Update drift file at most once per hour

Instead of writing to the file on every reference update, update it
on first update, on exit and otherwise only once per hour.

reference.c

@@ -76,6 +76,7 @@ static char *mail_change_user;
 
 /* Filename of the drift file. */
 static char *drift_file=NULL;
+static double drift_file_age;
 
 static void update_drift_file(double, double);
 
@@ -155,7 +156,7 @@ REF_Initialise(void)
           drift_file);
     }
 
-    update_drift_file(our_frequency_ppm,our_skew);
+    drift_file_age = 0.0;
   }
     
   LCL_SetAbsoluteFrequency(our_frequency_ppm);
@@ -201,6 +202,10 @@ REF_Finalise(void)
     LCL_SetLeap(0);
   }
 
+  if (drift_file && drift_file_age > 0.0) {
+    update_drift_file(LCL_ReadAbsoluteFrequency(), our_skew);
+  }
+
   Free(fb_drifts);
 
   initialised = 0;
@@ -531,6 +536,8 @@ REF_SetReference(int stratum,
 
   double abs_freq_ppm;
 
+  double update_interval;
+
   assert(initialised);
 
   /* Avoid getting NaNs */
@@ -630,21 +637,25 @@ REF_SetReference(int stratum,
             1.0e6*our_skew,
             our_offset);
 
+  UTI_DiffTimevalsToDouble(&update_interval, ref_time, &last_ref_update);
+
   if (drift_file) {
-    update_drift_file(abs_freq_ppm, our_skew);
+    /* Update drift file at most once per hour */
+    drift_file_age += update_interval;
+    if (drift_file_age < 0.0 || drift_file_age > 3600.0) {
+      update_drift_file(abs_freq_ppm, our_skew);
+      drift_file_age = 0.0;
+    }
   }
 
   /* Update fallback drifts */
   if (fb_drifts) {
-    double update_interval;
-
-    UTI_DiffTimevalsToDouble(&update_interval, ref_time, &last_ref_update);
-
     update_fb_drifts(abs_freq_ppm, update_interval);
-    last_ref_update = *ref_time;
-    last_ref_update_interval = update_interval;
   }
 
+  last_ref_update = *ref_time;
+  last_ref_update_interval = update_interval;
+
   /* And now set the freq and offset to zero */
   our_frequency = 0.0;
   our_offset = 0.0;