multithreading: Add AtomicValue to atomically set/get value

... and use it to replace the commonly used pattern in
Expression, LinkedMap, and GitTreeEntry. Furthermore, remove
assignment operators for Expression and LinkedMap as those
are considered to be used in an immutable manner anyway.

1 files changed, 8 insertions, 41 deletions

diff --git a/src/buildtool/build_engine/expression/linked_map.hpp b/src/buildtool/build_engine/expression/linked_map.hpp
index 5c6da558..6266c2ce 100644
--- a/src/buildtool/build_engine/expression/linked_map.hpp
+++ b/src/buildtool/build_engine/expression/linked_map.hpp
@@ -10,6 +10,7 @@
 #include <vector>
 
 #include "fmt/core.h"
+#include "src/buildtool/multithreading/atomic_value.hpp"
 #include "src/utils/cpp/atomic.hpp"
 #include "src/utils/cpp/hash_combine.hpp"
 
@@ -110,32 +111,12 @@ class LinkedMap {
     }
 
     LinkedMap() noexcept = default;
-    LinkedMap(LinkedMap const& other) noexcept
-        : next_{other.next_},
-          content_{other.content_},
-          map_{other.map_},
-          items_{other.items_.load()} {}
-    LinkedMap(LinkedMap&& other) noexcept
-        : next_{std::move(other.next_)},
-          content_{std::move(other.content_)},
-          map_{std::move(other.map_)},
-          items_{other.items_.load()} {}
+    LinkedMap(LinkedMap const& other) noexcept = default;
+    LinkedMap(LinkedMap&& other) noexcept = default;
     ~LinkedMap() noexcept = default;
 
-    auto operator=(LinkedMap const& other) noexcept -> LinkedMap& {
-        next_ = other.next_;
-        content_ = other.content_;
-        map_ = other.map_;
-        items_ = other.items_.load();
-        return *this;
-    }
-    auto operator=(LinkedMap&& other) noexcept -> LinkedMap& {
-        next_ = std::move(other.next_);
-        content_ = std::move(other.content_);
-        map_ = std::move(other.map_);
-        items_ = other.items_.load();
-        return *this;
-    }
+    auto operator=(LinkedMap const& other) noexcept = delete;
+    auto operator=(LinkedMap&& other) noexcept = delete;
 
     [[nodiscard]] auto contains(K const& key) const noexcept -> bool {
         return static_cast<bool>(Find(key));
@@ -276,23 +257,10 @@ class LinkedMap {
 
     // NOTE: Expensive, needs to compute sorted items.
     [[nodiscard]] auto Items() const& -> items_t const& {
-        if (items_.load() == nullptr) {
-            if (not items_loading_.exchange(true)) {
-                items_ = std::make_shared<items_t>(ComputeSortedItems());
-                items_.notify_all();
-            }
-            else {
-                items_.wait(nullptr);
-            }
-        }
-        return *items_.load();
+        return items_.SetOnceAndGet([this] { return ComputeSortedItems(); });
     }
 
-    // NOTE: Expensive, needs to compute sorted items.
-    [[nodiscard]] auto Items() && -> items_t {
-        return items_.load() == nullptr ? ComputeSortedItems()
-                                        : std::move(*items_.load());
-    }
+    [[nodiscard]] auto Items() && = delete;
 
     // NOTE: Expensive, needs to compute sorted items.
     [[nodiscard]] auto Keys() const -> keys_t {
@@ -323,8 +291,7 @@ class LinkedMap {
     Ptr content_{};           // content of this map if set
     underlying_map_t map_{};  // content of this map if content_ is not set
 
-    mutable atomic_shared_ptr<items_t> items_{};
-    mutable std::atomic<bool> items_loading_{};
+    AtomicValue<items_t> items_{};
 
     [[nodiscard]] auto ComputeSortedItems() const noexcept -> items_t {
         auto size =