Fetch export targets sequentially when creating local backup

Typically, the number of export targets is small compared to the
size of the respective export targets. Moreover, export targets are
often chained, resulting in overlapping artifacts that we want to
fetch only once. Therefore, fetch export targets sequentially, giving
room to introduce parallelism in the individual fetch steps later.

While there, also log the beginning of the artifact synchronisation
for the export targets.

1 files changed, 21 insertions, 19 deletions

diff --git a/src/buildtool/main/main.cpp b/src/buildtool/main/main.cpp
index 880c0e85..819d7470 100644
--- a/src/buildtool/main/main.cpp
+++ b/src/buildtool/main/main.cpp
@@ -836,32 +836,34 @@ void WriteTargetCacheEntries(
     std::unordered_map<TargetCacheKey, AnalysedTargetPtr> const& cache_targets,
     std::unordered_map<ArtifactDescription, Artifact::ObjectInfo> const&
         extra_infos,
-    std::size_t jobs,
+    std::size_t /* jobs */,
     gsl::not_null<IExecutionApi*> const& local_api,
     gsl::not_null<IExecutionApi*> const& remote_api) {
-    auto ts = TaskSystem{jobs};
+    if (!cache_targets.empty()) {
+        Logger::Log(LogLevel::Info,
+                    "Backing up artifacts of {} export targets",
+                    cache_targets.size());
+    }
     auto downloader = [&local_api, &remote_api](auto infos) {
         return remote_api->RetrieveToCas(infos, local_api);
     };
     for (auto const& [key, target] : cache_targets) {
-        ts.QueueTask(
-            [&key = key, &target = target, &extra_infos, &downloader]() {
-                if (auto entry =
-                        TargetCacheEntry::FromTarget(target, extra_infos)) {
-                    if (not Storage::Instance().TargetCache().Store(
-                            key, *entry, downloader)) {
-                        Logger::Log(LogLevel::Warning,
-                                    "Failed writing target cache entry for {}",
-                                    key.Id().ToString());
-                    }
-                }
-                else {
-                    Logger::Log(LogLevel::Warning,
-                                "Failed creating target cache entry for {}",
-                                key.Id().ToString());
-                }
-            });
+        if (auto entry = TargetCacheEntry::FromTarget(target, extra_infos)) {
+            if (not Storage::Instance().TargetCache().Store(
+                    key, *entry, downloader)) {
+                Logger::Log(LogLevel::Warning,
+                            "Failed writing target cache entry for {}",
+                            key.Id().ToString());
+            }
+        }
+        else {
+            Logger::Log(LogLevel::Warning,
+                        "Failed creating target cache entry for {}",
+                        key.Id().ToString());
+        }
     }
+    Logger::Log(LogLevel::Debug,
+                "Finished backing up artifacts of export targets");
 }
 
 #endif  // BOOTSTRAP_BUILD_TOOL