logind: introduce session "positions"

logind has no concept of session ordering. Sessions have a unique name,
some attributes about the capabilities and that's already it. There is
currently no stable+total order on sessions. If we use the logind API to
switch between sessions, we are faced with an unordered list of sessions
we have no clue of.

This used to be no problem on seats with VTs or on seats with only a
single active session. However, with the introduction of multi-session
capability for seats without VTs, we need to find a way to order sessions
in a stable way.

This patch introduces session "positions". A position is a simple integer
assigned to a session which is never changed implicitly (currently, we
also don't change it explicitly, but that may be changed someday). For
seats with VTs, we force the position to be the same as the VTnr. Without
VTs, we simply find the lowest unassigned number and use it as position.
If position-assignment fails or if, for any reason, we decide to not
assign a position to a session, the position is set to 0 (which is treated
as invalid position).
During session_load() or if two sessions have the same VTnr, we may end up
with two sessions with the same position (this shouldn't happen, but lets
be fail-safe in case some other part of the stack fails). This case is
dealt with gracefully by ignoring any session but the first session
assigned to the position. Thus, session->pos is a hint, seat->positions[i]
is the definite position-assignment. Always verify both match in case you
need to modify them!

Additionally, we introduce SwitchTo(unsigned int) on the seat-dbus-API.
You can call it with any integer value != 0 and logind will try to switch
to the request position. If you implement a compositor or any other
session-controller, you simply watch for ctrl+alt+F1 to F12 and call
SwitchTo(Fx). logind will figure a way out deal with this number.
For convenience, we also introduce SwitchToNext/Previous(). It should be
called on ctrl+alt+Left/Right (like the kernel-console used to support).

Note that the public API (SwitchTo*()) is *not* bound to the underlying
logic that is implemented now. We don't export "session-positions" on the
dbus/C API! They are an implementation detail. Instead, the SwitchTo*()
API is supposed to be a hint to let logind choose the session-switching
logic. Any foreground session-controller is free to enumerate/order
existing sessions according to their needs and call Session.Activate()
manually. But the SwitchTo*() API provides a uniform behavior across
session-controllers.

Background: Session-switching keys depend on the active keymap. The XKB
specification provides the XKB_KEY_XF86Switch_VT_1-12 key-symbols which
have to be mapped by all keymaps to allow session-switching. It is usually
bound to ctrl+alt+Fx but may be set differently. A compositor passes any
keyboard input to XKB before passing it to clients. In case a key-press
invokes the XKB_KEY_XF86Switch_VT_x action, the keypress is *not*
forwarded to clients, but instead a session-switch is scheduled.

This actually prevents us from handling these keys outside of the session.
If an active compositor has a keymap with a different mapping of these
keys, and logind itself tries to catch these combinations, we end up with
the key-press sent to the compositor's clients *and* handled by logind.
This is *bad* and we must avoid this. The only situation where a
background process is allowed to handle key-presses is debugging and
emergency-keys. In these cases, we don't care for keymap mismatches and
accept the double-event. Another exception is unmapped keys like
PowerOff/Suspend (even though this one is controversial).

5 files changed, 174 insertions, 0 deletions

diff --git a/src/login/logind-seat-dbus.c b/src/login/logind-seat-dbus.c
index 236af5eb9e..909007c30c 100644
--- a/src/login/logind-seat-dbus.c
+++ b/src/login/logind-seat-dbus.c
@@ -236,6 +236,59 @@ static int method_activate_session(sd_bus *bus, sd_bus_message *message, void *u
         return sd_bus_reply_method_return(message, NULL);
 }
 
+static int method_switch_to(sd_bus *bus, sd_bus_message *message, void *userdata, sd_bus_error *error) {
+        Seat *s = userdata;
+        unsigned int to;
+        int r;
+
+        assert(bus);
+        assert(message);
+        assert(s);
+
+        r = sd_bus_message_read(message, "u", &to);
+        if (r < 0)
+                return r;
+
+        if (to <= 0)
+                return -EINVAL;
+
+        r = seat_switch_to(s, to);
+        if (r < 0)
+                return r;
+
+        return sd_bus_reply_method_return(message, NULL);
+}
+
+static int method_switch_to_next(sd_bus *bus, sd_bus_message *message, void *userdata, sd_bus_error *error) {
+        Seat *s = userdata;
+        int r;
+
+        assert(bus);
+        assert(message);
+        assert(s);
+
+        r = seat_switch_to_next(s);
+        if (r < 0)
+                return r;
+
+        return sd_bus_reply_method_return(message, NULL);
+}
+
+static int method_switch_to_previous(sd_bus *bus, sd_bus_message *message, void *userdata, sd_bus_error *error) {
+        Seat *s = userdata;
+        int r;
+
+        assert(bus);
+        assert(message);
+        assert(s);
+
+        r = seat_switch_to_previous(s);
+        if (r < 0)
+                return r;
+
+        return sd_bus_reply_method_return(message, NULL);
+}
+
 const sd_bus_vtable seat_vtable[] = {
         SD_BUS_VTABLE_START(0),
 
