From 385d7e77bdbee44b5c7b66b53b1b8f096727ca0a Mon Sep 17 00:00:00 2001
From: Sergei Zimmerman <sergei@zimmerman.foo>
Date: Sat, 22 Nov 2025 22:48:40 +0300
Subject: [PATCH] libfetchers: Don't have a single shared tarball cache
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This partially reverts commit bc6b9ce.

This transformation is unsound and thread unsafe. Internal libgit2
structures must *never* be shared between threads. This causes
internal odb corruption with e.g.:

nix flake prefetch-inputs:

error:
       … while fetching the input 'github:nixos/nixpkgs/89c2b2330e733d6cdb5eae7b899326930c2c0648?narHash=sha256-Stk9ZYRkGrnnpyJ4eqt9eQtdFWRRIvMxpNRf4sIegnw%3D'

       error: adding a file to a tree builder: failed to insert entry: invalid object specified - upload-image.sh
error:
       … while fetching the input 'github:NixOS/nixpkgs/a8d610af3f1a5fb71e23e08434d8d61a466fc942?narHash=sha256-v5afmLjn/uyD9EQuPBn7nZuaZVV9r%2BJerayK/4wvdWA%3D'

       error: adding a file to a tree builder: failed to insert entry: invalid object specified - outline.nix
double free or corruption (!prev)

Thread 21 "nix" received signal SIGABRT, Aborted.
---
 src/libfetchers/git-utils.cc                           | 6 ++----
 src/libfetchers/include/nix/fetchers/fetch-settings.hh | 2 --
 2 files changed, 2 insertions(+), 6 deletions(-)

diff --git a/src/libfetchers/git-utils.cc b/src/libfetchers/git-utils.cc
index 14504ee0b..4a7fc3d0d 100644
--- a/src/libfetchers/git-utils.cc
+++ b/src/libfetchers/git-utils.cc
@@ -1334,10 +1334,8 @@ namespace fetchers {
 
 ref<GitRepo> Settings::getTarballCache() const
 {
-    auto tarballCache(_tarballCache.lock());
-    if (!*tarballCache)
-        *tarballCache = GitRepo::openRepo(std::filesystem::path(getCacheDir()) / "tarball-cache", true, true);
-    return ref<GitRepo>(*tarballCache);
+    static auto repoDir = std::filesystem::path(getCacheDir()) / "tarball-cache";
+    return GitRepo::openRepo(repoDir, true, true);
 }
 
 } // namespace fetchers
diff --git a/src/libfetchers/include/nix/fetchers/fetch-settings.hh b/src/libfetchers/include/nix/fetchers/fetch-settings.hh
index bddca3f9e..8cfa7f609 100644
--- a/src/libfetchers/include/nix/fetchers/fetch-settings.hh
+++ b/src/libfetchers/include/nix/fetchers/fetch-settings.hh
@@ -135,8 +135,6 @@ struct Settings : public Config
 
 private:
     mutable Sync<std::shared_ptr<Cache>> _cache;
-
-    mutable Sync<std::shared_ptr<GitRepo>> _tarballCache;
 };
 
 } // namespace nix::fetchers