core: complain if Before= dep on .device is declared

[Unit]
Before=foobar.device

[Service]
ExecStart=/bin/true
Type=oneshot

$ systemd-analyze verify before-device.service
before-device.service: Dependency Before=foobar.device ignored (.device units cannot be delayed)

2 files changed, 5 insertions, 2 deletions

diff --git a/TODO b/TODO
index 64d530b7d7..ff9e9be9fb 100644
--- a/TODO
+++ b/TODO
@@ -139,8 +139,6 @@ Features:
 
 * PID 1 should send out sd_notify("WATCHDOG=1") messages (for usage in the --user mode, and when run via nspawn)
 
-* consider throwing a warning if a service declares it wants to be "Before=" a .device unit.
-
 * there's probably something wrong with having user mounts below /sys,
   as we have for debugfs. for exmaple, src/core/mount.c handles mounts
   prefixed with /sys generally special.
diff --git a/src/core/unit.c b/src/core/unit.c
index 5d284a359d..693f75c928 100644
--- a/src/core/unit.c
+++ b/src/core/unit.c
@@ -2225,6 +2225,11 @@ int unit_add_dependency(Unit *u, UnitDependency d, Unit *other, bool add_referen
                 return 0;
         }
 
+        if (d == UNIT_BEFORE && other->type == UNIT_DEVICE) {
+                log_unit_warning(u, "Dependency Before=%s ignored (.device units cannot be delayed)", other->id);
+                return 0;
+        }
+
         r = set_ensure_allocated(&u->dependencies[d], NULL);
         if (r < 0)
                 return r;