Backport PR #27946: BUG: Merge with readonly arrays (#27948)

Author: meeseeksmachine

doc/source/whatsnew/v0.25.1.rst

@@ -127,9 +127,9 @@ Reshaping
 ^^^^^^^^^
 
 - A ``KeyError`` is now raised if ``.unstack()`` is called on a :class:`Series` or :class:`DataFrame` with a flat :class:`Index` passing a name which is not the correct one (:issue:`18303`)
--  Bug in :meth:`DataFrame.crosstab` when ``margins`` set to ``True`` and ``normalize`` is not ``False``, an error is raised. (:issue:`27500`)
+- Bug in :meth:`DataFrame.crosstab` when ``margins`` set to ``True`` and ``normalize`` is not ``False``, an error is raised. (:issue:`27500`)
 - :meth:`DataFrame.join` now suppresses the ``FutureWarning`` when the sort parameter is specified (:issue:`21952`)
--
+- Bug in :meth:`DataFrame.join` raising with readonly arrays (:issue:`27943`)
 
 Sparse
 ^^^^^^

pandas/_libs/hashtable.pyx

@@ -108,7 +108,7 @@ cdef class Int64Factorizer:
     def get_count(self):
         return self.count
 
-    def factorize(self, int64_t[:] values, sort=False,
+    def factorize(self, const int64_t[:] values, sort=False,
                   na_sentinel=-1, na_value=None):
         """
         Factorize values with nans replaced by na_sentinel

pandas/tests/reshape/merge/test_merge.py

@@ -1340,6 +1340,18 @@ def test_merge_take_missing_values_from_index_of_other_dtype(self):
         expected = expected.reindex(columns=["a", "key", "b"])
         tm.assert_frame_equal(result, expected)
 
+    def test_merge_readonly(self):
+        # https://github.com/pandas-dev/pandas/issues/27943
+        data1 = pd.DataFrame(
+            np.arange(20).reshape((4, 5)) + 1, columns=["a", "b", "c", "d", "e"]
+        )
+        data2 = pd.DataFrame(
+            np.arange(20).reshape((5, 4)) + 1, columns=["a", "b", "x", "y"]
+        )
+
+        data1._data.blocks[0].values.flags.writeable = False
+        data1.merge(data2)  # no error
+
 
 def _check_merge(x, y):
     for how in ["inner", "left", "outer"]: