core: reduce the number of stalled PIDs from the watched processes list when possible

Some PIDs can remain in the watched list even though their processes have
exited since a long time. It can easily happen if the main process of a forking
service manages to spawn a child before the control process exits for example.

However when a pid is about to be mapped to a unit by calling unit_watch_pid(),
the caller usually knows if the pid should belong to this unit exclusively: if
we just forked() off a child, then we can be sure that its PID is otherwise
unused. In this case we take this opportunity to remove any stalled PIDs from
the watched process list.

If we learnt about a PID in any other form (for example via PID file, via
searching, MAINPID= and so on), then we can't assume anything.

11 files changed, 43 insertions, 28 deletions

diff --git a/src/core/cgroup.c b/src/core/cgroup.c
index 8db044befa..ad67ba0438 100644
--- a/src/core/cgroup.c
+++ b/src/core/cgroup.c
@@ -2319,7 +2319,7 @@ static int unit_watch_pids_in_path(Unit *u, const char *path) {
                 pid_t pid;
 
                 while ((r = cg_read_pid(f, &pid)) > 0) {
-                        r = unit_watch_pid(u, pid);
+                        r = unit_watch_pid(u, pid, false);
                         if (r < 0 && ret >= 0)
                                 ret = r;
                 }
diff --git a/src/core/dbus-scope.c b/src/core/dbus-scope.c
index bb807df2e9..8eb915e508 100644
--- a/src/core/dbus-scope.c
+++ b/src/core/dbus-scope.c
@@ -106,7 +106,7 @@ static int bus_scope_set_transient_property(
                                 return r;
 
                         if (!UNIT_WRITE_FLAGS_NOOP(flags)) {
-                                r = unit_watch_pid(UNIT(s), pid);
+                                r = unit_watch_pid(UNIT(s), pid, false);
                                 if (r < 0 && r != -EEXIST)
                                         return r;
                         }
diff --git a/src/core/manager.c b/src/core/manager.c
index 22d1fda2dc..33785d905f 100644
--- a/src/core/manager.c
+++ b/src/core/manager.c
@@ -2129,6 +2129,16 @@ void manager_clear_jobs(Manager *m) {
                 job_finish_and_invalidate(j, JOB_CANCELED, false, false);
 }
 
+void manager_unwatch_pid(Manager *m, pid_t pid) {
+        assert(m);
+
+        /* First let's drop the unit keyed as "pid". */
+        (void) hashmap_remove(m->watch_pids, PID_TO_PTR(pid));
+
+        /* Then, let's also drop the array keyed by -pid. */
+        free(hashmap_remove(m->watch_pids, PID_TO_PTR(-pid)));
+}
+
 static int manager_dispatch_run_queue(sd_event_source *source, void *userdata) {
         Manager *m = userdata;
         Job *j;
diff --git a/src/core/manager.h b/src/core/manager.h
index bce8020cfd..c43c587a79 100644
--- a/src/core/manager.h
+++ b/src/core/manager.h
@@ -437,6 +437,8 @@ int manager_get_dump_string(Manager *m, char **ret);
 
 void manager_clear_jobs(Manager *m);
 
+void manager_unwatch_pid(Manager *m, pid_t pid);
+
 unsigned manager_dispatch_load_queue(Manager *m);
 
 int manager_default_environment(Manager *m);
diff --git a/src/core/mount.c b/src/core/mount.c
index a991bf8794..4fb4b0f81e 100644
--- a/src/core/mount.c
+++ b/src/core/mount.c
@@ -704,7 +704,7 @@ static int mount_coldplug(Unit *u) {
             pid_is_unwaited(m->control_pid) &&
             MOUNT_STATE_WITH_PROCESS(new_state)) {
 
-                r = unit_watch_pid(UNIT(m), m->control_pid);
+                r = unit_watch_pid(UNIT(m), m->control_pid, false);
                 if (r < 0)
                         return r;
 
@@ -810,9 +810,8 @@ static int mount_spawn(Mount *m, ExecCommand *c, pid_t *_pid) {
         if (r < 0)
                 return r;
 
-        r = unit_watch_pid(UNIT(m), pid);
+        r = unit_watch_pid(UNIT(m), pid, true);
         if (r < 0)
-                /* FIXME: we need to do something here */
                 return r;
 
         *_pid = pid;
diff --git a/src/core/service.c b/src/core/service.c
index 4682e86f8e..883ca7bbfe 100644
--- a/src/core/service.c
+++ b/src/core/service.c
@@ -990,7 +990,7 @@ static int service_load_pid_file(Service *s, bool may_warn) {
         if (r < 0)
                 return r;
 
-        r = unit_watch_pid(UNIT(s), pid);
+        r = unit_watch_pid(UNIT(s), pid, false);
         if (r < 0) /* FIXME: we need to do something here */
                 return log_unit_warning_errno(UNIT(s), r, "Failed to watch PID "PID_FMT" for service: %m", pid);
 
@@ -1020,7 +1020,7 @@ static void service_search_main_pid(Service *s) {
         if (service_set_main_pid(s, pid) < 0)
                 return;
 
-        r = unit_watch_pid(UNIT(s), pid);
+        r = unit_watch_pid(UNIT(s), pid, false);
         if (r < 0)
                 /* FIXME: we need to do something here */
                 log_unit_warning_errno(UNIT(s), r, "Failed to watch PID "PID_FMT" from: %m", pid);
@@ -1154,7 +1154,7 @@ static int service_coldplug(Unit *u) {
                     SERVICE_RUNNING, SERVICE_RELOAD,
                     SERVICE_STOP, SERVICE_STOP_WATCHDOG, SERVICE_STOP_SIGTERM, SERVICE_STOP_SIGKILL, SERVICE_STOP_POST,
                     SERVICE_FINAL_SIGTERM, SERVICE_FINAL_SIGKILL))) {
-                r = unit_watch_pid(UNIT(s), s->main_pid);
+                r = unit_watch_pid(UNIT(s), s->main_pid, false);
                 if (r < 0)
                         return r;
         }
@@ -1166,7 +1166,7 @@ static int service_coldplug(Unit *u) {
                    SERVICE_RELOAD,
                    SERVICE_STOP, SERVICE_STOP_WATCHDOG, SERVICE_STOP_SIGTERM, SERVICE_STOP_SIGKILL, SERVICE_STOP_POST,
                    SERVICE_FINAL_SIGTERM, SERVICE_FINAL_SIGKILL)) {
-                r = unit_watch_pid(UNIT(s), s->control_pid);
+                r = unit_watch_pid(UNIT(s), s->control_pid, false);
                 if (r < 0)
                         return r;
         }
@@ -1566,8 +1566,8 @@ static int service_spawn(
         s->exec_fd_event_source = TAKE_PTR(exec_fd_source);
         s->exec_fd_hot = false;
 
-        r = unit_watch_pid(UNIT(s), pid);
-        if (r < 0) /* FIXME: we need to do something here */
+        r = unit_watch_pid(UNIT(s), pid, true);
+        if (r < 0)
                 return r;
 
         *_pid = pid;
@@ -3705,7 +3705,7 @@ static void service_notify_message(
                         if (r > 0) {
                                 service_set_main_pid(s, new_main_pid);
 
-                                r = unit_watch_pid(UNIT(s), new_main_pid);
+                                r = unit_watch_pid(UNIT(s), new_main_pid, false);
                                 if (r < 0)
                                         log_unit_warning_errno(UNIT(s), r, "Failed to watch new main PID "PID_FMT" for service: %m", new_main_pid);
 
@@ -3923,7 +3923,7 @@ static void service_bus_name_owner_change(
                         log_unit_debug(u, "D-Bus name %s is now owned by process " PID_FMT, name, pid);
 
                         service_set_main_pid(s, pid);
-                        unit_watch_pid(UNIT(s), pid);
+                        unit_watch_pid(UNIT(s), pid, false);
                 }
         }
 }
diff --git a/src/core/socket.c b/src/core/socket.c
index fd550290be..1396842b26 100644
--- a/src/core/socket.c
+++ b/src/core/socket.c
@@ -1852,7 +1852,7 @@ static int socket_coldplug(Unit *u) {
                    SOCKET_FINAL_SIGTERM,
                    SOCKET_FINAL_SIGKILL)) {
 
-                r = unit_watch_pid(UNIT(s), s->control_pid);
+                r = unit_watch_pid(UNIT(s), s->control_pid, false);
                 if (r < 0)
                         return r;
 
@@ -1938,9 +1938,8 @@ static int socket_spawn(Socket *s, ExecCommand *c, pid_t *_pid) {
         if (r < 0)
                 return r;
 
-        r = unit_watch_pid(UNIT(s), pid);
+        r = unit_watch_pid(UNIT(s), pid, true);
         if (r < 0)
-                /* FIXME: we need to do something here */
                 return r;
 
         *_pid = pid;
@@ -2009,7 +2008,7 @@ static int socket_chown(Socket *s, pid_t *_pid) {
                 _exit(EXIT_SUCCESS);
         }
 
