commit-graph verify: detect inability to read the graph

Change "commit-graph verify" to error on open() failures other than
ENOENT. As noted in the third paragraph of 283e68c72f ("commit-graph:
add 'verify' subcommand", 2018-06-27) and the test it added it's
intentional that "commit-graph verify" doesn't error out when the file
doesn't exist.

But let's not be overly promiscuous in what we accept. If we can't
read the file for other reasons, e.g. permission errors, bad file
descriptor etc. we'd like to report an error to the user.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/commit-graph.c

@@ -62,8 +62,10 @@ static int graph_verify(int argc, const char **argv)
 
 	graph_name = get_commit_graph_filename(opts.obj_dir);
 	open_ok = open_commit_graph(graph_name, &fd, &st);
-	if (!open_ok)
+	if (!open_ok && errno == ENOENT)
 		return 0;
+	if (!open_ok)
+		die_errno(_("Could not open commit-graph '%s'"), graph_name);
 	graph = load_commit_graph_one_fd_st(fd, &st);
 	FREE_AND_NULL(graph_name);
 

t/t5318-commit-graph.sh

@@ -400,6 +400,12 @@ corrupt_graph_and_verify() {
 
 }
 
+test_expect_success POSIXPERM,SANITY 'detect permission problem' '
+	corrupt_graph_setup &&
+	chmod 000 $objdir/info/commit-graph &&
+	corrupt_graph_verify "Could not open"
+'
+
 test_expect_success 'detect too small' '
 	corrupt_graph_setup &&
 	echo "a small graph" >$objdir/info/commit-graph &&