timesync: increase retry interval more slowly

The new retry intervals are [15, 20, 26, 34, 45, 60, 80, 106, 141, 188, 250,
333, 360, ...]. This should allow graceful response if a transient network
failure is encountered. Growth is exponential, but with a small power and
capped to a non-too-large value so that we resynchronize within a few minutes
after network is restored. I made the minimum 15 s to make sure that we never
send packets more often than that.

Fixes #16492.

src/timesync/timesyncd-manager.c

@@ -137,11 +137,10 @@ static int manager_send_request(Manager *m) {
         }
 
         /* re-arm timer with increasing timeout, in case the packets never arrive back */
-        if (m->retry_interval > 0) {
-                if (m->retry_interval < m->poll_interval_max_usec)
-                        m->retry_interval *= 2;
-        } else
-                m->retry_interval = m->poll_interval_min_usec;
+        if (m->retry_interval == 0)
+                m->retry_interval = NTP_RETRY_INTERVAL_MIN_USEC;
+        else
+                m->retry_interval = MIN(m->retry_interval * 4/3, NTP_RETRY_INTERVAL_MAX_USEC);
 
         r = manager_arm_timer(m, m->retry_interval);
         if (r < 0)

src/timesync/timesyncd-manager.h

@@ -24,6 +24,9 @@ typedef struct Manager Manager;
 #define NTP_POLL_INTERVAL_MIN_USEC      (32 * USEC_PER_SEC)
 #define NTP_POLL_INTERVAL_MAX_USEC      (2048 * USEC_PER_SEC)
 
+#define NTP_RETRY_INTERVAL_MIN_USEC     (15 * USEC_PER_SEC)
+#define NTP_RETRY_INTERVAL_MAX_USEC     (6 * 60 * USEC_PER_SEC) /* 6 minutes */
+
 struct Manager {
         sd_bus *bus;
         sd_event *event;