sd-bus: add new sd_bus_set_connected_signal() API

With this new API sd-bus can synthesize a local "Connected" signal when
the connection is fully established. It mirrors the local "Disconnected"
signal that is already generated when the connection is terminated. This
is useful to be notified when connection setup is done, in order to
start method calls then, in particular when using "slow" connection
methods (for example slow TCP, or most importantly the "watch_bind"
inotify logic).

Note that one could also use hook into the initial NameAcquired signal
received from the bus broker, but that scheme works only if we actually
connect to a bus. The benefit of "Connected" OTOH is that it works with
any kind of connection.

Ideally, we'd just generate this message unconditionally, but in order
not to break clients that do not expect this message it is opt-in.

src/libsystemd/libsystemd.sym

@@ -541,4 +541,6 @@ global:
         sd_bus_match_signal;
         sd_bus_match_signal_async;
         sd_bus_is_ready;
+        sd_bus_set_connected_signal;
+        sd_bus_get_connected_signal;
 } LIBSYSTEMD_236;

src/libsystemd/sd-bus/bus-internal.h

@@ -219,6 +219,7 @@ struct sd_bus {
         bool is_monitor:1;
         bool accept_fd:1;
         bool attach_timestamp:1;
+        bool connected_signal:1;
 
         int use_memfd;
 

src/libsystemd/sd-bus/sd-bus.c

@@ -392,6 +392,66 @@ _public_ int sd_bus_get_watch_bind(sd_bus *bus) {
         return bus->watch_bind;
 }
 
+_public_ int sd_bus_set_connected_signal(sd_bus *bus, int b) {
+        assert_return(bus, -EINVAL);
+        assert_return(bus->state == BUS_UNSET, -EPERM);
+        assert_return(!bus_pid_changed(bus), -ECHILD);
+
+        bus->connected_signal = b;
+        return 0;
+}
+
+_public_ int sd_bus_get_connected_signal(sd_bus *bus) {
+        assert_return(bus, -EINVAL);
+        assert_return(!bus_pid_changed(bus), -ECHILD);
+
+        return bus->connected_signal;
+}
+
+static int synthesize_connected_signal(sd_bus *bus) {
+        _cleanup_(sd_bus_message_unrefp) sd_bus_message *m = NULL;
+        int r;
+
+        assert(bus);
+
+        /* If enabled, synthesizes a local "Connected" signal mirroring the local "Disconnected" signal. This is called
+         * whenever we fully established a connection, i.e. after the authorization phase, and after receiving the
+         * Hello() reply. Or in other words, whenver we enter BUS_RUNNING state.
+         *
+         * This is useful so that clients can start doing stuff whenver the connection is fully established in a way
+         * that works independently from whether we connected to a full bus or just a direct connection. */
+
+        if (!bus->connected_signal)
+                return 0;
+
+        r = sd_bus_message_new_signal(
+                        bus,
+                        &m,
+                        "/org/freedesktop/DBus/Local",
+                        "org.freedesktop.DBus.Local",
+                        "Connected");
+        if (r < 0)
+                return r;
+
+        bus_message_set_sender_local(bus, m);
+
+        r = bus_seal_synthetic_message(bus, m);
+        if (r < 0)
+                return r;
+
+        r = bus_rqueue_make_room(bus);
+        if (r < 0)
+                return r;
+
+        /* Insert at the very front */
+        memmove(bus->rqueue + 1, bus->rqueue, sizeof(sd_bus_message*) * bus->rqueue_size);
+        bus->rqueue[0] = m;
+        m = NULL;
+        bus->rqueue_size++;
+
+        return 0;
+}
+
 static int hello_callback(sd_bus_message *reply, void *userdata, sd_bus_error *error) {
         const char *s;
         sd_bus *bus;
@@ -417,9 +477,14 @@ static int hello_callback(sd_bus_message *reply, void *userdata, sd_bus_error *e
         if (!bus->unique_name)
                 return -ENOMEM;
 
-        if (bus->state == BUS_HELLO)
+        if (bus->state == BUS_HELLO) {
                 bus->state = BUS_RUNNING;
 
+                r = synthesize_connected_signal(bus);
+                if (r < 0)
+                        return r;
+        }
+
         return 1;
 }
 
@@ -449,6 +514,7 @@ int bus_start_running(sd_bus *bus) {
         struct reply_callback *c;
         Iterator i;
         usec_t n;
+        int r;
 
         assert(bus);
         assert(bus->state < BUS_HELLO);
@@ -471,6 +537,11 @@ int bus_start_running(sd_bus *bus) {
         }
 
         bus->state = BUS_RUNNING;
+
+        r = synthesize_connected_signal(bus);
+        if (r < 0)
+                return r;
+
         return 1;
 }