Merge pull request NixOS#11581 from Mic92/git-cache

create git caches atomically

src/libfetchers/git-utils.cc

@@ -205,6 +205,27 @@ static git_packbuilder_progress PACKBUILDER_PROGRESS_CHECK_INTERRUPT = &packBuil
 
 } // extern "C"
 
+static void initRepoAtomically(std::filesystem::path &path, bool bare) {
+ if (pathExists(path.string())) return;
+
+ Path tmpDir = createTempDir(std::filesystem::path(path).parent_path());
+ AutoDelete delTmpDir(tmpDir, true);
+ Repository tmpRepo;
+
+ if (git_repository_init(Setter(tmpRepo), tmpDir.c_str(), bare))
+ throw Error("creating Git repository %s: %s", path, git_error_last()->message);
+ try {
+ std::filesystem::rename(tmpDir, path);
+ } catch (std::filesystem::filesystem_error & e) {
+ if (e.code() == std::errc::file_exists) // Someone might race us to create the repository.
+ return;
+ else
+ throw SysError("moving temporary git repository from %s to %s", tmpDir, path);
+ }
+ // we successfully moved the repository, so the temporary directory no longer exists.
+ delTmpDir.cancel();
+}
+
 struct GitRepoImpl : GitRepo, std::enable_shared_from_this<GitRepoImpl>
 {
  /** Location of the repository on disk. */
@@ -226,13 +247,9 @@ struct GitRepoImpl : GitRepo, std::enable_shared_from_this<GitRepoImpl>
  {
  initLibGit2();
 
- if (pathExists(path.string())) {
- if (git_repository_open(Setter(repo), path.string().c_str()))
- throw Error("opening Git repository '%s': %s", path, git_error_last()->message);
- } else {
- if (git_repository_init(Setter(repo), path.string().c_str(), bare))
- throw Error("creating Git repository '%s': %s", path, git_error_last()->message);
- }
+ initRepoAtomically(path, bare);
+ if (git_repository_open(Setter(repo), path.string().c_str()))
+ throw Error("opening Git repository %s: %s", path, git_error_last()->message);
 
  ObjectDb odb;
  if (git_repository_odb(Setter(odb), repo.get()))