logind: update empty and "infinity" handling for [User]TasksMax (#3835)

The parsing functions for [User]TasksMax were inconsistent.  Empty string and
"infinity" were interpreted as no limit for TasksMax but not accepted for
UserTasksMax.  Update them so that they're consistent with other knobs.

* Empty string indicates the default value.
* "infinity" indicates no limit.

While at it, replace opencoded (uint64_t) -1 with CGROUP_LIMIT_MAX in TasksMax
handling.

v2: Update empty string to indicate the default value as suggested by Zbigniew
    Jędrzejewski-Szmek.

v3: Fixed empty UserTasksMax handling.

2 files changed, 14 insertions, 2 deletions

diff --git a/src/login/logind-user.c b/src/login/logind-user.c
index 11951aca5b..e0e73b034d 100644
--- a/src/login/logind-user.c
+++ b/src/login/logind-user.c
@@ -26,6 +26,7 @@
 #include "bus-common-errors.h"
 #include "bus-error.h"
 #include "bus-util.h"
+#include "cgroup-util.h"
 #include "clean-ipc.h"
 #include "conf-parser.h"
 #include "escape.h"
@@ -896,7 +897,17 @@ int config_parse_user_tasks_max(
         assert(rvalue);
         assert(data);
 
-        /* First, try to parse as percentage */
+        if (isempty(rvalue)) {
+                *m = system_tasks_max_scale(DEFAULT_USER_TASKS_MAX_PERCENTAGE, 100U);
+                return 0;
+        }
+
+        if (streq(rvalue, "infinity")) {
+                *m = CGROUP_LIMIT_MAX;
+                return 0;
+        }
+
+        /* Try to parse as percentage */
         r = parse_percent(rvalue);
         if (r >= 0)
                 k = system_tasks_max_scale(r, 100U);
diff --git a/src/login/logind.c b/src/login/logind.c
index 5ce36d28c7..bbbf4aef57 100644
--- a/src/login/logind.c
+++ b/src/login/logind.c
@@ -38,6 +38,7 @@
 #include "signal-util.h"
 #include "strv.h"
 #include "udev-util.h"
+#include "cgroup-util.h"
 
 static void manager_free(Manager *m);
 
@@ -62,7 +63,7 @@ static void manager_reset_config(Manager *m) {
         m->idle_action = HANDLE_IGNORE;
 
         m->runtime_dir_size = physical_memory_scale(10U, 100U); /* 10% */
-        m->user_tasks_max = system_tasks_max_scale(33U, 100U); /* 33% */
+        m->user_tasks_max = system_tasks_max_scale(DEFAULT_USER_TASKS_MAX_PERCENTAGE, 100U); /* 33% */
         m->sessions_max = 8192;
         m->inhibitors_max = 8192;