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authorPhilip Withnall <philip.withnall@collabora.co.uk>2015-05-29 10:49:21 +0100
committerDavid Herrmann <dh.herrmann@gmail.com>2015-06-05 12:48:16 +0200
commit7ffeb45cc63e1326690fd9461b7a4719a3d4f85c (patch)
tree32263060cb593f64459dd1d8c0934abe6d1c7d4a
parent3a3701f03360f9a292dcab31fda1f6e0e0629c64 (diff)
logind: Fix user_elect_display() to be more stable
The previous implementation of user_elect_display() could easily end up overwriting the user’s valid graphical session with a new TTY session. For example, consider the situation where there is one session: c1, type = SESSION_X11, !stopping, class = SESSION_USER it is initially elected as the user’s display (i.e. u->display = c1). If another session is started, on a different VT, the sessions_by_user list becomes: c1, type = SESSION_X11, !stopping, class = SESSION_USER c2, type = SESSION_TTY, !stopping, class = SESSION_USER In the previous code, graphical = c1 and text = c2, as expected. However, neither graphical nor text fulfil the conditions for setting u->display = graphical (because neither is better than u->display), so the code falls through to check the text variable. The conditions for this match, as u->display->type != SESSION_TTY (it’s actually SESSION_X11). Hence u->display is set to c2, which is incorrect, because session c1 is still valid. Refactor user_elect_display() to use a more explicit filter and pre-order comparison over the sessions. This can be demonstrated to be stable and only ever ‘upgrade’ the session to a more graphical one. https://bugs.freedesktop.org/show_bug.cgi?id=90769
-rw-r--r--src/login/logind-user.c90
1 files changed, 54 insertions, 36 deletions
diff --git a/src/login/logind-user.c b/src/login/logind-user.c
index dc3db9abda..373c9938e7 100644
--- a/src/login/logind-user.c
+++ b/src/login/logind-user.c
@@ -738,54 +738,72 @@ int user_kill(User *u, int signo) {
return manager_kill_unit(u->manager, u->slice, KILL_ALL, signo, NULL);
}
+static bool
+elect_display_filter(Session *s) {
+ /* Return true if the session is a candidate for the user’s ‘primary
+ * session’ or ‘display’. */
+ assert(s);
+
+ return (s->class == SESSION_USER && !s->stopping);
+}
+
+static int
+elect_display_compare(Session *s1, Session *s2) {
+ /* Indexed by SessionType. Lower numbers mean more preferred. */
+ const int type_ranks[_SESSION_TYPE_MAX] = {
+ [SESSION_UNSPECIFIED] = 0,
+ [SESSION_TTY] = -2,
+ [SESSION_X11] = -3,
+ [SESSION_WAYLAND] = -3,
+ [SESSION_MIR] = -3,
+ [SESSION_WEB] = -1,
+ };
+
+ /* Calculate the partial order relationship between s1 and s2,
+ * returning < 0 if s1 is preferred as the user’s ‘primary session’,
+ * 0 if s1 and s2 are equally preferred or incomparable, or > 0 if s2
+ * is preferred.
+ *
+ * s1 or s2 may be NULL. */
+ if ((s1 == NULL) != (s2 == NULL))
+ return (s1 == NULL) - (s2 == NULL);
+
+ if (s1->stopping != s2->stopping)
+ return s1->stopping - s2->stopping;
+
+ if ((s1->class != SESSION_USER) != (s2->class != SESSION_USER))
+ return (s1->class != SESSION_USER) - (s2->class != SESSION_USER);
+
+ if ((s1->type == _SESSION_TYPE_INVALID) != (s2->type == _SESSION_TYPE_INVALID))
+ return (s1->type == _SESSION_TYPE_INVALID) - (s2->type == _SESSION_TYPE_INVALID);
+
+ if (s1->type != s2->type)
+ return type_ranks[s1->type] - type_ranks[s2->type];
+
+ return 0;
+}
+
void user_elect_display(User *u) {
- Session *graphical = NULL, *text = NULL, *other = NULL, *s;
+ Session *s;
assert(u);
/* This elects a primary session for each user, which we call
* the "display". We try to keep the assignment stable, but we
* "upgrade" to better choices. */
+ log_debug("Electing new display for user %s", u->name);
LIST_FOREACH(sessions_by_user, s, u->sessions) {
-
- if (s->class != SESSION_USER)
- continue;
-
- if (s->stopping)
+ if (!elect_display_filter(s)) {
+ log_debug("Ignoring session %s", s->id);
continue;
+ }
- if (SESSION_TYPE_IS_GRAPHICAL(s->type))
- graphical = s;
- else if (s->type == SESSION_TTY)
- text = s;
- else
- other = s;
- }
-
- if (graphical &&
- (!u->display ||
- u->display->class != SESSION_USER ||
- u->display->stopping ||
- !SESSION_TYPE_IS_GRAPHICAL(u->display->type))) {
- u->display = graphical;
- return;
- }
-
- if (text &&
- (!u->display ||
- u->display->class != SESSION_USER ||
- u->display->stopping ||
- u->display->type != SESSION_TTY)) {
- u->display = text;
- return;
+ if (elect_display_compare(s, u->display) < 0) {
+ log_debug("Choosing session %s in preference to %s", s->id, u->display ? u->display->id : "-");
+ u->display = s;
+ }
}
-
- if (other &&
- (!u->display ||
- u->display->class != SESSION_USER ||
- u->display->stopping))
- u->display = other;
}
static const char* const user_state_table[_USER_STATE_MAX] = {