Add initial test for alignment

test/tst_alignment.py

@@ -0,0 +1,70 @@
+from netCDF4 import set_alignment, get_alignment, Dataset
+import netCDF4
+import tempfile, unittest, os
+import numpy as np
+
+libversion= tuple(int(v) for v in netCDF4.__netcdf4libversion__.split('.'))
+has_alignment = (libversion[0] > 4) or (
+    libversion[0] == 4 and (libversion[1] >= 9)
+)
+
+file_name = tempfile.NamedTemporaryFile(suffix='.nc', delete=False).name
+
+def is_aligned(dataset, offset=4096):
+    # Here we check if the dataset is aligned
+    return dataset.id.get_offset() % offset == 0
+
+class AlignmentTestCase(unittest.TestCase):
+    def setUp(self):
+        self.file = file_name
+
+        # This is a global variable in netcdf4, it must be set before File
+        # creation
+        if has_alignment:
+            set_alignment(1024, 4096)
+            assert get_alignment() == (1024, 4096)
+
+        f = Dataset(self.file, 'w')
+        f.createDimension('x', 4096)
+        # Create many datasets so that we decrease the chance of
+        # the dataset being randomly aligned
+        for i in range(10):
+            f.createVariable(f'data{i:02d}', np.float64, ('x',))
+            v = f.variables[f'data{i:02d}']
+            v[...] = 0
+        f.close()
+
+    def test_version_settings(self):
+        if has_alignment:
+            # One should always be able to set the alignment to 0, 0
+            set_alignment(0, 0)
+            assert get_alignment() == (0, 0)
+        else:
+            with self.assertRaises(RunetimeError):
+                set_alignment(0, 0)
+            with self.assertRaises(RunetimeError):
+                get_alignment(0, 0)
+
+
+    def test_setting_alignment(self):
+        # if we have no support for alignment, we have no guarantees on
+        # how the data can be aligned
+        if not has_alignment:
+            return
+
+        # TODO: ensure that the underlying alignment is set. but I'm not sure
+        # how to do this with netcdf
+        # https://github.com/h5py/h5py/pull/2040/files?diff=unified&w=0#diff-3166eca28ff7f5d816f07f37eaba428b4351077384d65a2630e7d85c1284698fR7
+        import h5py
+        with h5py.File(self.file, 'r') as h5file:
+            for i in range(10):
+                v = h5file[f'data{i:02d}']
+                assert is_aligned(v, offset=4096)
+
+
+    def tearDown(self):
+        # Remove the temporary files
+        os.remove(self.file)
+
+if __name__ == '__main__':
+    unittest.main()