-        r = unit_watch_pid(UNIT(s), pid);
+        r = unit_watch_pid(UNIT(s), pid, true);
         if (r < 0)
                 goto fail;
 
diff --git a/src/core/swap.c b/src/core/swap.c
index b836fdd304..28acef2373 100644
--- a/src/core/swap.c
+++ b/src/core/swap.c
@@ -550,7 +550,7 @@ static int swap_coldplug(Unit *u) {
             pid_is_unwaited(s->control_pid) &&
             SWAP_STATE_WITH_PROCESS(new_state)) {
 
-                r = unit_watch_pid(UNIT(s), s->control_pid);
+                r = unit_watch_pid(UNIT(s), s->control_pid, false);
                 if (r < 0)
                         return r;
 
@@ -657,9 +657,8 @@ static int swap_spawn(Swap *s, ExecCommand *c, pid_t *_pid) {
         if (r < 0)
                 goto fail;
 
-        r = unit_watch_pid(UNIT(s), pid);
+        r = unit_watch_pid(UNIT(s), pid, true);
         if (r < 0)
-                /* FIXME: we need to do something here */
                 goto fail;
 
         *_pid = pid;
diff --git a/src/core/unit.c b/src/core/unit.c
index 445cf6695c..5620fd9017 100644
--- a/src/core/unit.c
+++ b/src/core/unit.c
@@ -2525,7 +2525,7 @@ void unit_notify(Unit *u, UnitActiveState os, UnitActiveState ns, UnitNotifyFlag
         unit_add_to_gc_queue(u);
 }
 
-int unit_watch_pid(Unit *u, pid_t pid) {
+int unit_watch_pid(Unit *u, pid_t pid, bool exclusive) {
         int r;
 
         assert(u);
@@ -2533,6 +2533,12 @@ int unit_watch_pid(Unit *u, pid_t pid) {
 
         /* Watch a specific PID */
 
+        /* Caller might be sure that this PID belongs to this unit only. Let's take this
+         * opportunity to remove any stalled references to this PID as they can be created
+         * easily (when watching a process which is not our direct child). */
+        if (exclusive)
+                manager_unwatch_pid(u->manager, pid);
+
         r = set_ensure_allocated(&u->pids, NULL);
         if (r < 0)
                 return r;
diff --git a/src/core/unit.h b/src/core/unit.h
index 35b936771c..28878b5f4f 100644
--- a/src/core/unit.h
+++ b/src/core/unit.h
@@ -678,7 +678,7 @@ typedef enum UnitNotifyFlags {
 
 void unit_notify(Unit *u, UnitActiveState os, UnitActiveState ns, UnitNotifyFlags flags);
 
-int unit_watch_pid(Unit *u, pid_t pid);
+int unit_watch_pid(Unit *u, pid_t pid, bool exclusive);
 void unit_unwatch_pid(Unit *u, pid_t pid);
 void unit_unwatch_all_pids(Unit *u);
 
diff --git a/src/test/test-watch-pid.c b/src/test/test-watch-pid.c
index 2c6ca0a1a2..04d7e1fbd8 100644
--- a/src/test/test-watch-pid.c
+++ b/src/test/test-watch-pid.c
@@ -42,25 +42,25 @@ int main(int argc, char *argv[]) {
         assert_se(hashmap_isempty(m->watch_pids));
         assert_se(manager_get_unit_by_pid(m, 4711) == NULL);
 
-        assert_se(unit_watch_pid(a, 4711) >= 0);
+        assert_se(unit_watch_pid(a, 4711, false) >= 0);
         assert_se(manager_get_unit_by_pid(m, 4711) == a);
 
-        assert_se(unit_watch_pid(a, 4711) >= 0);
+        assert_se(unit_watch_pid(a, 4711, false) >= 0);
         assert_se(manager_get_unit_by_pid(m, 4711) == a);
 
-        assert_se(unit_watch_pid(b, 4711) >= 0);
+        assert_se(unit_watch_pid(b, 4711, false) >= 0);
         u = manager_get_unit_by_pid(m, 4711);
         assert_se(u == a || u == b);
 
-        assert_se(unit_watch_pid(b, 4711) >= 0);
+        assert_se(unit_watch_pid(b, 4711, false) >= 0);
         u = manager_get_unit_by_pid(m, 4711);
         assert_se(u == a || u == b);
 
-        assert_se(unit_watch_pid(c, 4711) >= 0);
+        assert_se(unit_watch_pid(c, 4711, false) >= 0);
         u = manager_get_unit_by_pid(m, 4711);
         assert_se(u == a || u == b || u == c);
 
-        assert_se(unit_watch_pid(c, 4711) >= 0);
+        assert_se(unit_watch_pid(c, 4711, false) >= 0);
         u = manager_get_unit_by_pid(m, 4711);
         assert_se(u == a || u == b || u == c);