[data-poll-manager] fix unexpected interval (#3780)

src/core/thread/data_poll_manager.cpp

@@ -392,18 +392,42 @@ exit:
 
 void DataPollManager::ScheduleNextPoll(PollPeriodSelector aPollPeriodSelector)
 {
+    uint32_t now;
+    uint32_t oldPeriod = mPollPeriod;
+
     if (aPollPeriodSelector == kRecalculatePollPeriod)
     {
         mPollPeriod = CalculatePollPeriod();
     }
 
+    now = TimerMilli::GetNow();
+
     if (mTimer.IsRunning())
     {
-        mTimer.StartAt(mTimerStartTime, mPollPeriod);
+        if (oldPeriod != mPollPeriod)
+        {
+            // If poll interval did change and re-starting the timer from
+            // last start time with new poll interval would fire quickly
+            // (i.e., fires within window `[now, now + kMinPollPeriod]`)
+            // add an extra minimum delay of `kMinPollPeriod`. This
+            // ensures that when an internal or external request triggers
+            // a switch to a shorter poll interval, the first data poll
+            // will not be sent too quickly (and possibly before the
+            // response is available/prepared on the parent node).
+            if (TimerScheduler::IsStrictlyBefore(mTimerStartTime + mPollPeriod, now + kMinPollPeriod))
+            {
+                mTimer.StartAt(now, kMinPollPeriod);
+            }
+            else
+            {
+                mTimer.StartAt(mTimerStartTime, mPollPeriod);
+            }
+        }
+        // Do nothing on the running poll timer if the poll interval doesn't change
     }
     else
     {
-        mTimerStartTime = TimerMilli::GetNow();
+        mTimerStartTime = now;
         mTimer.StartAt(mTimerStartTime, mPollPeriod);
     }
 }