commit-graph.c: no lazy fetch in lookup_commit_in_graph()

The commit-graph is used to opportunistically optimize accesses to
certain pieces of information on commit objects, and
lookup_commit_in_graph() tries to say "no" when the requested commit
does not locally exist by returning NULL, in which case the caller
can ask for (which may result in on-demand fetching from a promisor
remote) and parse the commit object itself.

However, it uses a wrong helper, repo_has_object_file(), to do so.
This helper not only checks if an object is mmediately available in
the local object store, but also tries to fetch from a promisor remote.
But the fetch machinery calls lookup_commit_in_graph(), thus causing an
infinite loop.

We should make lookup_commit_in_graph() expect that a commit given to it
can be legitimately missing from the local object store, by using the
has_object_file() helper instead.

Signed-off-by: Han Xin <hanxin.hx@bytedance.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit-graph.c

@@ -907,7 +907,7 @@ struct commit *lookup_commit_in_graph(struct repository *repo, const struct obje
 		return NULL;
 	if (!search_commit_pos_in_graph(id, repo->objects->commit_graph, &pos))
 		return NULL;
-	if (!repo_has_object_file(repo, id))
+	if (!has_object(repo, id, 0))
 		return NULL;
 
 	commit = lookup_commit(repo, id);

t/t5330-no-lazy-fetch-with-commit-graph.sh

@@ -0,0 +1,70 @@
+#!/bin/sh
+
+test_description='test for no lazy fetch with the commit-graph'
+
+. ./test-lib.sh
+
+run_with_limited_processses () {
+	# bash and ksh use "ulimit -u", dash uses "ulimit -p"
+	if test -n "$BASH_VERSION"
+	then
+		ulimit_max_process="-u"
+	elif test -n "$KSH_VERSION"
+	then
+		ulimit_max_process="-u"
+	fi
+	(ulimit ${ulimit_max_process-"-p"} 512 && "$@")
+}
+
+test_lazy_prereq ULIMIT_PROCESSES '
+	run_with_limited_processses true
+'
+
+if ! test_have_prereq ULIMIT_PROCESSES
+then
+	skip_all='skipping tests for no lazy fetch with the commit-graph, ulimit processes not available'
+	test_done
+fi
+
+test_expect_success 'setup: prepare a repository with a commit' '
+	git init with-commit &&
+	test_commit -C with-commit the-commit &&
+	oid=$(git -C with-commit rev-parse HEAD)
+'
+
+test_expect_success 'setup: prepare a repository with commit-graph contains the commit' '
+	git init with-commit-graph &&
+	echo "$(pwd)/with-commit/.git/objects" \
+		>with-commit-graph/.git/objects/info/alternates &&
+	# create a ref that points to the commit in alternates
+	git -C with-commit-graph update-ref refs/ref_to_the_commit "$oid" &&
+	# prepare some other objects to commit-graph
+	test_commit -C with-commit-graph something &&
+	git -c gc.writeCommitGraph=true -C with-commit-graph gc &&
+	test_path_is_file with-commit-graph/.git/objects/info/commit-graph
+'
+
+test_expect_success 'setup: change the alternates to what without the commit' '
+	git init --bare without-commit &&
+	git -C with-commit-graph cat-file -e $oid &&
+	echo "$(pwd)/without-commit/objects" \
+		>with-commit-graph/.git/objects/info/alternates &&
+	test_must_fail git -C with-commit-graph cat-file -e $oid
+'
+
+test_expect_success 'fetch any commit from promisor with the usage of the commit graph' '
+	# setup promisor and prepare any commit to fetch
+	git -C with-commit-graph remote add origin "$(pwd)/with-commit" &&
+	git -C with-commit-graph config remote.origin.promisor true &&
+	git -C with-commit-graph config remote.origin.partialclonefilter blob:none &&
+	test_commit -C with-commit any-commit &&
+	anycommit=$(git -C with-commit rev-parse HEAD) &&
+
+	run_with_limited_processses env GIT_TRACE="$(pwd)/trace.txt" \
+		git -C with-commit-graph fetch origin $anycommit 2>err &&
+	! grep "fatal: promisor-remote: unable to fork off fetch subprocess" err &&
+	grep "git fetch origin" trace.txt >actual &&
+	test_line_count = 1 actual
+'
+
+test_done