modules/cron: avoid rounding in timespec_to_double

Avoid implicit rounding in timespec_to_double in both cron.c and task.c.

For the cron task timestamps, rename the local function
round_timespec_to_double() to be honest about what we're doing,
and expand the comment to explain why.

For other cron timestamps, just drop the rounding for now. If it
an inconvenience in the future (unlikely), the rounding can be
copied from task.c as an example.

Resolves #1131

src/modules/cron/cron.c

@@ -85,18 +85,11 @@ void *cron_entry_type_data (cron_entry_t *e)
     return e->data;
 }
 
-static double timespec_to_double (struct timespec *tm)
-{
-    double s = tm->tv_sec;
-    double ns = tm->tv_nsec/1.0e9 + .5e-9; // round 1/2 epsilon
-    return (s + ns);
-}
-
 double get_timestamp (void)
 {
     struct timespec tm;
     clock_gettime (CLOCK_REALTIME, &tm);
-    return timespec_to_double (&tm);
+    return ((double) tm.tv_sec + (tm.tv_nsec/1.0e9));
 }
 
 static void timeout_cb (flux_t *h, cron_task_t *t, void *arg)

src/modules/cron/task.c

@@ -447,10 +447,15 @@ int cron_task_status (cron_task_t *t)
     return (t->status);
 }
 
-static double timespec_to_double (struct timespec *tm)
+static double round_timespec_to_double (struct timespec *tm)
 {
     double s = tm->tv_sec;
-    double ns = tm->tv_nsec/1.0e9 + .5e-9; // round 1/2 epsilon
+    /*  Add .5ns (1/2 the minumim possible value change) to avoid
+     *   underflow which represents something like .5 as .499999...
+     *   (we don't care about overflow since we'll truncate fractional
+     *    part to 9 significant digits *at the most* anyway)
+     */
+    double ns = tm->tv_nsec/1.0e9 + .5e-9;
     return (s + ns);
 }
 
@@ -480,7 +485,7 @@ static const char * cron_task_state_string (cron_task_t *t)
  */
 static int add_timespec (json_t *o, const char *name, struct timespec *tm)
 {
-    json_t *n = json_real (timespec_to_double (tm));
+    json_t *n = json_real (round_timespec_to_double (tm));
     if (n == NULL)
         return -1;
     return json_object_set_new (o, name, n);