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create git caches atomically

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When working on speeding up the CI,
I triggered a race condition in the creation of the tarball cache.
This code now instead will ensure that half-initialized repositories
are no longer visible to any other nix process.

This is the error message that I got before:

error: opening Git repository '"/Users/runner/.cache/nix/tarball-cache"': could not find repository at '/Users/runner/.cache/nix/tarball-cache'
This commit is contained in:
Jörg Thalheim 2024-09-24 08:02:57 +02:00
parent a5959aa121
commit 12d5b2cfa1
1 changed files with 24 additions and 7 deletions
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29
src/libfetchers/git-utils.cc
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@ -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())) {
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);
} else {
if (git_repository_init(Setter(repo), path.string().c_str(), bare))
throw Error("creating Git repository '%s': %s", path, git_error_last()->message);
}
throw Error("opening Git repository %s: %s", path, git_error_last()->message);
ObjectDb odb;
if (git_repository_odb(Setter(odb), repo.get()))