From 14c6648c71b4b8a12ac0905ff23fcd4de7f0556f Mon Sep 17 00:00:00 2001
From: Oliver Reiche <oliver.reiche@huawei.com>
Date: Mon, 13 Jun 2022 13:30:13 +0200
Subject: 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.
---
 src/buildtool/file_system/git_tree.cpp | 26 +++++++-------------------
 1 file changed, 7 insertions(+), 19 deletions(-)

(limited to 'src/buildtool/file_system/git_tree.cpp')

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> {
-- 
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