core: Try to prevent infinite recursive template instantiation

To prevent situations like in #17602 from happening, let's drop
direct recursive template dependencies. These will almost certainly
lead to infinite recursion so let's drop them immediately to avoid
instantiating potentially thousands of irrelevant units.

Example of a template that would lead to infinite recursion which
is caught by this check:

notify@.service:

```
[Unit]
Wants=notify@%n.service
```

1 files changed, 17 insertions, 0 deletions

diff --git a/src/test/test-unit-name.c b/src/test/test-unit-name.c
index 0077c4c5e3..665e7b7933 100644
--- a/src/test/test-unit-name.c
+++ b/src/test/test-unit-name.c
@@ -870,6 +870,22 @@ static void test_unit_name_from_dbus_path(void) {
         test_unit_name_from_dbus_path_one("/org/freedesktop/systemd1/unit/wpa_5fsupplicant_2eservice", 0, "wpa_supplicant.service");
 }
 
+static void test_unit_name_prefix_equal(void) {
+        log_info("/* %s */", __func__);
+
+        assert_se(unit_name_prefix_equal("a.service", "a.service"));
+        assert_se(unit_name_prefix_equal("a.service", "a.mount"));
+        assert_se(unit_name_prefix_equal("a@b.service", "a.service"));
+        assert_se(unit_name_prefix_equal("a@b.service", "a@c.service"));
+
+        assert_se(!unit_name_prefix_equal("a.service", "b.service"));
+        assert_se(!unit_name_prefix_equal("a.service", "b.mount"));
+        assert_se(!unit_name_prefix_equal("a@a.service", "b.service"));
+        assert_se(!unit_name_prefix_equal("a@a.service", "b@a.service"));
+        assert_se(!unit_name_prefix_equal("a", "b"));
+        assert_se(!unit_name_prefix_equal("a", "a"));
+}
+
 int main(int argc, char* argv[]) {
         _cleanup_(rm_rf_physical_and_freep) char *runtime_dir = NULL;
         int r, rc = 0;
@@ -902,6 +918,7 @@ int main(int argc, char* argv[]) {
         test_unit_name_path_unescape();
         test_unit_name_to_prefix();
         test_unit_name_from_dbus_path();
+        test_unit_name_prefix_equal();
 
         return rc;
 }