GitRepo: Guard fake repository odb wrapping

In the current libgit2 implementation, a fake repository wrapped
around an existing odb is being registered as owner the same way
as a normal repository object. Therefore, one has to guard both
the creation and destruction of the fake repository against all
other git operations that might access the internal cache during
this transfer of ownership.

1 files changed, 6 insertions, 0 deletions

diff --git a/src/buildtool/file_system/git_cas.hpp b/src/buildtool/file_system/git_cas.hpp
index 44b61cb4..f95cb403 100644
--- a/src/buildtool/file_system/git_cas.hpp
+++ b/src/buildtool/file_system/git_cas.hpp
@@ -18,6 +18,7 @@
 #include <filesystem>
 #include <memory>
 #include <optional>
+#include <shared_mutex>
 #include <unordered_map>
 #include <vector>
 
@@ -67,6 +68,11 @@ class GitCAS {
     // git folder path of repo
     std::filesystem::path git_path_{};
 
+    // mutex to guard odb while setting up a "fake" repository; it needs to be
+    // uniquely owned while wrapping the odb, but then git operations are free
+    // to share it.
+    mutable std::shared_mutex mutex_{};
+
     [[nodiscard]] auto OpenODB(std::filesystem::path const& repo_path) noexcept
         -> bool;