core: clearly refuse OnFailure= deps on units that can't fail

Similar, refuse triggering deps on units that cannot trigger.

And rework how we ignore After= dependencies on device units, to work
the same way.

See: #14142

src/core/automount.c

@@ -1106,6 +1106,10 @@ const UnitVTable automount_vtable = {
                 "Automount\0"
                 "Install\0",
 
+        .can_transient = true,
+        .can_fail = true,
+        .can_trigger = true,
+
         .init = automount_init,
         .load = automount_load,
         .done = automount_done,
@@ -1132,8 +1136,6 @@ const UnitVTable automount_vtable = {
         .bus_vtable = bus_automount_vtable,
         .bus_set_property = bus_automount_set_property,
 
-        .can_transient = true,
-
         .shutdown = automount_shutdown,
         .supported = automount_supported,
 

src/core/device.c

@@ -1064,6 +1064,7 @@ const UnitVTable device_vtable = {
                 "Device\0"
                 "Install\0",
 
+        .refuse_after = true,
         .gc_jobs = true,
 
         .init = device_init,

src/core/mount.c

@@ -2065,6 +2065,9 @@ const UnitVTable mount_vtable = {
                 "Install\0",
         .private_section = "Mount",
 
+        .can_transient = true,
+        .can_fail = true,
+
         .init = mount_init,
         .load = mount_load,
         .done = mount_done,
@@ -2103,8 +2106,6 @@ const UnitVTable mount_vtable = {
 
         .get_timeout = mount_get_timeout,
 
-        .can_transient = true,
-
         .enumerate_perpetual = mount_enumerate_perpetual,
         .enumerate = mount_enumerate,
         .shutdown = mount_shutdown,

src/core/path.c

@@ -806,6 +806,8 @@ const UnitVTable path_vtable = {
         .private_section = "Path",
 
         .can_transient = true,
+        .can_fail = true,
+        .can_trigger = true,
 
         .init = path_init,
         .done = path_done,

src/core/scope.c

@@ -619,6 +619,7 @@ const UnitVTable scope_vtable = {
 
         .can_transient = true,
         .can_delegate = true,
+        .can_fail = true,
         .once_only = true,
 
         .init = scope_init,

src/core/service.c

@@ -4391,6 +4391,7 @@ const UnitVTable service_vtable = {
 
         .can_transient = true,
         .can_delegate = true,
+        .can_fail = true,
 
         .init = service_init,
         .done = service_done,

src/core/socket.c

@@ -3417,6 +3417,8 @@ const UnitVTable socket_vtable = {
         .private_section = "Socket",
 
         .can_transient = true,
+        .can_trigger = true,
+        .can_fail = true,
 
         .init = socket_init,
         .done = socket_done,

src/core/swap.c

@@ -1593,6 +1593,8 @@ const UnitVTable swap_vtable = {
                 "Install\0",
         .private_section = "Swap",
 
+        .can_fail = true,
+
         .init = swap_init,
         .load = swap_load,
         .done = swap_done,

src/core/timer.c

@@ -895,6 +895,10 @@ const UnitVTable timer_vtable = {
                 "Install\0",
         .private_section = "Timer",
 
+        .can_transient = true,
+        .can_fail = true,
+        .can_trigger = true,
+
         .init = timer_init,
         .done = timer_done,
         .load = timer_load,
@@ -923,6 +927,4 @@ const UnitVTable timer_vtable = {
 
         .bus_vtable = bus_timer_vtable,
         .bus_set_property = bus_timer_set_property,
-
-        .can_transient = true,
 };

src/core/unit.c

@@ -2937,12 +2937,28 @@ int unit_add_dependency(
                 return 0;
         }
 
-        if ((d == UNIT_BEFORE && other->type == UNIT_DEVICE) ||
-            (d == UNIT_AFTER && u->type == UNIT_DEVICE)) {
-                log_unit_warning(u, "Dependency Before=%s ignored (.device units cannot be delayed)", other->id);
+        if (d == UNIT_AFTER && UNIT_VTABLE(u)->refuse_after) {
+                log_unit_warning(u, "Requested dependency After=%s ignored (%s units cannot be delayed).", other->id, unit_type_to_string(u->type));
                 return 0;
         }
 
+        if (d == UNIT_BEFORE && UNIT_VTABLE(other)->refuse_after) {
+                log_unit_warning(u, "Requested dependency Before=%s ignored (%s units cannot be delayed).", other->id, unit_type_to_string(other->type));
+                return 0;
+        }
+
+        if (d == UNIT_ON_FAILURE && !UNIT_VTABLE(u)->can_fail) {
+                log_unit_warning(u, "Requested dependency OnFailure=%s ignored (%s units cannot fail).", other->id, unit_type_to_string(u->type));
+                return 0;
+        }
+
+        if (d == UNIT_TRIGGERS && !UNIT_VTABLE(u)->can_trigger)
+                return log_unit_error_errno(u, SYNTHETIC_ERRNO(EINVAL),
+                                            "Requested dependency Triggers=%s refused (%s units cannot trigger other units).", other->id, unit_type_to_string(u->type));
+        if (d == UNIT_TRIGGERED_BY && !UNIT_VTABLE(other)->can_trigger)
+                return log_unit_error_errno(u, SYNTHETIC_ERRNO(EINVAL),
+                                            "Requested dependency TriggeredBy=%s refused (%s units cannot trigger other units).", other->id, unit_type_to_string(other->type));
+
         r = unit_add_dependency_hashmap(u->dependencies + d, other, mask, 0);
         if (r < 0)
                 return r;

src/core/unit.h

@@ -600,6 +600,15 @@ typedef struct UnitVTable {
         /* True if cgroup delegation is permissible */
         bool can_delegate:1;
 
+        /* True if the unit type triggers other units, i.e. can have a UNIT_TRIGGERS dependency */
+        bool can_trigger:1;
+
+        /* True if the unit type knows a failure state, and thus can be source of an OnFailure= dependency */
+        bool can_fail:1;
+
+        /* True if After= dependencies should be refused */
+        bool refuse_after:1;
+
         /* True if units of this type shall be startable only once and then never again */
         bool once_only:1;