core: watch SIGCHLD more closely to track processes of units with no reliable cgroup empty notifier

When a process dies that we can associate with a specific unit, start
watching all other processes of that unit, so that we can associate
those processes with the unit too.

Also, for service units start doing this as soon as we get the first
SIGCHLD for either control or main process, so that we can follow the
processes of the service from one to the other, as long as process that
remain are processes of the ones we watched that died and got reassigned
to us as parent.

Similar, for scope units start doing this as soon as the scope
controller abandons the unit, and thus management entirely reverts to
systemd. To abandon a unit introduce a new Abandon() scope unit method
call.

1 files changed, 66 insertions, 32 deletions

diff --git a/src/core/scope.c b/src/core/scope.c
index 0c1d17e68c..940e40dda7 100644
--- a/src/core/scope.c
+++ b/src/core/scope.c
@@ -35,6 +35,7 @@
 static const UnitActiveState state_translation_table[_SCOPE_STATE_MAX] = {
         [SCOPE_DEAD] = UNIT_INACTIVE,
         [SCOPE_RUNNING] = UNIT_ACTIVE,
+        [SCOPE_ABANDONED] = UNIT_ACTIVE,
         [SCOPE_STOP_SIGTERM] = UNIT_DEACTIVATING,
         [SCOPE_STOP_SIGKILL] = UNIT_DEACTIVATING,
         [SCOPE_FAILED] = UNIT_FAILED
@@ -66,9 +67,6 @@ static void scope_done(Unit *u) {
 
         free(s->controller);
 
-        set_free(s->pids);
-        s->pids = NULL;
-
         s->timer_event_source = sd_event_source_unref(s->timer_event_source);
 }
 
@@ -100,15 +98,14 @@ static void scope_set_state(Scope *s, ScopeState state) {
         old_state = s->state;
         s->state = state;
 
-        if (state != SCOPE_STOP_SIGTERM &&
-            state != SCOPE_STOP_SIGKILL)
+        if (!IN_SET(state, SCOPE_STOP_SIGTERM, SCOPE_STOP_SIGKILL))
                 s->timer_event_source = sd_event_source_unref(s->timer_event_source);
 
+        if (IN_SET(state, SCOPE_DEAD, SCOPE_FAILED))
+                unit_unwatch_all_pids(UNIT(s));
+
         if (state != old_state)
-                log_debug("%s changed %s -> %s",
-                          UNIT(s)->id,
-                          scope_state_to_string(old_state),
-                          scope_state_to_string(state));
+                log_debug("%s changed %s -> %s", UNIT(s)->id, scope_state_to_string(old_state), scope_state_to_string(state));
 
         unit_notify(UNIT(s), state_translation_table[old_state], state_translation_table[state], true);
 }
@@ -135,7 +132,7 @@ static int scope_verify(Scope *s) {
         if (UNIT(s)->load_state != UNIT_LOADED)
                 return 0;
 
-        if (set_size(s->pids) <= 0 && UNIT(s)->manager->n_reloading <= 0) {
+        if (set_isempty(UNIT(s)->pids) && UNIT(s)->manager->n_reloading <= 0) {
                 log_error_unit(UNIT(s)->id, "Scope %s has no PIDs. Refusing.", UNIT(s)->id);
                 return -EINVAL;
         }
@@ -181,12 +178,15 @@ static int scope_coldplug(Unit *u) {
 
         if (s->deserialized_state != s->state) {
 
-                if (s->deserialized_state == SCOPE_STOP_SIGKILL || s->deserialized_state == SCOPE_STOP_SIGTERM) {
+                if (IN_SET(s->deserialized_state, SCOPE_STOP_SIGKILL, SCOPE_STOP_SIGTERM)) {
                         r = scope_arm_timer(s);
                         if (r < 0)
                                 return r;
                 }
 
+                if (!IN_SET(s->deserialized_state, SCOPE_DEAD, SCOPE_FAILED))
+                        unit_watch_all_pids(UNIT(s));
+
                 scope_set_state(s, s->deserialized_state);
         }
 
@@ -227,6 +227,8 @@ static void scope_enter_signal(Scope *s, ScopeState state, ScopeResult f) {
         if (f != SCOPE_SUCCESS)
                 s->result = f;
 
+        unit_watch_all_pids(UNIT(s));
+
         /* If we have a controller set let's ask the controller nicely
          * to terminate the scope, instead of us going directly into
          * SIGTERM beserk mode */
@@ -288,13 +290,10 @@ static int scope_start(Unit *u) {
                 return r;
         }
 
-        r = cg_attach_many_everywhere(u->manager->cgroup_supported, u->cgroup_path, s->pids);
+        r = cg_attach_many_everywhere(u->manager->cgroup_supported, u->cgroup_path, UNIT(s)->pids);
         if (r < 0)
                 return r;
 
