Fix stock CDL headers for unlimited time dimension. (#4827)

* Fix CDL headers for tests.stock.netcdf.

* Whats new entry.

* Improved unlimited dimension check in stock NetCDF generation.

docs/src/whatsnew/latest.rst

@@ -226,6 +226,11 @@ This document explains the changes made to Iris for this release
    bin in the system PATH.
    (:pull:`4794`)
 
+#. `@trexfeathers`_ and `@pp-mo`_ fixed the CDL headers for
+   :mod:`iris.tests.stock.netcdf` to allow generation of NetCDF-4 files with an
+   unlimited time dimension.
+   (:pull:`4827`)
+
 
 .. comment
     Whatsnew author names (@github name) in alphabetical order. Note that,

lib/iris/tests/stock/file_headers/xios_2D_face_half_levels.cdl

@@ -55,4 +55,8 @@ variables:
 		:name = "${DATASET_NAME}" ;
 //		original name = "lfric_ngvat_2D_1t_face_half_levels_main_conv_rain"
 		:Conventions = "UGRID" ;
+
+// data
+data:
+  time_instant = 0 ;
 }

lib/iris/tests/stock/file_headers/xios_3D_face_full_levels.cdl

@@ -58,4 +58,8 @@ variables:
 		:name = "${DATASET_NAME}" ;
 //		original name = "lfric_ngvat_3D_1t_full_level_face_grid_main_u3"
 		:Conventions = "UGRID" ;
+
+// data
+data:
+  time_instant = 0 ;
 }

lib/iris/tests/stock/file_headers/xios_3D_face_half_levels.cdl

@@ -58,4 +58,8 @@ variables:
 		:name = "${DATASET_NAME}" ;
 //		original name = "lfric_ngvat_3D_1t_half_level_face_grid_derived_theta_in_w3"
 		:Conventions = "UGRID" ;
+
+// data
+data:
+  time_instant = 0 ;
 }

lib/iris/tests/stock/netcdf.py

@@ -103,7 +103,7 @@ def _add_standard_data(nc_path, unlimited_dim_size=0):
     ds = netCDF4.Dataset(nc_path, "r+")
 
     unlimited_dim_names = [
-        dim for dim in ds.dimensions if ds.dimensions[dim].size == 0
+        dim for dim in ds.dimensions if ds.dimensions[dim].isunlimited()
     ]
     # Data addition dependent on this assumption:
     assert len(unlimited_dim_names) < 2