logind: change TasksMax= value for user logins to 33%

Let's change from a fixed value of 12288 tasks per user to a relative value of
33%, which with the kernel's default of 32768 translates to 10813. This is a
slight decrease of the limit, for no other reason than "33%" sounding like a nice
round number that is close enough to 12288 (which would translate to 37.5%).
(Well, it also has the nice effect of still leaving a bit of room in the PID
space if there are 3 cooperating evil users that try to consume all PIDs...
Also, I like my bikesheds blue).

Since the new value is taken relative, and machined's TasksMax= setting
defaults to 16384, 33% inside of containers is usually equivalent to 5406,
which should still be ample space.

To summarize:

               | on the host | in the container
   old default |       12288 |            12288
   new default |       10813 |             5406

1 files changed, 45 insertions, 0 deletions

diff --git a/src/login/logind-user.c b/src/login/logind-user.c
index de44d369cf..f7d83909a0 100644
--- a/src/login/logind-user.c
+++ b/src/login/logind-user.c
@@ -870,3 +870,48 @@ int config_parse_tmpfs_size(
 
         return 0;
 }
+
+int config_parse_user_tasks_max(
+                const char* unit,
+                const char *filename,
+                unsigned line,
+                const char *section,
+                unsigned section_line,
+                const char *lvalue,
+                int ltype,
+                const char *rvalue,
+                void *data,
+                void *userdata) {
+
+        uint64_t *m = data;
+        uint64_t k;
+        int r;
+
+        assert(filename);
+        assert(lvalue);
+        assert(rvalue);
+        assert(data);
+
+        /* First, try to parse as percentage */
+        r = parse_percent(rvalue);
+        if (r > 0 && r < 100)
+                k = system_tasks_max_scale(r, 100U);
+        else {
+
+                /* If the passed argument was not a percentage, or out of range, parse as byte size */
+
+                r = safe_atou64(rvalue, &k);
+                if (r < 0) {
+                        log_syntax(unit, LOG_ERR, filename, line, r, "Failed to parse tasks maximum, ignoring: %s", rvalue);
+                        return 0;
+                }
+        }
+
+        if (k <= 0 || k >= UINT64_MAX) {
+                log_syntax(unit, LOG_ERR, filename, line, 0, "Tasks maximum out of range, ignoring: %s", rvalue);
+                return 0;
+        }
+
+        *m = k;
+        return 0;
+}