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.

3 files changed, 15 insertions, 9 deletions

diff --git a/src/core/cgroup.c b/src/core/cgroup.c
index 910a64b4da..ca3c3366f3 100644
--- a/src/core/cgroup.c
+++ b/src/core/cgroup.c
@@ -952,7 +952,7 @@ static void cgroup_context_apply(Unit *u, CGroupMask mask, ManagerState state) {
 
         if ((mask & CGROUP_MASK_PIDS) && !is_root) {
 
-                if (c->tasks_max != (uint64_t) -1) {
+                if (c->tasks_max != CGROUP_LIMIT_MAX) {
                         char buf[DECIMAL_STR_MAX(uint64_t) + 2];
 
                         sprintf(buf, "%" PRIu64 "\n", c->tasks_max);
diff --git a/src/core/load-fragment.c b/src/core/load-fragment.c
index 4ad6c4d79b..d5185cf6a0 100644
--- a/src/core/load-fragment.c
+++ b/src/core/load-fragment.c
@@ -2999,30 +2999,36 @@ int config_parse_tasks_max(
                 void *data,
                 void *userdata) {
 
-        uint64_t *tasks_max = data, u;
+        uint64_t *tasks_max = data, v;
+        Unit *u = userdata;
         int r;
 
-        if (isempty(rvalue) || streq(rvalue, "infinity")) {
-                *tasks_max = (uint64_t) -1;
+        if (isempty(rvalue)) {
+                *tasks_max = u->manager->default_tasks_max;
+                return 0;
+        }
+
+        if (streq(rvalue, "infinity")) {
+                *tasks_max = CGROUP_LIMIT_MAX;
                 return 0;
         }
 
         r = parse_percent(rvalue);
         if (r < 0) {
-                r = safe_atou64(rvalue, &u);
+                r = safe_atou64(rvalue, &v);
                 if (r < 0) {
                         log_syntax(unit, LOG_ERR, filename, line, r, "Maximum tasks value '%s' invalid. Ignoring.", rvalue);
                         return 0;
                 }
         } else
-                u = system_tasks_max_scale(r, 100U);
+                v = system_tasks_max_scale(r, 100U);
 
-        if (u <= 0 || u >= UINT64_MAX) {
+        if (v <= 0 || v >= UINT64_MAX) {
                 log_syntax(unit, LOG_ERR, filename, line, 0, "Maximum tasks value '%s' out of range. Ignoring.", rvalue);
                 return 0;
         }
 
-        *tasks_max = u;
+        *tasks_max = v;
         return 0;
 }
 
diff --git a/src/core/main.c b/src/core/main.c
index 02324d325e..c7c6df3447 100644
--- a/src/core/main.c
+++ b/src/core/main.c
@@ -1559,7 +1559,7 @@ int main(int argc, char *argv[]) {
         (void) reset_all_signal_handlers();
         (void) ignore_signals(SIGNALS_IGNORE, -1);
 
-        arg_default_tasks_max = system_tasks_max_scale(15U, 100U); /* 15% the system PIDs equals 4915 by default. */
+        arg_default_tasks_max = system_tasks_max_scale(DEFAULT_TASKS_MAX_PERCENTAGE, 100U);
 
         if (parse_config_file() < 0) {
                 error_message = "Failed to parse config file";