terminal/sysview: introduce SETTLE events

This introduces a new SYSVIEW_EVENT_SETTLE notification that is sent after
initial scanning via sysview is done. This is very handy to let the
application raise warnings in case requested resources are not found
during startup.

The SETTLE event is sent after systemd-logind and udev enumerations are
done. This event does in no way guarantee that a given resource is
available. All it does is notify the application that scanning is done!
You must not react to SETTLE if you don't have external synchronization
with the resource you're waiting for.

The main use-case for SETTLE is to run applications _inside_ of logind
sessions and startup sysview. You really want to make sure that the own
session you're running in was found during enumeration. If not, something
is seriously wrong.

1 files changed, 58 insertions, 8 deletions

diff --git a/src/libsystemd-terminal/sysview.c b/src/libsystemd-terminal/sysview.c
index cd61ea44e7..550f0305af 100644
--- a/src/libsystemd-terminal/sysview.c
+++ b/src/libsystemd-terminal/sysview.c
@@ -474,6 +474,14 @@ static int context_raise(sysview_context *c, sysview_event *event, int def) {
         return c->running ? c->event_fn(c, c->userdata, event) : def;
 }
 
+static int context_raise_settle(sysview_context *c) {
+        sysview_event event = {
+                .type = SYSVIEW_EVENT_SETTLE,
+        };
+
+        return context_raise(c, &event, 0);
+}
+
 static int context_raise_seat_add(sysview_context *c, sysview_seat *seat) {
         sysview_event event = {
                 .type = SYSVIEW_EVENT_SEAT_ADD,
@@ -585,6 +593,21 @@ static int context_raise_session_refresh(sysview_context *c, sysview_session *se
         return context_raise(c, &event, 0);
 }
 
+static void context_settle(sysview_context *c) {
+        int r;
+
+        if (c->n_probe <= 0 || --c->n_probe > 0)
+                return;
+
+        log_debug("sysview: settle");
+
+        c->settled = true;
+
+        r = context_raise_settle(c);
+        if (r < 0)
+                log_debug_errno(r, "sysview: callback failed on settle: %m");
+}
+
 static void context_add_device(sysview_context *c, sysview_device *device) {
         sysview_session *session;
         Iterator i;
@@ -1251,7 +1274,8 @@ static int context_ld_list_seats_fn(sd_bus *bus,
 
                 log_debug("sysview: ListSeats on logind failed: %s: %s",
                           error->name, error->message);
-                return -sd_bus_error_get_errno(error);
+                r = -sd_bus_error_get_errno(error);
+                goto settle;
         }
 
         r = sd_bus_message_enter_container(reply, 'a', "(so)");
@@ -1277,12 +1301,16 @@ static int context_ld_list_seats_fn(sd_bus *bus,
 
         r = sd_bus_message_exit_container(reply);
         if (r < 0)
-                return r;
+                goto error;
 
-        return 0;
+        r = 0;
+        goto settle;
 
 error:
-        return log_debug_errno(r, "sysview: erroneous ListSeats response from logind: %m");
+        log_debug_errno(r, "sysview: erroneous ListSeats response from logind: %m");
+settle:
+        context_settle(c);
+        return r;
 }
 
 static int context_ld_list_sessions_fn(sd_bus *bus,
@@ -1299,7 +1327,8 @@ static int context_ld_list_sessions_fn(sd_bus *bus,
 
                 log_debug("sysview: ListSessions on logind failed: %s: %s",
                           error->name, error->message);
-                return -sd_bus_error_get_errno(error);
+                r = -sd_bus_error_get_errno(error);
+                goto settle;
         }
 
         r = sd_bus_message_enter_container(reply, 'a', "(susso)");
@@ -1341,12 +1370,16 @@ static int context_ld_list_sessions_fn(sd_bus *bus,
 
         r = sd_bus_message_exit_container(reply);
         if (r < 0)
-                return r;
+                goto error;
 
-        return 0;
+        r = 0;
+        goto settle;
 
 error:
-        return log_debug_errno(r, "sysview: erroneous ListSessions response from logind: %m");
+        log_debug_errno(r, "sysview: erroneous ListSessions response from logind: %m");
+settle:
+        context_settle(c);
+        return r;
 }
 
 static int context_ld_scan(sysview_context *c) {
@@ -1376,6 +1409,9 @@ static int context_ld_scan(sysview_context *c) {
         if (r < 0)
                 return r;
 
+        if (!c->settled)
+                ++c->n_probe;
+
         /* request session list */
 
         m = sd_bus_message_unref(m);
@@ -1397,6 +1433,9 @@ static int context_ld_scan(sysview_context *c) {
         if (r < 0)
                 return r;
 
+        if (!c->settled)
+                ++c->n_probe;
+
         return 0;
 }
 
@@ -1461,6 +1500,8 @@ void sysview_context_stop(sysview_context *c) {
 
         c->running = false;
         c->scanned = false;
+        c->settled = false;
+        c->n_probe = 0;
         c->event_fn = NULL;
         c->userdata = NULL;
         c->scan_src = sd_event_source_unref(c->scan_src);
@@ -1474,6 +1515,8 @@ static int context_scan_fn(sd_event_source *s, void *userdata) {
         Iterator i;
         int r;
 
+        c->rescan = false;
+
         if (!c->scanned) {
                 r = context_ld_scan(c);
                 if (r < 0)
@@ -1491,6 +1534,7 @@ static int context_scan_fn(sd_event_source *s, void *userdata) {
         }
 
         c->scanned = true;
+        context_settle(c);
 
         return 0;
 }
@@ -1501,6 +1545,12 @@ int sysview_context_rescan(sysview_context *c) {
         if (!c->running)
                 return 0;
 
+        if (!c->rescan) {
+                c->rescan = true;
+                if (!c->settled)
+                        ++c->n_probe;
+        }
+
         if (c->scan_src)
                 return sd_event_source_set_enabled(c->scan_src, SD_EVENT_ONESHOT);
         else