Error on memoryview argument capture on 0.29.x (GH-4849)

I don't believe it's easy to fix
https://github.com/cython/cython/issues/4798 on 0.29.x Therefore,
generate an error message that explains two possible workarounds.

This at least makes sure that people don't end up with mysterious crashes.

2 files changed, 33 insertions, 2 deletions

diff --git a/Cython/Compiler/Nodes.py b/Cython/Compiler/Nodes.py
index 339b1fa04..743d6959b 100644
--- a/Cython/Compiler/Nodes.py
+++ b/Cython/Compiler/Nodes.py
@@ -2637,8 +2637,11 @@ class CFuncDefNode(FuncDefNode):
         def put_into_closure(entry):
             if entry.in_closure and not arg.default:
                 code.putln('%s = %s;' % (entry.cname, entry.original_cname))
-                code.put_var_incref(entry)
-                code.put_var_giveref(entry)
+                if entry.type.is_memoryviewslice:
+                    code.put_incref_memoryviewslice(entry.cname, have_gil=True)
+                else:
+                    code.put_var_incref(entry)
+                    code.put_var_giveref(entry)
         for arg in self.args:
             put_into_closure(scope.lookup_here(arg.name))
 
@@ -3234,6 +3237,14 @@ class DefNode(FuncDefNode):
         # Move arguments into closure if required
         def put_into_closure(entry):
             if entry.in_closure:
+                if entry.type.is_memoryviewslice:
+                    error(
+                        self.pos,
+                        "Referring to a memoryview typed argument directly in a nested closure function "
+                        "is not supported in Cython 0.x. "
+                        "Either upgrade to Cython 3, or assign the argument to a local variable "
+                        "and use that in the nested function."
+                    )
                 code.putln('%s = %s;' % (entry.cname, entry.original_cname))
                 if entry.xdecref_cleanup:
                     # mostly applies to the starstar arg - this can sometimes be NULL
diff --git a/tests/memoryview/memslice.pyx b/tests/memoryview/memslice.pyx
index 24af61e17..ccf760c21 100644
--- a/tests/memoryview/memslice.pyx
+++ b/tests/memoryview/memslice.pyx
@@ -2549,3 +2549,23 @@ def test_const_buffer(const int[:] a):
     cdef const int[:] c = a
     print(a[0])
     print(c[-1])
+
+cdef arg_in_closure_cdef(int [:] a):
+    def inner():
+        return (a[0], a[1])
+    return inner
+
+def test_arg_in_closure_cdef(a):
+    """
+    >>> A = IntMockBuffer("A", range(6), shape=(6,))
+    >>> inner = test_arg_in_closure_cdef(A)
+    acquired A
+    >>> inner()
+    (0, 1)
+
+    The assignment below is just to avoid printing what was collected
+    >>> del inner; ignore_me = gc.collect()
+    released A
+    """
+    return arg_in_closure_cdef(a)
+