-        set_free(s->pids);
-        s->pids = NULL;
-
         s->result = SCOPE_SUCCESS;
 
         scope_set_state(s, SCOPE_RUNNING);
@@ -305,13 +304,13 @@ static int scope_stop(Unit *u) {
         Scope *s = SCOPE(u);
 
         assert(s);
-        assert(s->state == SCOPE_RUNNING);
 
         if (s->state == SCOPE_STOP_SIGTERM ||
             s->state == SCOPE_STOP_SIGKILL)
                 return 0;
 
-        assert(s->state == SCOPE_RUNNING);
+        assert(s->state == SCOPE_RUNNING ||
+               s->state == SCOPE_ABANDONED);
 
         scope_enter_signal(s, SCOPE_STOP_SIGTERM, SCOPE_SUCCESS);
         return 0;
@@ -389,8 +388,8 @@ static bool scope_check_gc(Unit *u) {
         /* Never clean up scopes that still have a process around,
          * even if the scope is formally dead. */
 
-        if (UNIT(s)->cgroup_path) {
-                r = cg_is_empty_recursive(SYSTEMD_CGROUP_CONTROLLER, UNIT(s)->cgroup_path, true);
+        if (u->cgroup_path) {
+                r = cg_is_empty_recursive(SYSTEMD_CGROUP_CONTROLLER, u->cgroup_path, true);
                 if (r <= 0)
                         return true;
         }
@@ -398,6 +397,32 @@ static bool scope_check_gc(Unit *u) {
         return false;
 }
 
+static void scope_notify_cgroup_empty_event(Unit *u) {
+        Scope *s = SCOPE(u);
+        assert(u);
+
+        log_debug_unit(u->id, "%s: cgroup is empty", u->id);
+
+        if (IN_SET(s->state, SCOPE_RUNNING, SCOPE_ABANDONED, SCOPE_STOP_SIGTERM, SCOPE_STOP_SIGKILL))
+                scope_enter_dead(s, SCOPE_SUCCESS);
+}
+
+static void scope_sigchld_event(Unit *u, pid_t pid, int code, int status) {
+
+        /* If we get a SIGCHLD event for one of the processes we were
+           interested in, then we look for others to watch, under the
+           assumption that we'll sooner or later get a SIGCHLD for
+           them, as the original process we watched was probably the
+           parent of them, and they are hence now our children. */
+
+        unit_tidy_watch_pids(u, 0, 0);
+        unit_watch_all_pids(u);
+
+        /* If the PID set is empty now, then let's finish this off */
+        if (set_isempty(u->pids))
+                scope_notify_cgroup_empty_event(u);
+}
+
 static int scope_dispatch_timer(sd_event_source *source, usec_t usec, void *userdata) {
         Scope *s = SCOPE(userdata);
 
@@ -429,24 +454,30 @@ static int scope_dispatch_timer(sd_event_source *source, usec_t usec, void *user
         return 0;
 }
 
-static void scope_notify_cgroup_empty_event(Unit *u) {
-        Scope *s = SCOPE(u);
-        assert(u);
+int scope_abandon(Scope *s) {
+        assert(s);
 
-        log_debug_unit(u->id, "%s: cgroup is empty", u->id);
+        if (!IN_SET(s->state, SCOPE_RUNNING, SCOPE_ABANDONED))
+                return -ESTALE;
 
-        switch (s->state) {
+        free(s->controller);
+        s->controller = NULL;
 
-        case SCOPE_RUNNING:
-        case SCOPE_STOP_SIGTERM:
-        case SCOPE_STOP_SIGKILL:
-                scope_enter_dead(s, SCOPE_SUCCESS);
+        /* The client is no longer watching the remaining processes,
+         * so let's step in here, under the assumption that the
+         * remaining processes will be sooner or later reassigned to
+         * us as parent. */
 
-                break;
+        unit_tidy_watch_pids(UNIT(s), 0, 0);
+        unit_watch_all_pids(UNIT(s));
 
-        default:
-                ;
-        }
+        /* If the PID set is empty now, then let's finish this off */
+        if (set_isempty(UNIT(s)->pids))
+                scope_notify_cgroup_empty_event(UNIT(s));
+        else
+                scope_set_state(s, SCOPE_ABANDONED);
+
+        return 0;
 }
 
 _pure_ static UnitActiveState scope_active_state(Unit *u) {
@@ -464,6 +495,7 @@ _pure_ static const char *scope_sub_state_to_string(Unit *u) {
 static const char* const scope_state_table[_SCOPE_STATE_MAX] = {
         [SCOPE_DEAD] = "dead",
         [SCOPE_RUNNING] = "running",
+        [SCOPE_ABANDONED] = "abandoned",
         [SCOPE_STOP_SIGTERM] = "stop-sigterm",
         [SCOPE_STOP_SIGKILL] = "stop-sigkill",
         [SCOPE_FAILED] = "failed",
@@ -516,6 +548,8 @@ const UnitVTable scope_vtable = {
 
         .check_gc = scope_check_gc,
 
+        .sigchld_event = scope_sigchld_event,
+
         .reset_failed = scope_reset_failed,
 
         .notify_cgroup_empty = scope_notify_cgroup_empty_event,