change parallel access to whole dataset (issue #122, thanks @pgf)

ext/NCDatasetsMPIExt.jl

@@ -3,13 +3,16 @@ using MPI
 using NCDatasets
 using NCDatasets:
     NC_COLLECTIVE,
+    NC_FORMAT_NETCDF4,
+    NC_FORMAT_NETCDF4_CLASSIC,
     NC_GLOBAL,
     NC_INDEPENDENT,
     Variable,
     _dataset_ncmode,
     check,
     dataset,
-    libnetcdf
+    libnetcdf,
+    nc_inq_format
 
 import NCDatasets:
     NCDataset,
@@ -58,14 +61,27 @@ Change the parallel access mode of the variable `ncv` or all variables of the da
 More information is available in the [NetCDF documentation](https://web.archive.org/web/20240414204638/https://docs.unidata.ucar.edu/netcdf-c/current/parallel_io.html).
 """
 function access(ncv::Variable,par_access::Symbol)
+    ds = dataset(ncv)
+    ncid = ds.ncid
     varid = ncv.varid
-    ncid = dataset(ncv).ncid
-    nc_var_par_access(ncid,varid,parallel_access_mode(par_access))
+
+    if nc_inq_format(ncid) in (NC_FORMAT_NETCDF4, NC_FORMAT_NETCDF4_CLASSIC)
+        nc_var_par_access(ncid,varid,parallel_access_mode(par_access))
+    else
+        error("The netCDF 3 and 5 formats do not allow different access methods per variable. You need to call this function for the whole data set: NCDatasets.access(ds,$par_access)")
+    end
 end
 
 # set collective or independent IO globally (for all variables)
 function access(ds::NCDataset,par_access::Symbol)
-    nc_var_par_access(ds.ncid,NC_GLOBAL,parallel_access_mode(par_access))
+    if nc_inq_format(ds.ncid) in (NC_FORMAT_NETCDF4, NC_FORMAT_NETCDF4_CLASSIC)
+        for (varname,ncv) in ds
+            access(ncv.var,par_access)
+        end
+    else
+        # only for PnetCDF
+        nc_var_par_access(ds.ncid,NC_GLOBAL,parallel_access_mode(par_access))
+    end
 end
 
 """

test/test_mpi_script.jl

@@ -25,7 +25,7 @@ ncv = defVar(ds,"temp",Int32,("lon","lat"))
 # see
 # https://web.archive.org/web/20240414204638/https://docs.unidata.ucar.edu/netcdf-c/current/parallel_io.html
 NCDatasets.access(ncv.var,:collective)
-
+NCDatasets.access(ds,:collective)
 
 @debug("rank $(mpi_rank) writing to netCDF variable")
 ncv[:,i] .= mpi_rank