packed-backend: fsck should warn when "packed-refs" file is empty

We assume the "packed-refs" won't be empty and instead has at least one
line in it (even when there are no refs packed, there is the file header
line). Because there is no terminating LF in the empty file, we will
report "packedRefEntryNotTerminated(ERROR)" to the user.

However, the runtime code paths would accept an empty "packed-refs"
file, for example, "create_snapshot" would simply return the "snapshot"
without checking the content of "packed-refs". So, we should skip
checking the content of "packed-refs" when it is empty during fsck.

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. So, versions of Git that are not too ancient never
write such an empty "packed-refs" file.

As an empty file often indicates a sign of a filesystem-level issue, the
way we want to resolve this inconsistency is not make everybody totally
silent but notice and report the anomaly.

Let's create a "FSCK_INFO" message id "EMPTY_PACKED_REFS_FILE" to report
to the users that "packed-refs" is empty.

Signed-off-by: shejialuo <shejialuo@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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
+	git@vger.kernel.org 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`.
 

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) \

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;

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 &&