bus: introduce concept of "const" properties

This way we have four kinds of properties:

a) those which are constant as long as an object exists
b) those which can change and PropertiesChange messages with contents are generated
c) those which can change and where the PropertesChange merely includes invalidation
d) those which can change but for which no events are generated

Clients (through code generators run on the introspection XML) can thus
aggressively cache a, b, c, with only d excluded.

4 files changed, 21 insertions, 17 deletions

diff --git a/src/libsystemd-bus/bus-introspect.c b/src/libsystemd-bus/bus-introspect.c
index 4d5c25a23f..d528ab2a04 100644
--- a/src/libsystemd-bus/bus-introspect.c
+++ b/src/libsystemd-bus/bus-introspect.c
@@ -82,10 +82,12 @@ static void introspect_write_flags(struct introspect *i, int type, int flags) {
                 fputs("   <annotation name=\"org.freedesktop.DBus.Method.NoReply\" value=\"true\"/>\n", i->f);
 
         if (type == _SD_BUS_VTABLE_PROPERTY || type == _SD_BUS_VTABLE_WRITABLE_PROPERTY) {
-                if (!(flags & SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE))
-                        fputs("   <annotation name=\"org.freedesktop.DBus.Property.EmitsChangedSignal\" value=\"false\"/>\n", i->f);
-                else if (flags & SD_BUS_VTABLE_PROPERTY_INVALIDATE_ONLY)
+                if (flags & SD_BUS_VTABLE_PROPERTY_CONST)
+                        fputs("   <annotation name=\"org.freedesktop.DBus.Property.EmitsChangedSignal\" value=\"const\"/>\n", i->f);
+                else if (flags & SD_BUS_VTABLE_PROPERTY_EMITS_INVALIDATION)
                         fputs("   <annotation name=\"org.freedesktop.DBus.Property.EmitsChangedSignal\" value=\"invalidates\"/>\n", i->f);
+                else if (!(flags & SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE))
+                        fputs("   <annotation name=\"org.freedesktop.DBus.Property.EmitsChangedSignal\" value=\"false\"/>\n", i->f);
         }
 
         if (!i->trusted &&
diff --git a/src/libsystemd-bus/bus-objects.c b/src/libsystemd-bus/bus-objects.c
index c3889b7949..e468025a81 100644
--- a/src/libsystemd-bus/bus-objects.c
+++ b/src/libsystemd-bus/bus-objects.c
@@ -1680,7 +1680,7 @@ static int add_object_vtable_internal(
                             !signature_is_valid(strempty(v->x.method.signature), false) ||
                             !signature_is_valid(strempty(v->x.method.result), false) ||
                             !(v->x.method.handler || (isempty(v->x.method.signature) && isempty(v->x.method.result))) ||
-                            v->flags & (SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE|SD_BUS_VTABLE_PROPERTY_INVALIDATE_ONLY)) {
+                            v->flags & (SD_BUS_VTABLE_PROPERTY_CONST|SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE|SD_BUS_VTABLE_PROPERTY_EMITS_INVALIDATION)) {
                                 r = -EINVAL;
                                 goto fail;
                         }
@@ -1722,13 +1722,12 @@ static int add_object_vtable_internal(
                             !signature_is_single(v->x.property.signature, false) ||
                             !(v->x.property.get || bus_type_is_basic(v->x.property.signature[0]) || streq(v->x.property.signature, "as")) ||
                             v->flags & SD_BUS_VTABLE_METHOD_NO_REPLY ||
-                            (v->flags & SD_BUS_VTABLE_PROPERTY_INVALIDATE_ONLY && !(v->flags & SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE)) ||
+                            (!!(v->flags & SD_BUS_VTABLE_PROPERTY_CONST) + !!(v->flags & SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE) + !!(v->flags & SD_BUS_VTABLE_PROPERTY_EMITS_INVALIDATION)) > 1 ||
                             (v->flags & SD_BUS_VTABLE_UNPRIVILEGED && v->type == _SD_BUS_VTABLE_PROPERTY)) {
                                 r = -EINVAL;
                                 goto fail;
                         }
 
-
                         m = new0(struct vtable_member, 1);
                         if (!m) {
                                 r = -ENOMEM;
@@ -2013,9 +2012,10 @@ static int emit_properties_changed_on_interface(
                         if (c != v->parent)
                                 continue;
 
