From 48083028ac075f251442bab34013c92394903aee Mon Sep 17 00:00:00 2001
From: Farid Zakaria <farid.m.zakaria@gmail.com>
Date: Fri, 11 Jul 2025 15:49:27 -0700
Subject: [PATCH 1/2] Add a test case for failing git cache

builtins.fetchGit is not using the cached Git directory if
packed-references are used.

This is because the ref file for the fetchGit `refs/heads/master` is
used to check the mtime for whether to cache or not.

Let's at least codify this failure in a test case.
---
 tests/functional/git/meson.build             |  6 ++
 tests/functional/git/packed-refs-no-cache.sh | 70 ++++++++++++++++++++
 tests/functional/meson.build                 |  1 +
 3 files changed, 77 insertions(+)
 create mode 100644 tests/functional/git/meson.build
 create mode 100644 tests/functional/git/packed-refs-no-cache.sh

diff --git a/tests/functional/git/meson.build b/tests/functional/git/meson.build
new file mode 100644
index 000000000..af6882698
--- /dev/null
+++ b/tests/functional/git/meson.build
@@ -0,0 +1,6 @@
+suites += {
+  'name' : 'git',
+  'deps' : [],
+  'tests' : [ 'packed-refs-no-cache.sh' ],
+  'workdir' : meson.current_source_dir(),
+}
diff --git a/tests/functional/git/packed-refs-no-cache.sh b/tests/functional/git/packed-refs-no-cache.sh
new file mode 100644
index 000000000..0f39da775
--- /dev/null
+++ b/tests/functional/git/packed-refs-no-cache.sh
@@ -0,0 +1,70 @@
+#!/usr/bin/env bash
+
+source ../common.sh
+
+requireGit
+
+clearStoreIfPossible
+
+# Intentionally not in a canonical form
+# See https://github.com/NixOS/nix/issues/6195
+repo=$TEST_ROOT/./git
+
+export _NIX_FORCE_HTTP=1
+
+rm -rf "$repo" "${repo}-tmp" "$TEST_HOME/.cache/nix"
+
+git init --initial-branch="master" "$repo"
+git -C "$repo" config user.email "nix-tests@example.com"
+git -C "$repo" config user.name "Nix Tests"
+
+echo "hello world" > "$repo/hello_world"
+git -C "$repo" add hello_world
+git -C "$repo" commit -m 'My first commit.'
+
+# We now do an eval
+nix eval --impure --raw --expr "builtins.fetchGit { url = file://$repo; }"
+
+# test that our eval even worked by checking for the presence of the file
+[[ $(nix eval --impure --raw --expr "builtins.readFile ((builtins.fetchGit { url = file://$repo; }) + \"/hello_world\")") = 'hello world' ]]
+
+# Validate that refs/heads/master exists
+shopt -s nullglob
+matches=("$TEST_HOME/.cache/nix/gitv3/*/refs/heads/master")
+shopt -u nullglob
+
+if [[ ${#matches[@]} -eq 0 ]]; then
+  echo "refs/heads/master does not exist."
+  exit 1
+fi
+# pack refs
+git -C "$TEST_HOME"/.cache/nix/gitv3/*/ pack-refs --all
+
+shopt -s nullglob
+matches=("$TEST_HOME"/.cache/nix/gitv3/*/refs/heads/master)
+shopt -u nullglob
+
+# ensure refs/heads/master is now gone
+if [[ ${#matches[@]} -ne 0 ]]; then
+  echo "refs/heads/master still exists after pack-refs"
+  exit 1
+fi
+
+# create a new commit
+echo "hello again" > "$repo/hello_again"
+git -C "$repo" add hello_again
+git -C "$repo" commit -m 'Second commit.'
+
+# re-eval — this should return the path to the cached version
+store_path=$(nix eval --tarball-ttl 3600 --impure --raw --expr "(builtins.fetchGit { url = file://$repo; }).outPath")
+echo "Fetched store path: $store_path"
+
+# Validate that the new file is *not* there
+# FIXME: This is a broken test case and we should swap the assertion here.
+if [[ -e "$store_path/hello_again" ]]; then
+  echo "ERROR: Cached fetchGit should not include the new commit."
+  exit 0
+else
+  echo "PASS: New commit was not fetched due to caching (as expected)."
+  exit 1
+fi
\ No newline at end of file
diff --git a/tests/functional/meson.build b/tests/functional/meson.build
index b03507c91..0e2004219 100644
--- a/tests/functional/meson.build
+++ b/tests/functional/meson.build
@@ -213,6 +213,7 @@ endif
 subdir('ca')
 subdir('dyn-drv')
 subdir('flakes')
+subdir('git')
 subdir('git-hashing')
 subdir('local-overlay-store')
 

From 0c32b0c8c32b558f6b2c3782172181ba2f2c3558 Mon Sep 17 00:00:00 2001
From: Farid Zakaria <farid.m.zakaria@gmail.com>
Date: Fri, 11 Jul 2025 15:57:27 -0700
Subject: [PATCH 2/2] Added comment to test case

---
 tests/functional/git/packed-refs-no-cache.sh | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/tests/functional/git/packed-refs-no-cache.sh b/tests/functional/git/packed-refs-no-cache.sh
index 0f39da775..54e0ab901 100644
--- a/tests/functional/git/packed-refs-no-cache.sh
+++ b/tests/functional/git/packed-refs-no-cache.sh
@@ -1,5 +1,16 @@
 #!/usr/bin/env bash
 
+# Please see https://github.com/NixOS/nix/issues/13457
+# for a higher description of the purpose of the test.
+# tl;dr;fetchGit will utilize the git cache and avoid refetching when possible.
+# It relies on the presence of either the commit when rev is provided
+# or checks if the ref refs/heads/<ref_name> if ref is provided.
+#
+# Unfortunately, git can occasionally "pack references" which moves the references
+# from individual files to a single unifies file.
+# When this occurs, nix can no longer check for the presence of the ref to check
+# for the mtime and will refetch unnecessarily.
+
 source ../common.sh
 
 requireGit
@@ -67,4 +78,4 @@ if [[ -e "$store_path/hello_again" ]]; then
 else
   echo "PASS: New commit was not fetched due to caching (as expected)."
   exit 1
-fi
\ No newline at end of file
+fi