sd-login: get rid of seat_can_multi_session()

Follow-up for fa2cf64a91.
Backwards-compat is retained. A short note is added in docs, in case
people see sd_seat_can_multi_session() mentioned somewhere and wonder what
happened to it.

Also see https://github.com/systemd/systemd/pull/15337#issuecomment-610369404.

src/libsystemd/sd-login/sd-login.c

@@ -746,7 +746,7 @@ static int seat_get_can(const char *seat, const char *variable) {
 }
 
 _public_ int sd_seat_can_multi_session(const char *seat) {
-        return seat_get_can(seat, "CAN_MULTI_SESSION");
+        return true;
 }
 
 _public_ int sd_seat_can_tty(const char *seat) {

src/libsystemd/sd-login/test-login.c

@@ -142,8 +142,11 @@ static void test_login(void) {
 
                         log_info("sd_session_get_seat(\"%s\") → \"%s\"", session, seat);
 
+#pragma GCC diagnostic push
+#pragma GCC diagnostic ignored "-Wdeprecated-declarations"
                         r = sd_seat_can_multi_session(seat);
-                        assert_se(r >= 0);
+#pragma GCC diagnostic pop
+                        assert_se(r == 1);
                         log_info("sd_session_can_multi_seat(\"%s\") → %s", seat, yes_no(r));
 
                         r = sd_seat_can_tty(seat);

src/login/logind-seat-dbus.c

@@ -17,7 +17,7 @@
 #include "user-util.h"
 #include "util.h"
 
-static BUS_DEFINE_PROPERTY_GET(property_get_can_multi_session, "b", Seat, seat_can_multi_session);
+static BUS_DEFINE_PROPERTY_GET_GLOBAL(property_get_const_true, "b", true);
 static BUS_DEFINE_PROPERTY_GET(property_get_can_tty, "b", Seat, seat_can_tty);
 static BUS_DEFINE_PROPERTY_GET(property_get_can_graphical, "b", Seat, seat_can_graphical);
 
@@ -296,7 +296,7 @@ const sd_bus_vtable seat_vtable[] = {
 
         SD_BUS_PROPERTY("Id", "s", NULL, offsetof(Seat, id), SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_PROPERTY("ActiveSession", "(so)", property_get_active_session, 0, SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE),
-        SD_BUS_PROPERTY("CanMultiSession", "b", property_get_can_multi_session, 0, SD_BUS_VTABLE_PROPERTY_CONST),
+        SD_BUS_PROPERTY("CanMultiSession", "b", property_get_const_true, 0, SD_BUS_VTABLE_PROPERTY_CONST|SD_BUS_VTABLE_HIDDEN),
         SD_BUS_PROPERTY("CanTTY", "b", property_get_can_tty, 0, SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_PROPERTY("CanGraphical", "b", property_get_can_graphical, 0, SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE),
         SD_BUS_PROPERTY("Sessions", "a(so)", property_get_sessions, 0, 0),

src/login/logind-seat.c

@@ -104,11 +104,10 @@ int seat_save(Seat *s) {
         fprintf(f,
                 "# This is private data. Do not parse.\n"
                 "IS_SEAT0=%i\n"
-                "CAN_MULTI_SESSION=%i\n"
+                "CAN_MULTI_SESSION=1\n"
                 "CAN_TTY=%i\n"
                 "CAN_GRAPHICAL=%i\n",
                 seat_is_seat0(s),
-                seat_can_multi_session(s),
                 seat_can_tty(s),
                 seat_can_graphical(s));
 
@@ -558,13 +557,6 @@ bool seat_is_seat0(Seat *s) {
         return s->manager->seat0 == s;
 }
 
-bool seat_can_multi_session(Seat *s) {
-        assert(s);
-
-        /* multiple sessions are supported on all seats now */
-        return true;
-}
-
 bool seat_can_tty(Seat *s) {
         assert(s);
 

src/login/logind-seat.h

@@ -51,7 +51,6 @@ void seat_claim_position(Seat *s, Session *session, unsigned pos);
 
 bool seat_has_vts(Seat *s);
 bool seat_is_seat0(Seat *s);
-bool seat_can_multi_session(Seat *s);
 bool seat_can_tty(Seat *s);
 bool seat_has_master_device(Seat *s);
 bool seat_can_graphical(Seat *s);

src/systemd/sd-login.h

@@ -183,7 +183,7 @@ int sd_seat_get_active(const char *seat, char **session, uid_t *uid);
 int sd_seat_get_sessions(const char *seat, char ***sessions, uid_t **uid, unsigned *n_uids);
 
 /* Return whether the seat is multi-session capable */
-int sd_seat_can_multi_session(const char *seat);
+int sd_seat_can_multi_session(const char *seat) _sd_deprecated_;
 
 /* Return whether the seat is TTY capable, i.e. suitable for showing console UIs */
 int sd_seat_can_tty(const char *seat);