Fix aggregated_by mdtol/masked constant problem (#4246)

* add failing test

* pass test

* whatsnew

* remove redundant copy call

docs/src/whatsnew/dev.rst

@@ -45,6 +45,10 @@ This document explains the changes made to Iris for this release
 #. `@rcomer`_ enabled passing of scalar objects to :func:`~iris.plot.plot` and 
    :func:`~iris.plot.scatter`. (:pull:`4616`)
 
+#. `@rcomer`_ fixed :meth:`~iris.cube.Cube.aggregated_by` with `mdtol` for 1D
+   cubes where an aggregated section is entirely masked, reported at
+   :issue:`3190`.  (:pull:`4246`)
+
 
 💣 Incompatible Changes
 =======================

lib/iris/analysis/__init__.py

@@ -582,7 +582,11 @@ def aggregate(self, data, axis, **kwargs):
         mdtol = kwargs.pop("mdtol", None)
 
         result = self.call_func(data, axis=axis, **kwargs)
-        if mdtol is not None and ma.isMaskedArray(data):
+        if (
+            mdtol is not None
+            and ma.is_masked(data)
+            and result is not ma.masked
+        ):
             fraction_not_missing = data.count(axis=axis) / data.shape[axis]
             mask_update = 1 - mdtol > fraction_not_missing
             if ma.isMaskedArray(result):

lib/iris/tests/unit/analysis/test_Aggregator.py

@@ -156,6 +156,19 @@ def test_unmasked(self):
             self.assertArrayAlmostEqual(result, mock_return.copy())
         mock_method.assert_called_once_with(data, axis=axis)
 
+    def test_allmasked_1D_with_mdtol(self):
+        data = ma.masked_all((3,))
+        axis = 0
+        mdtol = 0.5
+        mock_return = ma.masked
+        with mock.patch.object(
+            self.TEST, "call_func", return_value=mock_return
+        ) as mock_method:
+            result = self.TEST.aggregate(data, axis, mdtol=mdtol)
+
+        self.assertIs(result, mock_return)
+        mock_method.assert_called_once_with(data, axis=axis)
+
     def test_returning_scalar_mdtol(self):
         # Test the case when the data aggregation function returns a scalar and
         # turns it into a masked array.