test.
+TEST(Int64CounterImplTest, ConcurrentAdds) {
+    CounterImpl<int64_t> counter("name", "description", "unit");
+    constexpr int kNumThreads = 10;
+    constexpr int kIncrementsPerThread = 1000;
+
+    std::vector<stdx::thread> threads;
+    for (int i = 0; i < kNumThreads; ++i) {
+        threads.emplace_back([&counter]() {
+            for (int j = 0; j < kIncrementsPerThread; ++j) {
+                counter.add(1);
+            }
+        });
+    }
+
+    for (stdx::thread& thread : threads) {
+        thread.join();
+    }
+
+    ASSERT_EQ(counter.value(), kNumThreads * kIncrementsPerThread);
+}
+
+}  // namespace mongo::otel::metrics