cgroup: Allow empty assignments of Memory{Low,Min}=

Currently, an empty assignment of Memory{Low,Min}= directives would be
interpretted as setting it to global default, i.e. zero. However, if we
set a runtime protection value on a unit that inherits parent's
DefaultMemory{Low,Min}=, it is not possible to revert it back to the
state where the DefaultMemory{Low,Min}= is propagated from parent
slice(s).

This patch changes the semantics of the empty assignments to explicitly
nullify any value set by the user previously. Since DBus API uses
uint64_t where 0 is a valid configuration, the patch modifies DBus API
by exploiting the variant type of property value to pass the NULL value.

1 files changed, 4 insertions, 2 deletions

diff --git a/src/core/load-fragment.c b/src/core/load-fragment.c
index 781fe66f5e..266bd68cb6 100644
--- a/src/core/load-fragment.c
+++ b/src/core/load-fragment.c
@@ -3396,6 +3396,8 @@ int config_parse_memory_limit(
                 }
         }
 
+        /* Keep Memory{Low,Min} unset with empty assignment so that we fall back to DefaultMemory* which in
+         * contrast means zeroing the property. */
         if (streq(lvalue, "DefaultMemoryLow")) {
                 c->default_memory_low = bytes;
                 c->default_memory_low_set = true;
@@ -3404,10 +3406,10 @@ int config_parse_memory_limit(
                 c->default_memory_min_set = true;
         } else if (streq(lvalue, "MemoryMin")) {
                 c->memory_min = bytes;
-                c->memory_min_set = true;
+                c->memory_min_set = !isempty(rvalue);
         } else if (streq(lvalue, "MemoryLow")) {
                 c->memory_low = bytes;
-                c->memory_low_set = true;
+                c->memory_low_set = !isempty(rvalue);
         } else if (streq(lvalue, "MemoryHigh"))
                 c->memory_high = bytes;
         else if (streq(lvalue, "MemoryMax"))