core: add new RandomSec= setting for time units

This allows configuration of a random time on top of the elapse events,
in order to spread time events in a network evenly across a range.

5 files changed, 64 insertions, 0 deletions

diff --git a/src/core/dbus-timer.c b/src/core/dbus-timer.c
index 2e6c479c3a..4bee82df07 100644
--- a/src/core/dbus-timer.c
+++ b/src/core/dbus-timer.c
@@ -180,6 +180,7 @@ const sd_bus_vtable bus_timer_vtable[] = {
         BUS_PROPERTY_DUAL_TIMESTAMP("LastTriggerUSec", offsetof(Timer, last_trigger), SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE),
         SD_BUS_PROPERTY("Result", "s", property_get_result, offsetof(Timer, result), SD_BUS_VTABLE_PROPERTY_EMITS_CHANGE),
         SD_BUS_PROPERTY("AccuracyUSec", "t", bus_property_get_usec, offsetof(Timer, accuracy_usec), SD_BUS_VTABLE_PROPERTY_CONST),
+        SD_BUS_PROPERTY("RandomUSec", "t", bus_property_get_usec, offsetof(Timer, random_usec), SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_PROPERTY("Persistent", "b", bus_property_get_bool, offsetof(Timer, persistent), SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_PROPERTY("WakeSystem", "b", bus_property_get_bool, offsetof(Timer, wake_system), SD_BUS_VTABLE_PROPERTY_CONST),
         SD_BUS_PROPERTY("RemainAfterElapse", "b", bus_property_get_bool, offsetof(Timer, remain_after_elapse), SD_BUS_VTABLE_PROPERTY_CONST),
@@ -283,6 +284,22 @@ static int bus_timer_set_transient_property(
 
                 return 1;
 
+        } else if (streq(name, "RandomUSec")) {
+                usec_t u = 0;
+
+                r = sd_bus_message_read(message, "t", &u);
+                if (r < 0)
+                        return r;
+
+                if (mode != UNIT_CHECK) {
+                        char time[FORMAT_TIMESPAN_MAX];
+
+                        t->random_usec = u;
+                        unit_write_drop_in_private_format(UNIT(t), mode, name, "RandomSec=%s\n", format_timespan(time, sizeof(time), u, USEC_PER_MSEC));
+                }
+
+                return 1;
+
         } else if (streq(name, "WakeSystem")) {
                 int b;
 
diff --git a/src/core/load-fragment-gperf.gperf.m4 b/src/core/load-fragment-gperf.gperf.m4
index 0dbe38d7e5..c64850802e 100644
--- a/src/core/load-fragment-gperf.gperf.m4
+++ b/src/core/load-fragment-gperf.gperf.m4
@@ -347,6 +347,7 @@ Timer.Persistent,                config_parse_bool,                  0,
 Timer.WakeSystem,                config_parse_bool,                  0,                             offsetof(Timer, wake_system)
 Timer.RemainAfterElapse,         config_parse_bool,                  0,                             offsetof(Timer, remain_after_elapse)
 Timer.AccuracySec,               config_parse_sec,                   0,                             offsetof(Timer, accuracy_usec)
+Timer.RandomSec,                 config_parse_sec,                   0,                             offsetof(Timer, random_usec)
 Timer.Unit,                      config_parse_trigger_unit,          0,                             0
 m4_dnl
 Path.PathExists,                 config_parse_path_spec,             0,                             0
diff --git a/src/core/timer.c b/src/core/timer.c
index 0587452cfb..6b0f8e8616 100644
--- a/src/core/timer.c
+++ b/src/core/timer.c
@@ -27,6 +27,7 @@
 #include "dbus-timer.h"
 #include "fs-util.h"
 #include "parse-util.h"
+#include "random-util.h"
 #include "special.h"
 #include "string-table.h"
 #include "string-util.h"
@@ -330,6 +331,28 @@ static usec_t monotonic_to_boottime(usec_t t) {
                 return 0;
 }
 
+static void add_random(Timer *t, usec_t *v) {
+        char s[FORMAT_TIMESPAN_MAX];
+        usec_t add;
+
+        assert(t);
+        assert(*v);
+
+        if (t->random_usec == 0)
+                return;
+        if (*v == USEC_INFINITY)
+                return;
+
+        add = random_u64() % t->random_usec;
+
+        if (*v + add < *v) /* overflow */
+                *v = (usec_t) -2; /* Highest possible value, that is not USEC_INFINITY */
+        else
+                *v += add;
+
+        log_unit_info(UNIT(t), "Adding %s random time.", format_timespan(s, sizeof(s), add, 0));
+}
+
 static void timer_enter_waiting(Timer *t, bool initial) {
         bool found_monotonic = false, found_realtime = false;
         usec_t ts_realtime, ts_monotonic;
@@ -452,6 +475,8 @@ static void timer_enter_waiting(Timer *t, bool initial) {
                 char buf[FORMAT_TIMESPAN_MAX];
                 usec_t left;
 
+                add_random(t, &t->next_elapse_monotonic_or_boottime);
+
                 left = t->next_elapse_monotonic_or_boottime > ts_monotonic ? t->next_elapse_monotonic_or_boottime - ts_monotonic : 0;
                 log_unit_debug(UNIT(t), "Monotonic timer elapses in %s.", format_timespan(buf, sizeof(buf), left, 0));
 
@@ -486,6 +511,9 @@ static void timer_enter_waiting(Timer *t, bool initial) {
 
         if (found_realtime) {
                 char buf[FORMAT_TIMESTAMP_MAX];
+
+                add_random(t, &t->next_elapse_realtime);
+
                 log_unit_debug(UNIT(t), "Realtime timer elapses at %s.", format_timestamp(buf, sizeof(buf), t->next_elapse_realtime));
 
                 if (t->realtime_event_source) {
diff --git a/src/core/timer.h b/src/core/timer.h
index 6bc9fbed3d..0599f07818 100644
--- a/src/core/timer.h
+++ b/src/core/timer.h
@@ -58,6 +58,7 @@ struct Timer {
         Unit meta;
 
         usec_t accuracy_usec;
+        usec_t random_usec;
 
         LIST_HEAD(TimerValue, values);
         usec_t next_elapse_realtime;
diff --git a/src/shared/bus-util.c b/src/shared/bus-util.c
index dab4934901..38281045b8 100644
--- a/src/shared/bus-util.c
+++ b/src/shared/bus-util.c
@@ -1442,6 +1442,23 @@ int bus_append_unit_property_assignment(sd_bus_message *m, const char *assignmen
                         return bus_log_create_error(r);
 
                 return 0;
+
+        } else if (streq(field, "RandomSec")) {
+                usec_t t;
+
+                r = parse_sec(eq, &t);
+                if (r < 0)
+                        return log_error_errno(r, "Failed to parse RandomSec= parameter: %s", eq);
+
+                r = sd_bus_message_append_basic(m, SD_BUS_TYPE_STRING, "RandomUSec");
+                if (r < 0)
+                        return bus_log_create_error(r);
+
+                r = sd_bus_message_append(m, "v", "t", t);
+                if (r < 0)
+                        return bus_log_create_error(r);
+
+                return 0;
         }
 
         r = sd_bus_message_append_basic(m, SD_BUS_TYPE_STRING, field);