thodg/libgit2

diff --git a/src/remote.c b/src/remote.c
index 5f6ba5b..ec68cc0 100644
--- a/src/remote.c
+++ b/src/remote.c
@@ -1460,6 +1460,11 @@ static int update_tips_for_spec(
 		if (error < 0 && error != GIT_ENOTFOUND)
 			goto on_error;
 
+		if (!(error || error == GIT_ENOTFOUND)
+				&& !spec->force
+				&& !git_graph_descendant_of(remote->repo, &head->oid, &old))
+			continue;
+
 		if (error == GIT_ENOTFOUND) {
 			memset(&old, 0, GIT_OID_RAWSZ);
 
diff --git a/tests/remote/fetch.c b/tests/remote/fetch.c
new file mode 100644
index 0000000..392801e
--- /dev/null
+++ b/tests/remote/fetch.c
@@ -0,0 +1,169 @@
+#include "../clar_libgit2.h"
+
+#include "remote.h"
+#include "repository.h"
+
+static git_repository *repo1;
+static git_repository *repo2;
+static char* repo1_path;
+static char* repo2_path;
+
+static const char *REPO1_REFNAME = "refs/heads/main";
+static const char *REPO2_REFNAME = "refs/remotes/repo1/main";
+static char *FORCE_FETCHSPEC = "+refs/heads/main:refs/remotes/repo1/main";
+static char *NON_FORCE_FETCHSPEC = "refs/heads/main:refs/remotes/repo1/main";
+
+void test_remote_fetch__initialize(void) {
+	git_config *c;
+	git_buf repo1_path_buf = GIT_BUF_INIT;
+	git_buf repo2_path_buf = GIT_BUF_INIT;
+	const char *sandbox = clar_sandbox_path();
+
+	cl_git_pass(git_buf_joinpath(&repo1_path_buf, sandbox, "fetchtest_repo1"));
+	repo1_path = git_buf_detach(&repo1_path_buf);
+	cl_git_pass(git_repository_init(&repo1, repo1_path, true));
+
+	cl_git_pass(git_buf_joinpath(&repo2_path_buf, sandbox, "fetchtest_repo2"));
+	repo2_path = git_buf_detach(&repo2_path_buf);
+	cl_git_pass(git_repository_init(&repo2, repo2_path, true));
+
+	cl_git_pass(git_repository_config(&c, repo1));
+	cl_git_pass(git_config_set_string(c, "user.email", "some@email"));
+	cl_git_pass(git_config_set_string(c, "user.name", "some@name"));
+	git_config_free(c);
+	git_buf_dispose(&repo1_path_buf);
+	git_buf_dispose(&repo2_path_buf);
+}
+
+void test_remote_fetch__cleanup(void) {
+	git_repository_free(repo1);
+	git_repository_free(repo2);
+
+	cl_git_pass(git_futils_rmdir_r(repo1_path, NULL, GIT_RMDIR_REMOVE_FILES));
+	free(repo1_path);
+
+	cl_git_pass(git_futils_rmdir_r(repo2_path, NULL, GIT_RMDIR_REMOVE_FILES));
+	free(repo2_path);
+}
+
+
+/**
+ * This checks that the '+' flag on fetchspecs is respected. We create a
+ * repository that has a reference to two commits, one a child of the other.
+ * We fetch this repository into a second repository. Then we reset the
+ * reference in the first repository and run the fetch again. If the '+' flag
+ * is used then the reference in the second repository will change, but if it
+ * is not then it should stay the same.
+ *
+ * @param commit1id A pointer to an OID which will be populated with the first
+ *                  commit.
+ * @param commit2id A pointer to an OID which will be populated with the second
+ *                  commit, which is a descendant of the first.
+ * @param force     Whether to use a spec with '+' prefixed to force the refs
+ *                  to update
+ */