@@ -251,6 +304,9 @@ const sd_bus_vtable seat_vtable[] = {
 
         SD_BUS_METHOD("Terminate", NULL, NULL, method_terminate, SD_BUS_VTABLE_CAPABILITY(CAP_KILL)),
         SD_BUS_METHOD("ActivateSession", "s", NULL, method_activate_session, SD_BUS_VTABLE_UNPRIVILEGED),
+        SD_BUS_METHOD("SwitchTo", "u", NULL, method_switch_to, SD_BUS_VTABLE_UNPRIVILEGED),
+        SD_BUS_METHOD("SwitchToNext", NULL, NULL, method_switch_to_next, SD_BUS_VTABLE_UNPRIVILEGED),
+        SD_BUS_METHOD("SwitchToPrevious", NULL, NULL, method_switch_to_previous, SD_BUS_VTABLE_UNPRIVILEGED),
 
         SD_BUS_VTABLE_END
 };
diff --git a/src/login/logind-seat.c b/src/login/logind-seat.c
index c73860478d..b8f18c47a3 100644
--- a/src/login/logind-seat.c
+++ b/src/login/logind-seat.c
@@ -79,6 +79,7 @@ void seat_free(Seat *s) {
 
         hashmap_remove(s->manager->seats, s->id);
 
+        free(s->positions);
         free(s->state_file);
         free(s);
 }
@@ -270,6 +271,60 @@ int seat_set_active(Seat *s, Session *session) {
         return 0;
 }
 
+int seat_switch_to(Seat *s, unsigned int num) {
+        /* Public session positions skip 0 (there is only F1-F12). Maybe it
+         * will get reassigned in the future, so return error for now. */
+        if (!num)
+                return -EINVAL;
+
+        if (num >= s->position_count || !s->positions[num])
+                return -EINVAL;
+
+        return session_activate(s->positions[num]);
+}
+
+int seat_switch_to_next(Seat *s) {
+        unsigned int start, i;
+
+        if (!s->position_count)
+                return -EINVAL;
+
+        start = 1;
+        if (s->active && s->active->pos > 0)
+                start = s->active->pos;
+
+        for (i = start + 1; i < s->position_count; ++i)
+                if (s->positions[i])
+                        return session_activate(s->positions[i]);
+
+        for (i = 1; i < start; ++i)
+                if (s->positions[i])
+                        return session_activate(s->positions[i]);
+
+        return -EINVAL;
+}
+
+int seat_switch_to_previous(Seat *s) {
+        unsigned int start, i;
+
+        if (!s->position_count)
+                return -EINVAL;
+
+        start = 1;
+        if (s->active && s->active->pos > 0)
+                start = s->active->pos;
+
+        for (i = start - 1; i > 0; --i)
+                if (s->positions[i])
+                        return session_activate(s->positions[i]);
+
+        for (i = s->position_count - 1; i > start; --i)
+                if (s->positions[i])
+                        return session_activate(s->positions[i]);
+
+        return -EINVAL;
+}
+
 int seat_active_vt_changed(Seat *s, unsigned int vtnr) {
         Session *i, *new_active = NULL;
         int r;
@@ -403,6 +458,46 @@ int seat_stop_sessions(Seat *s) {
         return r;
 }
 
+void seat_evict_position(Seat *s, Session *session) {
+        unsigned int pos = session->pos;
+
+        session->pos = 0;
+
+        if (!pos)
+                return;
+
+        if (pos < s->position_count && s->positions[pos] == session)
+                s->positions[pos] = NULL;
+}
+
+void seat_claim_position(Seat *s, Session *session, unsigned int pos) {
+        /* with VTs, the position is always the same as the VTnr */
+        if (seat_has_vts(s))
+                pos = session->vtnr;
+
+        if (!GREEDY_REALLOC0(s->positions, s->position_count, pos + 1))
+                return;
+
+        seat_evict_position(s, session);
+
+        session->pos = pos;
+        if (pos > 0 && !s->positions[pos])
+                s->positions[pos] = session;
+}
+
+static void seat_assign_position(Seat *s, Session *session) {
+        unsigned int pos;
+
+        if (session->pos > 0)
+                return;
+
+        for (pos = 1; pos < s->position_count; ++pos)
+                if (!s->positions[pos])
+                        break;
+
+        seat_claim_position(s, session, pos);
+}
+
 int seat_attach_session(Seat *s, Session *session) {
         assert(s);
         assert(session);
@@ -413,6 +508,7 @@ int seat_attach_session(Seat *s, Session *session) {
 
