core: hook up timer unit type with clean operation

timer units maintain state on disk (the persistent touch file), hence
let's expose cleaning it up generically with the new cleaning operation
for units.

This is a much simpler implementation as for the service unit type:
instead of forking out a worker process we just remove the touch file
directly. That should be OK since we only need to remove a single
(empty) file, instead of a recursive user-controlled directory tree.

Fixes: #4930

src/core/timer.c

@@ -833,6 +833,41 @@ static void timer_timezone_change(Unit *u) {
         }
 }
 
+static int timer_clean(Unit *u, ExecCleanMask mask) {
+        Timer *t = TIMER(u);
+        int r;
+
+        assert(t);
+        assert(mask != 0);
+
+        if (t->state != TIMER_DEAD)
+                return -EBUSY;
+
+        if (!IN_SET(mask, EXEC_CLEAN_STATE))
+                return -EUNATCH;
+
+        r = timer_setup_persistent(t);
+        if (r < 0)
+                return r;
+
+        if (!t->stamp_path)
+                return -EUNATCH;
+
+        if (unlink(t->stamp_path) && errno != ENOENT)
+                return log_unit_error_errno(u, errno, "Failed to clean stamp file of timer: %m");
+
+        return 0;
+}
+
+static int timer_can_clean(Unit *u, ExecCleanMask *ret) {
+        Timer *t = TIMER(u);
+
+        assert(t);
+
+        *ret = t->persistent ? EXEC_CLEAN_STATE : 0;
+        return 0;
+}
+
 static const char* const timer_base_table[_TIMER_BASE_MAX] = {
         [TIMER_ACTIVE] = "OnActiveSec",
         [TIMER_BOOT] = "OnBootSec",
@@ -872,6 +907,9 @@ const UnitVTable timer_vtable = {
         .start = timer_start,
         .stop = timer_stop,
 
+        .clean = timer_clean,
+        .can_clean = timer_can_clean,
+
         .serialize = timer_serialize,
         .deserialize_item = timer_deserialize_item,