+void do_time_travelling_fetch(git_oid *commit1id, git_oid *commit2id,
+		bool force) {
+	char *refspec_strs = {
+		force ? FORCE_FETCHSPEC : NON_FORCE_FETCHSPEC,
+	};
+	git_strarray refspecs = {
+		.count = 1,
+		.strings = &refspec_strs,
+	};
+
+	// create two commits in repo 1 and a reference to them
+	{
+		git_oid empty_tree_id;
+		git_tree *empty_tree;
+		git_signature *sig;
+		git_treebuilder *tb;
+		cl_git_pass(git_treebuilder_new(&tb, repo1, NULL));
+		cl_git_pass(git_treebuilder_write(&empty_tree_id, tb));
+		cl_git_pass(git_tree_lookup(&empty_tree, repo1, &empty_tree_id));
+		cl_git_pass(git_signature_default(&sig, repo1));
+		cl_git_pass(git_commit_create(commit1id, repo1, REPO1_REFNAME, sig, 
+					sig, NULL, "one", empty_tree, 0, NULL));
+		cl_git_pass(git_commit_create_v(commit2id, repo1, REPO1_REFNAME, sig, 
+					sig, NULL, "two", empty_tree, 1, commit1id));
+
+		git_tree_free(empty_tree);
+		git_signature_free(sig);
+		git_treebuilder_free(tb);
+	}
+
+	// fetch the reference via the remote
+	{
+		git_remote *remote;
+
+		cl_git_pass(git_remote_create_anonymous(&remote, repo2,
+					git_repository_path(repo1)));
+		cl_git_pass(git_remote_fetch(remote, &refspecs, NULL, "some message"));
+
+		git_remote_free(remote);
+	}
+
+	// assert that repo2 references the second commit
+	{
+		const git_oid *target;
+		git_reference *ref;
+		cl_git_pass(git_reference_lookup(&ref, repo2, REPO2_REFNAME));
+		target = git_reference_target(ref);
+		cl_assert_equal_b(git_oid_cmp(target, commit2id), 0);
+		git_reference_free(ref);
+	}
+
+	// set the reference in repo1 to point to the older commit
+	{
+		git_reference *ref;
+		git_reference *ref2;
+		cl_git_pass(git_reference_lookup(&ref, repo1, REPO1_REFNAME));
+		cl_git_pass(git_reference_set_target(&ref2, ref, commit1id, 
+					"rollback"));
+		git_reference_free(ref);
+		git_reference_free(ref2);
+	}
+
+	// fetch the reference again
+	{
+		git_remote *remote;
+
+		cl_git_pass(git_remote_create_anonymous(&remote, repo2,
+					git_repository_path(repo1)));
+		cl_git_pass(git_remote_fetch(remote, &refspecs, NULL, "some message"));
+
+		git_remote_free(remote);
+	}
+}
+
+void test_remote_fetch__dont_update_refs_if_not_descendant_and_not_force(void) {
+	const git_oid *target;
+	git_oid commit1id;
+	git_oid commit2id;
+	git_reference *ref;
+
+	do_time_travelling_fetch(&commit1id, &commit2id, false);
+
+	// assert that the reference in repo2 has not changed
+	cl_git_pass(git_reference_lookup(&ref, repo2, REPO2_REFNAME));
+	target = git_reference_target(ref);
+	cl_assert_equal_b(git_oid_cmp(target, &commit2id), 0);
+
+	git_reference_free(ref);
+}
+
+void test_remote_fetch__do_update_refs_if_not_descendant_and_force(void) {
+	const git_oid *target;
+	git_oid commit1id;
+	git_oid commit2id;
+	git_reference *ref;
+
+	do_time_travelling_fetch(&commit1id, &commit2id, true);
+
+	// assert that the reference in repo2 has changed
+	cl_git_pass(git_reference_lookup(&ref, repo2, REPO2_REFNAME));
+	target = git_reference_target(ref);
+	cl_assert_equal_b(git_oid_cmp(target, &commit1id), 0);
+
+	git_reference_free(ref);
+}