Serve service: properly lock git operations against each other

... by using an exclusive lock. A lock of which only ever shared instances
are requested has no synchronisation effect. Fix this.

(cherry-picked from dfb7ad5c7d5dcca13d9728534434079a2b60bdea)

2 files changed, 3 insertions, 4 deletions

diff --git a/src/buildtool/serve_api/serve_service/source_tree.cpp b/src/buildtool/serve_api/serve_service/source_tree.cpp
index 560d96d3..9fb2dae4 100644
--- a/src/buildtool/serve_api/serve_service/source_tree.cpp
+++ b/src/buildtool/serve_api/serve_service/source_tree.cpp
@@ -17,7 +17,6 @@
 #include "src/buildtool/serve_api/serve_service/source_tree.hpp"
 
 #include <algorithm>
-#include <shared_mutex>
 #include <thread>
 
 #include "fmt/core.h"
@@ -539,7 +538,7 @@ auto SourceTreeService::ResolveContentTree(
             });
         {
             // this is a non-thread-safe Git operation, so it must be guarded!
-            std::shared_lock slock{mutex_};
+            std::unique_lock slock{mutex_};
             // open real repository at Git CAS location
             auto git_repo =
                 GitRepo::Open(native_context_->storage_config->GitRoot());
@@ -661,7 +660,7 @@ auto SourceTreeService::CommonImportToGit(
     // tag commit and keep it in Git CAS
     {
         // this is a non-thread-safe Git operation, so it must be guarded!
-        std::shared_lock slock{mutex_};
+        std::unique_lock slock{mutex_};
         // open real repository at Git CAS location
         auto git_repo =
             GitRepo::Open(native_context_->storage_config->GitRoot());
diff --git a/src/buildtool/serve_api/serve_service/source_tree.hpp b/src/buildtool/serve_api/serve_service/source_tree.hpp
index bf3cd9ef..24ef59c2 100644
--- a/src/buildtool/serve_api/serve_service/source_tree.hpp
+++ b/src/buildtool/serve_api/serve_service/source_tree.hpp
@@ -137,7 +137,7 @@ class SourceTreeService final
     ApiBundle const& apis_;
     gsl::not_null<LocalContext const*> native_context_;
     LocalContext const* compat_context_;
-    mutable std::shared_mutex mutex_;
+    mutable std::mutex mutex_;
     std::shared_ptr<Logger> logger_{std::make_shared<Logger>("serve-service")};
     // symlinks resolver map
     ResolveSymlinksMap resolve_symlinks_map_{CreateResolveSymlinksMap()};