sd-bus: add API to optionally set a sender field on all outgoing messages

This is useful on direct connections to generate messages with valid
sender fields.

This is particularly useful for services that are accessible both
through direct connections and the broker, as it allows clients to
install matches on the sender service name, and they work the same in
both cases.

src/libsystemd/libsystemd.sym

@@ -543,4 +543,7 @@ global:
         sd_bus_is_ready;
         sd_bus_set_connected_signal;
         sd_bus_get_connected_signal;
+        sd_bus_set_sender;
+        sd_bus_get_sender;
+        sd_bus_message_set_sender;
 } LIBSYSTEMD_236;

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

@@ -316,6 +316,7 @@ struct sd_bus {
         char *cgroup_root;
 
         char *description;
+        char *patch_sender;
 
         sd_bus_track *track_queue;
 

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

@@ -5487,6 +5487,15 @@ _public_ int sd_bus_message_set_destination(sd_bus_message *m, const char *desti
         return message_append_field_string(m, BUS_MESSAGE_HEADER_DESTINATION, SD_BUS_TYPE_STRING, destination, &m->destination);
 }
 
+_public_ int sd_bus_message_set_sender(sd_bus_message *m, const char *sender) {
+        assert_return(m, -EINVAL);
+        assert_return(sender, -EINVAL);
+        assert_return(!m->sealed, -EPERM);
+        assert_return(!m->sender, -EEXIST);
+
+        return message_append_field_string(m, BUS_MESSAGE_HEADER_SENDER, SD_BUS_TYPE_STRING, sender, &m->sender);
+}
+
 int bus_message_get_blob(sd_bus_message *m, void **buffer, size_t *sz) {
         size_t total;
         void *p, *e;

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

@@ -154,6 +154,7 @@ static void bus_free(sd_bus *b) {
         free(b->machine);
         free(b->cgroup_root);
         free(b->description);
+        free(b->patch_sender);
 
         free(b->exec_path);
         strv_free(b->exec_argv);
@@ -274,6 +275,7 @@ _public_ int sd_bus_set_exec(sd_bus *bus, const char *path, char *const argv[])
 _public_ int sd_bus_set_bus_client(sd_bus *bus, int b) {
         assert_return(bus, -EINVAL);
         assert_return(bus->state == BUS_UNSET, -EPERM);
+        assert_return(!bus->patch_sender, -EPERM);
         assert_return(!bus_pid_changed(bus), -ECHILD);
 
         bus->bus_client = !!b;
@@ -1537,6 +1539,8 @@ _public_ int sd_bus_get_bus_id(sd_bus *bus, sd_id128_t *id) {
 }
 
 static int bus_seal_message(sd_bus *b, sd_bus_message *m, usec_t timeout) {
+        int r;
+
         assert(b);
         assert(m);
 
@@ -1551,6 +1555,12 @@ static int bus_seal_message(sd_bus *b, sd_bus_message *m, usec_t timeout) {
         if (timeout == 0)
                 timeout = BUS_DEFAULT_TIMEOUT;
 
+        if (!m->sender && b->patch_sender) {
+                r = sd_bus_message_set_sender(m, b->patch_sender);
+                if (r < 0)
+                        return r;
+        }
+
         return sd_bus_message_seal(m, ++b->cookie, timeout);
 }
 
@@ -3959,3 +3969,22 @@ _public_ int sd_bus_get_exit_on_disconnect(sd_bus *bus) {
 
         return bus->exit_on_disconnect;
 }
+
+_public_ int sd_bus_set_sender(sd_bus *bus, const char *sender) {
+        assert_return(bus, -EINVAL);
+        assert_return(!bus->bus_client, -EPERM);
+        assert_return(!sender || service_name_is_valid(sender), -EINVAL);
+
+        return free_and_strdup(&bus->patch_sender, sender);
+}
+
+_public_ int sd_bus_get_sender(sd_bus *bus, const char **ret) {
+        assert_return(bus, -EINVAL);
+        assert_return(ret, -EINVAL);
+
+        if (!bus->patch_sender)
+                return -ENODATA;
+
+        *ret = bus->patch_sender;
+        return 0;
+}

src/systemd/sd-bus.h

@@ -154,6 +154,8 @@ int sd_bus_set_watch_bind(sd_bus *bus, int b);
 int sd_bus_get_watch_bind(sd_bus *bus);
 int sd_bus_set_connected_signal(sd_bus *bus, int b);
 int sd_bus_get_connected_signal(sd_bus *bus);
+int sd_bus_set_sender(sd_bus *bus, const char *sender);
+int sd_bus_get_sender(sd_bus *bus, const char **ret);
 
 int sd_bus_start(sd_bus *bus);
 
@@ -273,6 +275,7 @@ int sd_bus_message_set_auto_start(sd_bus_message *m, int b);
 int sd_bus_message_set_allow_interactive_authorization(sd_bus_message *m, int b);
 
 int sd_bus_message_set_destination(sd_bus_message *m, const char *destination);
+int sd_bus_message_set_sender(sd_bus_message *m, const char *sender);
 int sd_bus_message_set_priority(sd_bus_message *m, int64_t priority);
 
 int sd_bus_message_append(sd_bus_message *m, const char *types, ...);