Improve handling of masks in concatenate (#6187)

* Improve handling of masks

* Add whatsnew

docs/src/whatsnew/latest.rst

@@ -82,7 +82,7 @@ This document explains the changes made to Iris for this release
    may happen when there are very many or large auxiliary coordinates, derived
    coordinates, cell measures, or ancillary variables to be checked that span
    the concatenation axis. This issue can be avoided by disabling the
-   problematic check. (:pull:`5926`)
+   problematic check. (:pull:`5926` and :pull:`6187`)
 
 🔥 Deprecations
 ===============

lib/iris/_concatenate.py

@@ -310,7 +310,7 @@ def _hash_ndarray(a: np.ndarray) -> np.ndarray:
 
     # Hash the bytes representing the array data.
     hash.update(b"data=")
-    if isinstance(a, np.ma.MaskedArray):
+    if np.ma.is_masked(a):
         # Hash only the unmasked data
         hash.update(a.compressed().tobytes())
         # Hash the mask

lib/iris/tests/unit/concatenate/test_hashing.py

@@ -21,6 +21,8 @@
         (np.array([np.nan, 1.0]), np.array([np.nan, 1.0]), True),
         (np.ma.array([1, 2], mask=[0, 1]), np.ma.array([1, 2], mask=[0, 1]), True),
         (np.ma.array([1, 2], mask=[0, 1]), np.ma.array([1, 2], mask=[0, 0]), False),
+        (np.ma.array([1, 2], mask=[1, 1]), np.ma.array([1, 2], mask=[1, 1]), True),
+        (np.ma.array([1, 2], mask=[0, 0]), np.ma.array([1, 2], mask=[0, 0]), True),
         (da.arange(6).reshape((2, 3)), da.arange(6, chunks=1).reshape((2, 3)), True),
         (da.arange(20, chunks=1), da.arange(20, chunks=2), True),
         (
@@ -33,6 +35,21 @@
             da.ma.masked_array([1, 3], mask=[0, 1]),
             True,
         ),
+        (
+            np.arange(2),
+            da.ma.masked_array(np.arange(2), mask=[0, 0]),
+            True,
+        ),
+        (
+            np.arange(2),
+            da.ma.masked_array(np.arange(2), mask=[0, 1]),
+            False,
+        ),
+        (
+            da.ma.masked_array(np.arange(10), mask=np.zeros(10)),
+            da.ma.masked_array(np.arange(10), mask=np.ma.nomask),
+            True,
+        ),
         (
             np.ma.array([1, 2], mask=[0, 1]),
             np.ma.array([1, 3], mask=[0, 1], fill_value=10),