[systemd-devel] [RFC] logind: introduce session "positions"
David Herrmann
dh.herrmann at gmail.com
Sun Dec 1 03:43:35 PST 2013
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).
Future ideas: As this commit-msg isn't long enough, yet, some notes on
future ideas. The current position-assignment is compatible with the
legacy VT numbers. However, it is a rather outdated way of addressing
sessions. Instead, we can make use of session-classes of logind. We
already tag session with one of the classes "greeter", "user",
"background", "lock-screen". So one of my ideas is to make
"position-assignment" a "per-class" thing. And instead of mapping F1-F12
directly to the positions, we map it as follows:
- F1: Activate the last-spawned session in the "greeter" class. Usually,
only a single greeter should be active, but in case
systemd-welcomed runs and gdm is spawned later, this will switch to
gdm (actually gdm.service should stop systemd-welcomed.service but
lets be overly pedantic here).
- F2: Activate the session from the "user" class which has been active
last. So if you switch to F1 and back to F2, you're guaranteed to
get back to your last active user-session.
- F3-F11: Direct mapping to "user" sessions. So F3 maps to the
user-session with position 3, F11 to position 11 (or apply a
"-2" offset, so F3=>1 and F11=>9..)
- F12: Switch to the last-spawned session in the "emergency" class. This
doesn't exist, yet, but we could start sessions showing the
kernel-log or an emergency-console here.
Inside of each group, you can use SwitchToNext/Previous() to switch to the
next/previous session. As only the "user" group has a way of directly
addressing sessions, this is needed for the "greeter" and "emergency"
groups. Note that both groups are expected to only have one session, so
this is really just a fallback for odd cases. However, for the "user"
group the Next/Previous can come in quite handy.
Comments?
---
src/login/logind-seat-dbus.c | 56 ++++++++++++++++++++++++++
src/login/logind-seat.c | 96 ++++++++++++++++++++++++++++++++++++++++++++
src/login/logind-seat.h | 8 ++++
src/login/logind-session.c | 13 ++++++
src/login/logind-session.h | 1 +
5 files changed, 174 insertions(+)
diff --git a/src/login/logind-seat-dbus.c b/src/login/logind-seat-dbus.c
index 23f975b..ff9e2e4 100644
--- a/src/login/logind-seat-dbus.c
+++ b/src/login/logind-seat-dbus.c
@@ -235,6 +235,59 @@ 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)
+ 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),
@@ -250,6 +303,9 @@ const sd_bus_vtable seat_vtable[] = {
SD_BUS_METHOD("Terminate", NULL, NULL, method_terminate, 0),
SD_BUS_METHOD("ActivateSession", "s", NULL, method_activate_session, 0),
+ SD_BUS_METHOD("SwitchTo", "u", NULL, method_switch_to, 0),
+ SD_BUS_METHOD("SwitchToNext", NULL, NULL, method_switch_to_next, 0),
+ SD_BUS_METHOD("SwitchToPrevious", NULL, NULL, method_switch_to_previous, 0),
SD_BUS_VTABLE_END
};
diff --git a/src/login/logind-seat.c b/src/login/logind-seat.c
index b1a5ec3..928e2c5 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 1254268..9e21e3a 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 beaa601..c24e573 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 ee93101..d724e20 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;
--
1.8.4.2
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