CFL restarts: don't overwrite the restart checkpoint on the first step

CFL mode saved when abs(mod(mytime, t_save)) < dt. A restart starts at
exactly n_start*t_save, so after the first step mod(mytime, t_save) is
within an ulp of dt and falls below it about half the time. The save then
writes index int(mytime/t_save) = n_start and replaces the restart
checkpoint with the state one step later.

Save instead when int(mytime/t_save), the index s_save_data writes, moves
past the last saved index (n_start at start), or at t_stop. Fresh-run
saves happen at the same steps as before.

The CFL restart test restarts from save indices 1-4 and checks that each
restart leaves its own checkpoint byte-identical.

Author: sbryngelson

src/simulation/p_main.fpp

@@ -12,14 +12,15 @@ program p_main
 
     implicit none
 
-    integer                             :: t_step  !< Iterator for the time-stepping loop
+    integer                             :: t_step     !< Iterator for the time-stepping loop
     real(wp)                            :: time_avg, time_final
     real(wp)                            :: io_time_avg, io_time_final
     real(wp), allocatable, dimension(:) :: proc_time
     real(wp), allocatable, dimension(:) :: io_proc_time
     logical                             :: file_exists
     real(wp)                            :: start, finish
     integer                             :: nt
+    integer                             :: last_save  !< Index of the last CFL-mode save
 
     call system_clock(COUNT=cpu_start, COUNT_RATE=cpu_rate)
 
@@ -47,6 +48,7 @@ program p_main
         t_step = 0
         t_step_start = 0
         mytime = t_save*n_start
+        last_save = n_start
     else
         t_step = t_step_start
         if (t_step == 0) then
@@ -78,8 +80,10 @@ program p_main
         call s_perform_time_step(t_step, time_avg)
 
         if (cfl_dt) then
-            if (abs(mod(mytime, t_save)) < dt .or. mytime >= t_stop) then
+            ! Save once per index; a mod(mytime, t_save) < dt test can refire on a restart and overwrite n_start
+            if (int(mytime/t_save) > last_save .or. mytime >= t_stop) then
                 call s_save_data(t_step, start, finish, io_time_avg, nt)
+                last_save = int(mytime/t_save)
             end if
         else
             if (mod(t_step - t_step_start, t_step_save) == 0 .or. t_step == t_step_stop) then