core: add new ConditionNeedsUpdate= unit condition

This new condition allows checking whether /etc or /var are out-of-date
relative to /usr. This is the counterpart for the update flag managed by
systemd-update-done.service. Services that want to be started once after
/usr got updated should use:

        [Unit]
        ConditionNeedsUpdate=/etc
        Before=systemd-update-done.service

This makes sure that they are only run if /etc is out-of-date relative
to /usr. And that it will be executed after systemd-update-done.service
which is responsible for marking /etc up-to-date relative to the current
/usr.

ConditionNeedsUpdate= will also checks whether /etc is actually
writable, and not trigger if it isn't, since no update is possible then.

1 files changed, 1 insertions, 0 deletions

diff --git a/src/shared/condition-util.c b/src/shared/condition-util.c
index 1c7d1f6a5e..7b89b0fbaf 100644
--- a/src/shared/condition-util.c
+++ b/src/shared/condition-util.c
@@ -256,6 +256,7 @@ static const char* const condition_type_table[_CONDITION_TYPE_MAX] = {
         [CONDITION_HOST] = "ConditionHost",
         [CONDITION_AC_POWER] = "ConditionACPower",
         [CONDITION_ARCHITECTURE] = "ConditionArchitecture",
+        [CONDITION_NEEDS_UPDATE] = "ConditionNeedsUpdate",
         [CONDITION_NULL] = "ConditionNull"
 };