fsck: verify commit graph when implicitly enabled

Change fsck to check the "core_commit_graph" variable set in
"repo-settings.c" instead of reading the "core.commitGraph" variable.
This fixes a bug where we wouldn't verify the commit-graph if the
config key was missing. This bug was introduced in
31b1de6a09 (commit-graph: turn on commit-graph by default, 2019-08-13),
where core.commitGraph was turned on by default.

Add tests to "t5318-commit-graph.sh" to verify that fsck checks the
commit-graph as expected for the 3 values of core.commitGraph. Also,
disable GIT_TEST_COMMIT_GRAPH in t/t0410-partial-clone.sh because some
test cases use fsck in ways that assume that commit-graph checking is
disabled.

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

builtin/fsck.c

@@ -803,6 +803,7 @@ int cmd_fsck(int argc, const char **argv, const char *prefix)
 		fsck_enable_object_names(&fsck_walk_options);
 
 	git_config(git_fsck_config, &fsck_obj_options);
+	prepare_repo_settings(the_repository);
 
 	if (connectivity_only) {
 		for_each_loose_object(mark_loose_for_connectivity, NULL, 0);
@@ -908,7 +909,7 @@ int cmd_fsck(int argc, const char **argv, const char *prefix)
 
 	check_connectivity();
 
-	if (!git_config_get_bool("core.commitgraph", &i) && i) {
+	if (the_repository->settings.core_commit_graph) {
 		struct child_process commit_graph_verify = CHILD_PROCESS_INIT;
 		const char *verify_argv[] = { "commit-graph", "verify", NULL, NULL, NULL };
 

t/t0410-partial-clone.sh

@@ -6,6 +6,10 @@ test_description='partial clone'
 
 # missing promisor objects cause repacks which write bitmaps to fail
 GIT_TEST_MULTI_PACK_INDEX_WRITE_BITMAP=0
+# When enabled, some commands will write commit-graphs. This causes fsck
+# to fail when delete_object() is called because fsck will attempt to
+# verify the out-of-sync commit graph.
+GIT_TEST_COMMIT_GRAPH=0
 
 delete_object () {
 	rm $1/.git/objects/$(echo $2 | sed -e 's|^..|&/|')
@@ -322,7 +326,7 @@ test_expect_success 'rev-list stops traversal at missing and promised commit' '
 
 	git -C repo config core.repositoryformatversion 1 &&
 	git -C repo config extensions.partialclone "arbitrary string" &&
-	GIT_TEST_COMMIT_GRAPH=0 git -C repo -c core.commitGraph=false rev-list --exclude-promisor-objects --objects bar >out &&
+	git -C repo rev-list --exclude-promisor-objects --objects bar >out &&
 	grep $(git -C repo rev-parse bar) out &&
 	! grep $FOO out
 '

t/t5318-commit-graph.sh

@@ -694,12 +694,33 @@ test_expect_success 'detect incorrect chunk count' '
 		$GRAPH_CHUNK_LOOKUP_OFFSET
 '
 
-test_expect_success 'git fsck (checks commit-graph)' '
+test_expect_success 'git fsck (checks commit-graph when config set to true)' '
 	cd "$TRASH_DIRECTORY/full" &&
 	git fsck &&
 	corrupt_graph_and_verify $GRAPH_BYTE_FOOTER "\00" \
 		"incorrect checksum" &&
 	cp commit-graph-pre-write-test $objdir/info/commit-graph &&
+	test_must_fail git -c core.commitGraph=true fsck
+'
+
+test_expect_success 'git fsck (ignores commit-graph when config set to false)' '
+	cd "$TRASH_DIRECTORY/full" &&
+	git fsck &&
+	corrupt_graph_and_verify $GRAPH_BYTE_FOOTER "\00" \
+		"incorrect checksum" &&
+	cp commit-graph-pre-write-test $objdir/info/commit-graph &&
+	git -c core.commitGraph=false fsck
+'
+
+test_expect_success 'git fsck (checks commit-graph when config unset)' '
+	cd "$TRASH_DIRECTORY/full" &&
+	test_when_finished "git config core.commitGraph true" &&
+
+	git fsck &&
+	corrupt_graph_and_verify $GRAPH_BYTE_FOOTER "\00" \
+		"incorrect checksum" &&
+	test_unconfig core.commitGraph &&
+	cp commit-graph-pre-write-test $objdir/info/commit-graph &&
 	test_must_fail git fsck
 '