Subject: [PATCH v4 1/3] packed-backend: fsck should warn when "packed-refs" file is empty

From: shejialuo <[email-masked]>
Date: Tue, 13 May 2025 19:07:40 +0800

During fsck, an empty "packed-refs" gives an error; this is unwarranted.
The runtime code paths would accept an empty "packed-refs" file, such as
"create_snapshot" would simply return the "snapshot" without checking
the content of "packed-refs".

And we should also skip checking the content of "packed-refs" when it is
empty during fsck. However, we should think about whether we need to
report something to the users in this case.

After 694b7a1999 (repack_without_ref(): write peeled refs in the
rewritten file, 2013-04-22), we would always write a header into the
"packed-refs" file where we would never create empty file since then.
Because we only create empty "packed-refs" in the very early versions,
we may tighten this rule in the future. In order to notify the users
about this, we should at least report an warning to the users.

But we need to consider the fsck message type carefully, it is not
appropriate that we use "FSCK_ERROR". This is because we would
definitely break the compatibility. Let's create a "FSCK_INFO" message
id EMPTY_PACKED_REFS_FILE" to indicate that "packed-refs" is empty.

Signed-off-by: shejialuo <[email-masked]>
---
 Documentation/fsck-msgids.adoc |  6 ++++++
 fsck.h                         |  1 +
 refs/packed-backend.c          |  9 +++++++++
 t/t0602-reffiles-fsck.sh       | 17 +++++++++++++++++
 4 files changed, 33 insertions(+)

diff --git a/Documentation/fsck-msgids.adoc b/Documentation/fsck-msgids.adoc
index 9601fff228..0ba4f9a27e 100644
--- a/Documentation/fsck-msgids.adoc
+++ b/Documentation/fsck-msgids.adoc
@@ -59,6 +59,12 @@
 `emptyName`::
 	(WARN) A path contains an empty name.
 
+`emptyPackedRefsFile`::
+	(INFO) "packed-refs" file is empty. Report to the
+	[email-masked] mailing list if you see this error. As only
+	very early versions of Git would create such an empty
+	"packed_refs" file, we might tighten this rule in the future.
+
 `extraHeaderEntry`::
 	(IGNORE) Extra headers found after `tagger`.
 
diff --git a/fsck.h b/fsck.h
index b1deae61ee..0c5869ac34 100644
--- a/fsck.h
+++ b/fsck.h
@@ -84,6 +84,7 @@ enum fsck_msg_type {
 	FUNC(LARGE_PATHNAME, WARN) \
 	/* infos (reported as warnings, but ignored by default) */ \
 	FUNC(BAD_FILEMODE, INFO) \
+	FUNC(EMPTY_PACKED_REFS_FILE, INFO) \
 	FUNC(GITMODULES_PARSE, INFO) \
 	FUNC(GITIGNORE_SYMLINK, INFO) \
 	FUNC(GITATTRIBUTES_SYMLINK, INFO) \
diff --git a/refs/packed-backend.c b/refs/packed-backend.c
index 3ad1ed0787..fb91833e76 100644
--- a/refs/packed-backend.c
+++ b/refs/packed-backend.c
@@ -2103,6 +2103,15 @@ static int packed_fsck(struct ref_store *ref_store,
 		goto cleanup;
 	}
 
+	if (!st.st_size) {
+		struct fsck_ref_report report = { 0 };
+		report.path = "packed-refs";
+		ret = fsck_report_ref(o, &report,
+				      FSCK_MSG_EMPTY_PACKED_REFS_FILE,
+				      "file is empty");
+		goto cleanup;
+	}
+
 	if (strbuf_read(&packed_ref_content, fd, 0) < 0) {
 		ret = error_errno(_("unable to read '%s'"), refs->path);
 		goto cleanup;
diff --git a/t/t0602-reffiles-fsck.sh b/t/t0602-reffiles-fsck.sh
index 9d1dc2144c..f671ac4d3a 100755
--- a/t/t0602-reffiles-fsck.sh
+++ b/t/t0602-reffiles-fsck.sh
@@ -647,6 +647,23 @@ test_expect_success SYMLINKS 'the filetype of packed-refs should be checked' '
 	)
 '
 
+test_expect_success 'empty packed-refs should be reported' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		test_commit default &&
+
+		>.git/packed-refs &&
+		git refs verify 2>err &&
+		cat >expect <<-EOF &&
+		warning: packed-refs: emptyPackedRefsFile: file is empty
+		EOF
+		rm .git/packed-refs &&
+		test_cmp expect err
+	)
+'
+
 test_expect_success 'packed-refs header should be checked' '
 	test_when_finished "rm -rf repo" &&
 	git init repo &&
-- 
2.49.0