remote api: support otpional parallel CAS synchronisation

Allow implementations to use a given number of threads to carry out
the synchronisation. In this way parallelism can be achieved even
in situations where batch reading degrades as objects of unknown
size have to be fetched; this situation typically occurs if a tree
object has a large number of direct children that are blobs.

If no implementation is provided, the default implementation is to
fall back to the normal (sequential) CAS synchronisation.

1 files changed, 9 insertions, 0 deletions

diff --git a/src/buildtool/execution_api/common/execution_api.hpp b/src/buildtool/execution_api/common/execution_api.hpp
index a8ba8009..e30c1276 100644
--- a/src/buildtool/execution_api/common/execution_api.hpp
+++ b/src/buildtool/execution_api/common/execution_api.hpp
@@ -87,6 +87,15 @@ class IExecutionApi {
         std::vector<Artifact::ObjectInfo> const& artifacts_info,
         gsl::not_null<IExecutionApi*> const& api) noexcept -> bool = 0;
 
+    /// \brief A variant of RetrieveToCas that is allowed to internally use
+    /// the specified number of threads to carry out the task in parallel.
+    [[nodiscard]] virtual auto ParallelRetrieveToCas(
+        std::vector<Artifact::ObjectInfo> const& artifacts_info,
+        gsl::not_null<IExecutionApi*> const& api,
+        std::size_t /* jobs */) noexcept -> bool {
+        return RetrieveToCas(artifacts_info, api);
+    }
+
     /// \brief Retrieve one artifact from CAS and make it available for
     /// furter in-memory processing
     [[nodiscard]] virtual auto RetrieveToMemory(