Abort when post_process is given a different grid than the restart files

post_process checks that the restart files exist but never that they hold the
grid the case file asks for. Give it a case whose resolution no longer matches
the run and it reads past the end of every file, post-processes the overrun,
and exits 0 with NaN-filled output. Nothing reports a problem until someone
plots it, or until a diagnostic built on that output starts returning NaN and
gets blamed instead.

That is what happened here: a 349 M cell run (m = 1644) post-processed with a
case file that had since moved to m = 2056, so 681 M cells were read from files
holding 349 M.

x_cb.dat holds exactly m_glb + 2 cell boundaries, so its size states which grid
wrote the restart. The check costs one inquire on a file already being opened,
fires before any bulk read, and names both grids:

  Restart grid mismatch: this case has m = 2056 but
  .../restart_data/x_cb.dat was written with m = 1644. Post-processing must use
  the same grid as the run that wrote the restart files, or it reads past the
  end of every file and writes NaN.

Claude-Session: https://claude.ai/code/session_01HMJ7cycfo7kTFSFq5yhHLG

Author: sbryngelson

src/post_process/m_data_input.f90

@@ -276,6 +276,8 @@ impure subroutine s_read_parallel_data_files(t_step)
         integer(kind=MPI_OFFSET_KIND)        :: offset
         character(LEN=path_len + 2*name_len) :: file_loc
         logical                              :: file_exist
+        integer(kind=8)                      :: file_bytes
+        character(len=10)                    :: case_m_str, file_m_str
         character(len=10)                    :: t_step_string
         integer                              :: i
 
@@ -290,9 +292,21 @@ impure subroutine s_read_parallel_data_files(t_step)
         end if
 
         file_loc = trim(case_dir) // '/restart_data' // trim(mpiiofs) // 'x_cb.dat'
-        inquire (FILE=trim(file_loc), EXIST=file_exist)
+        inquire (FILE=trim(file_loc), EXIST=file_exist, SIZE=file_bytes)
 
+        ! The grid file holds exactly m_glb + 2 cell boundaries, so its size says which grid wrote the restart.
+        ! Without this check a case file whose resolution no longer matches the run reads past the end of every
+        ! restart file and post-processes silently, exiting 0 with NaN-filled output -- which is indistinguishable
+        ! from success until someone plots it.
         if (file_exist) then
+            if (file_bytes /= int(m_glb + 2, 8)*8_8) then
+                call s_int_to_str(m_glb, case_m_str)
+                call s_int_to_str(int(file_bytes/8_8) - 2, file_m_str)
+                call s_mpi_abort('Restart grid mismatch: this case has m = ' // trim(case_m_str) // &
+                                 ' but ' // trim(file_loc) // ' was written with m = ' // trim(file_m_str) // &
+                                 '. Post-processing must use the same grid as the run that wrote the ' // &
+                                 'restart files, or it reads past the end of every file and writes NaN.')
+            end if
             data_size = m_glb + 2
             call MPI_FILE_OPEN(MPI_COMM_WORLD, file_loc, MPI_MODE_RDONLY, mpi_info_int, ifile, ierr)