-                        assert_return(v->vtable->flags & SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE, -EDOM);
+                        assert_return(v->vtable->flags & SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE ||
+                                      v->vtable->flags & SD_BUS_VTABLE_PROPERTY_EMITS_INVALIDATION, -EDOM);
 
-                        if (v->vtable->flags & SD_BUS_VTABLE_PROPERTY_INVALIDATE_ONLY) {
+                        if (v->vtable->flags & SD_BUS_VTABLE_PROPERTY_EMITS_INVALIDATION) {
                                 has_invalidating = true;
                                 continue;
                         }
@@ -2084,7 +2084,7 @@ static int emit_properties_changed_on_interface(
                                 assert_se(v = hashmap_get(bus->vtable_properties, &key));
                                 assert(c == v->parent);
 
-                                if (!(v->vtable->flags & SD_BUS_VTABLE_PROPERTY_INVALIDATE_ONLY))
+                                if (!(v->vtable->flags & SD_BUS_VTABLE_PROPERTY_EMITS_INVALIDATION))
                                         continue;
 
                                 r = sd_bus_message_append(m, "s", *property);
diff --git a/src/libsystemd-bus/test-bus-introspect.c b/src/libsystemd-bus/test-bus-introspect.c
index 574479dd6c..67b6461f30 100644
--- a/src/libsystemd-bus/test-bus-introspect.c
+++ b/src/libsystemd-bus/test-bus-introspect.c
@@ -41,7 +41,8 @@ static const sd_bus_vtable vtable[] = {
         SD_BUS_WRITABLE_PROPERTY("AProperty", "s", prop_get, prop_set, 0, 0),
         SD_BUS_PROPERTY("AReadOnlyDeprecatedProperty", "(ut)", prop_get, 0, SD_BUS_VTABLE_DEPRECATED),
         SD_BUS_PROPERTY("ChangingProperty", "t", prop_get, 0, SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE),
-        SD_BUS_PROPERTY("Invalidating", "t", prop_get, 0, SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE|SD_BUS_VTABLE_PROPERTY_INVALIDATE_ONLY),
+        SD_BUS_PROPERTY("Invalidating", "t", prop_get, 0, SD_BUS_VTABLE_PROPERTY_EMITS_INVALIDATION),
+        SD_BUS_PROPERTY("Constant", "t", prop_get, 0, SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_VTABLE_END
 };
 
diff --git a/src/systemd/sd-bus-vtable.h b/src/systemd/sd-bus-vtable.h
index cc029eb1a7..a03221502a 100644
--- a/src/systemd/sd-bus-vtable.h
+++ b/src/systemd/sd-bus-vtable.h
@@ -40,13 +40,14 @@ enum {
 };
 
 enum {
-        SD_BUS_VTABLE_DEPRECATED               = 1ULL << 0,
-        SD_BUS_VTABLE_METHOD_NO_REPLY          = 1ULL << 1,
-        SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE    = 1ULL << 2,
-        SD_BUS_VTABLE_PROPERTY_INVALIDATE_ONLY = 1ULL << 3,
-        SD_BUS_VTABLE_UNPRIVILEGED             = 1ULL << 4,
-        SD_BUS_VTABLE_HIDDEN                   = 1ULL << 5,
-        _SD_BUS_VTABLE_CAPABILITY_MASK         = 0xFFFFULL << 40
+        SD_BUS_VTABLE_DEPRECATED                   = 1ULL << 0,
+        SD_BUS_VTABLE_HIDDEN                       = 1ULL << 1,
+        SD_BUS_VTABLE_UNPRIVILEGED                 = 1ULL << 2,
+        SD_BUS_VTABLE_METHOD_NO_REPLY              = 1ULL << 3,
+        SD_BUS_VTABLE_PROPERTY_CONST               = 1ULL << 4,
+        SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE        = 1ULL << 5,
+        SD_BUS_VTABLE_PROPERTY_EMITS_INVALIDATION  = 1ULL << 6,
+        _SD_BUS_VTABLE_CAPABILITY_MASK             = 0xFFFFULL << 40
 };
 
 #define SD_BUS_VTABLE_CAPABILITY(x) ((uint64_t) (((x)+1) & 0xFFFF) << 40)