multi: round the timeout up to prevent early wakeups

Curl_timediff rounds down to the millisecond, so curl_multi_perform can
be called too early, then we get a timeout of 0 and call it again.

The code already handled the case of timeouts which expired less than
1ms in the future.  By rounding up, we make sure we will never ask the
platform to wake up too early.

Closes #11938

lib/multi.c

@@ -3422,20 +3422,10 @@ static CURLMcode multi_timeout(struct Curl_multi *multi,
 
     if(Curl_splaycomparekeys(multi->timetree->key, now) > 0) {
       /* some time left before expiration */
-      timediff_t diff = Curl_timediff(multi->timetree->key, now);
-      if(diff <= 0)
-        /*
-         * Since we only provide millisecond resolution on the returned value
-         * and the diff might be less than one millisecond here, we don't
-         * return zero as that may cause short bursts of busyloops on fast
-         * processors while the diff is still present but less than one
-         * millisecond! instead we return 1 until the time is ripe.
-         */
-        *timeout_ms = 1;
-      else
-        /* this should be safe even on 64 bit archs, as we don't use that
-           overly long timeouts */
-        *timeout_ms = (long)diff;
+      timediff_t diff = Curl_timediff_ceil(multi->timetree->key, now);
+      /* this should be safe even on 32 bit archs, as we don't use that
+         overly long timeouts */
+      *timeout_ms = (long)diff;
     }
     else
       /* 0 means immediately */

lib/timeval.c

@@ -209,6 +209,20 @@ timediff_t Curl_timediff(struct curltime newer, struct curltime older)
   return diff * 1000 + (newer.tv_usec-older.tv_usec)/1000;
 }
 
+/*
+ * Returns: time difference in number of milliseconds, rounded up.
+ * For too large diffs it returns max value.
+ */
+timediff_t Curl_timediff_ceil(struct curltime newer, struct curltime older)
+{
+  timediff_t diff = (timediff_t)newer.tv_sec-older.tv_sec;
+  if(diff >= (TIMEDIFF_T_MAX/1000))
+    return TIMEDIFF_T_MAX;
+  else if(diff <= (TIMEDIFF_T_MIN/1000))
+    return TIMEDIFF_T_MIN;
+  return diff * 1000 + (newer.tv_usec - older.tv_usec + 999)/1000;
+}
+
 /*
  * Returns: time difference in number of microseconds. For too large diffs it
  * returns max value.

lib/timeval.h

@@ -36,16 +36,24 @@ struct curltime {
 struct curltime Curl_now(void);
 
 /*
- * Make sure that the first argument (t1) is the more recent time and t2 is
- * the older time, as otherwise you get a weird negative time-diff back...
+ * Make sure that the first argument (newer) is the more recent time and older
+ * is the older time, as otherwise you get a weird negative time-diff back...
  *
  * Returns: the time difference in number of milliseconds.
  */
-timediff_t Curl_timediff(struct curltime t1, struct curltime t2);
+timediff_t Curl_timediff(struct curltime newer, struct curltime older);
 
 /*
- * Make sure that the first argument (t1) is the more recent time and t2 is
- * the older time, as otherwise you get a weird negative time-diff back...
+ * Make sure that the first argument (newer) is the more recent time and older
+ * is the older time, as otherwise you get a weird negative time-diff back...
+ *
+ * Returns: the time difference in number of milliseconds, rounded up.
+ */
+timediff_t Curl_timediff_ceil(struct curltime newer, struct curltime older);
+
+/*
+ * Make sure that the first argument (newer) is the more recent time and older
+ * is the older time, as otherwise you get a weird negative time-diff back...
  *
  * Returns: the time difference in number of microseconds.
  */