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, 7 insertions, 19 deletions

diff --git a/src/buildtool/file_system/git_tree.cpp b/src/buildtool/file_system/git_tree.cpp
index 98281707..f4912a7b 100644
--- a/src/buildtool/file_system/git_tree.cpp
+++ b/src/buildtool/file_system/git_tree.cpp
@@ -13,9 +13,6 @@ namespace {
 
 constexpr auto kOIDRawSize{GIT_OID_RAWSZ};
 
-auto const kLoadTreeError =
-    std::make_shared<std::optional<GitTree>>(std::nullopt);
-
 [[nodiscard]] auto PermToType(std::string const& perm_str) noexcept
     -> std::optional<ObjectType> {
     constexpr auto kPermBase = 8;
@@ -154,26 +151,17 @@ auto GitTreeEntry::Blob() const noexcept -> std::optional<std::string> {
 }
 
 auto GitTreeEntry::Tree() const& noexcept -> std::optional<GitTree> const& {
-    auto ptr = tree_cached_.load();
-    if (not ptr) {
-        if (not tree_loading_.exchange(true)) {
-            ptr = kLoadTreeError;
-            std::optional<std::string> obj{};
-            if (IsTree() and (obj = cas_->ReadObject(raw_id_))) {
+    return tree_cached_.SetOnceAndGet([this]() -> std::optional<GitTree> {
+        std::optional<std::string> obj{};
+        if (IsTree()) {
+            if (auto obj = cas_->ReadObject(raw_id_)) {
                 if (auto entries = ParseRawTreeObject(cas_, *obj)) {
-                    ptr = std::make_shared<std::optional<GitTree>>(
-                        GitTree{cas_, std::move(*entries), raw_id_});
+                    return GitTree{cas_, std::move(*entries), raw_id_};
                 }
             }
-            tree_cached_.store(ptr);
-            tree_cached_.notify_all();
-        }
-        else {
-            tree_cached_.wait(nullptr);
-            ptr = tree_cached_.load();
         }
-    }
-    return *ptr;
+        return std::nullopt;
+    });
 }
 
 auto GitTreeEntry::Size() const noexcept -> std::optional<std::size_t> {