core: replace slice dependencies as they get added

Defines a "UNIT_DEPENDENCY_SLICE_PROPERTY" UnitDependencyMask type that
is used when adding slices to the dependencies hashmap. This type is
used to remove slice dependencies when they get overridden by new ones.

Fixes #20182

5 files changed, 15 insertions, 7 deletions

diff --git a/src/core/dbus-unit.c b/src/core/dbus-unit.c
index fe320f1b05..d4ec789a7c 100644
--- a/src/core/dbus-unit.c
+++ b/src/core/dbus-unit.c
@@ -2273,7 +2273,7 @@ static int bus_unit_set_transient_property(
                         return sd_bus_error_setf(error, SD_BUS_ERROR_INVALID_ARGS, "Unit name '%s' is not a slice", s);
 
                 if (!UNIT_WRITE_FLAGS_NOOP(flags)) {
-                        r = unit_set_slice(u, slice, UNIT_DEPENDENCY_FILE);
+                        r = unit_set_slice(u, slice);
                         if (r < 0)
                                 return r;
 
diff --git a/src/core/load-fragment.c b/src/core/load-fragment.c
index 7ef6bcc41d..6efcba3265 100644
--- a/src/core/load-fragment.c
+++ b/src/core/load-fragment.c
@@ -3793,7 +3793,7 @@ int config_parse_unit_slice(
                 return 0;
         }
 
-        r = unit_set_slice(u, slice, UNIT_DEPENDENCY_FILE);
+        r = unit_set_slice(u, slice);
         if (r < 0) {
                 log_syntax(unit, LOG_WARNING, filename, line, r, "Failed to assign slice %s to unit %s, ignoring: %m", slice->id, u->id);
                 return 0;
diff --git a/src/core/unit-serialize.c b/src/core/unit-serialize.c
index 3458d7017b..7d2e6bc130 100644
--- a/src/core/unit-serialize.c
+++ b/src/core/unit-serialize.c
@@ -593,6 +593,7 @@ static void print_unit_dependency_mask(FILE *f, const char *kind, UnitDependency
                 { UNIT_DEPENDENCY_MOUNTINFO_IMPLICIT, "mountinfo-implicit" },
                 { UNIT_DEPENDENCY_MOUNTINFO_DEFAULT,  "mountinfo-default"  },
                 { UNIT_DEPENDENCY_PROC_SWAP,          "proc-swap"          },
+                { UNIT_DEPENDENCY_SLICE_PROPERTY,     "slice-property"     },
         };
 
         assert(f);
diff --git a/src/core/unit.c b/src/core/unit.c
index 59be3b78eb..77d4ceaf24 100644
--- a/src/core/unit.c
+++ b/src/core/unit.c
@@ -3285,7 +3285,7 @@ reset:
         return r;
 }
 
-int unit_set_slice(Unit *u, Unit *slice, UnitDependencyMask mask) {
+int unit_set_slice(Unit *u, Unit *slice) {
         int r;
 
         assert(u);
@@ -3318,7 +3318,11 @@ int unit_set_slice(Unit *u, Unit *slice, UnitDependencyMask mask) {
         if (UNIT_GET_SLICE(u) && u->cgroup_realized)
                 return -EBUSY;
 
-        r = unit_add_dependency(u, UNIT_IN_SLICE, slice, true, mask);
+        /* Remove any slices assigned prior; we should only have one UNIT_IN_SLICE dependency */
+        if (UNIT_GET_SLICE(u))
+                unit_remove_dependencies(u, UNIT_DEPENDENCY_SLICE_PROPERTY);
+
+        r = unit_add_dependency(u, UNIT_IN_SLICE, slice, true, UNIT_DEPENDENCY_SLICE_PROPERTY);
         if (r < 0)
                 return r;
 
@@ -3374,7 +3378,7 @@ int unit_set_default_slice(Unit *u) {
         if (r < 0)
                 return r;
 
-        return unit_set_slice(u, slice, UNIT_DEPENDENCY_FILE);
+        return unit_set_slice(u, slice);
 }
 
 const char *unit_slice_name(Unit *u) {
diff --git a/src/core/unit.h b/src/core/unit.h
index b49ae7c1b8..3f3a75d33b 100644
--- a/src/core/unit.h
+++ b/src/core/unit.h
@@ -89,7 +89,10 @@ typedef enum UnitDependencyMask {
         /* A dependency created because of data read from /proc/swaps and no other configuration source */
         UNIT_DEPENDENCY_PROC_SWAP          = 1 << 7,
 
-        _UNIT_DEPENDENCY_MASK_FULL         = (1 << 8) - 1,
+        /* A dependency for units in slices assigned by directly setting Slice= */
+        UNIT_DEPENDENCY_SLICE_PROPERTY     = 1 << 8,
+
+        _UNIT_DEPENDENCY_MASK_FULL         = (1 << 9) - 1,
 } UnitDependencyMask;
 
 /* The Unit's dependencies[] hashmaps use this structure as value. It has the same size as a void pointer, and thus can
@@ -782,7 +785,7 @@ Unit *unit_follow_merge(Unit *u) _pure_;
 int unit_load_fragment_and_dropin(Unit *u, bool fragment_required);
 int unit_load(Unit *unit);
 
-int unit_set_slice(Unit *u, Unit *slice, UnitDependencyMask mask);
+int unit_set_slice(Unit *u, Unit *slice);
 int unit_set_default_slice(Unit *u);
 
 const char *unit_description(Unit *u) _pure_;