         session->seat = s;
         LIST_PREPEND(sessions_by_seat, s->sessions, session);
+        seat_assign_position(s, session);
 
         seat_send_changed(s, "Sessions", NULL);
 
diff --git a/src/login/logind-seat.h b/src/login/logind-seat.h
index 1254268f10..9e21e3a8a3 100644
--- a/src/login/logind-seat.h
+++ b/src/login/logind-seat.h
@@ -41,6 +41,9 @@ struct Seat {
         Session *pending_switch;
         LIST_HEAD(Session, sessions);
 
+        Session **positions;
+        size_t position_count;
+
         bool in_gc_queue:1;
         bool started:1;
 
@@ -55,12 +58,17 @@ int seat_load(Seat *s);
 
 int seat_apply_acls(Seat *s, Session *old_active);
 int seat_set_active(Seat *s, Session *session);
+int seat_switch_to(Seat *s, unsigned int num);
+int seat_switch_to_next(Seat *s);
+int seat_switch_to_previous(Seat *s);
 int seat_active_vt_changed(Seat *s, unsigned int vtnr);
 int seat_read_active_vt(Seat *s);
 int seat_preallocate_vts(Seat *s);
 
 int seat_attach_session(Seat *s, Session *session);
 void seat_complete_switch(Seat *s);
+void seat_evict_position(Seat *s, Session *session);
+void seat_claim_position(Seat *s, Session *session, unsigned int pos);
 
 bool seat_has_vts(Seat *s);
 bool seat_is_seat0(Seat *s);
diff --git a/src/login/logind-session.c b/src/login/logind-session.c
index 4476f5820a..ff0a7a4f2e 100644
--- a/src/login/logind-session.c
+++ b/src/login/logind-session.c
@@ -125,6 +125,7 @@ void session_free(Session *s) {
                 if (s->seat->pending_switch == s)
                         s->seat->pending_switch = NULL;
 
+                seat_evict_position(s->seat, s);
                 LIST_REMOVE(sessions_by_seat, s->seat->sessions, s);
         }
 
@@ -231,6 +232,9 @@ int session_save(Session *s) {
         if (s->seat && seat_has_vts(s->seat))
                 fprintf(f, "VTNR=%u\n", s->vtnr);
 
+        if (!s->vtnr)
+                fprintf(f, "POS=%u\n", s->pos);
+
         if (s->leader > 0)
                 fprintf(f, "LEADER=%lu\n", (unsigned long) s->leader);
 
@@ -266,6 +270,7 @@ int session_load(Session *s) {
         _cleanup_free_ char *remote = NULL,
                 *seat = NULL,
                 *vtnr = NULL,
+                *pos = NULL,
                 *leader = NULL,
                 *type = NULL,
                 *class = NULL,
@@ -290,6 +295,7 @@ int session_load(Session *s) {
                            "REMOTE_USER",    &s->remote_user,
                            "SERVICE",        &s->service,
                            "VTNR",           &vtnr,
+                           "POS",            &pos,
                            "LEADER",         &leader,
                            "TYPE",           &type,
                            "CLASS",          &class,
@@ -350,6 +356,13 @@ int session_load(Session *s) {
         if (!s->seat || !seat_has_vts(s->seat))
                 s->vtnr = 0;
 
+        if (pos && s->seat) {
+                unsigned int npos;
+
+                safe_atou(pos, &npos);
+                seat_claim_position(s->seat, s, npos);
+        }
+
         if (leader) {
                 k = parse_pid(leader, &s->leader);
                 if (k >= 0)
diff --git a/src/login/logind-session.h b/src/login/logind-session.h
index ee931013dd..d724e20e7f 100644
--- a/src/login/logind-session.h
+++ b/src/login/logind-session.h
@@ -69,6 +69,7 @@ struct Session {
         Manager *manager;
 
         char *id;
+        unsigned int pos;
         SessionType type;
         SessionClass class;