Fix crash on memoryview self-assignments (GH-3874)

* Fixed crash on memoryview self-assignment
* Rename memoryview decref function to match what it does

Cython/Compiler/FusedNode.py

@@ -414,7 +414,7 @@ class FusedCFuncDefNode(StatListNode):
                 if itemsize == -1 or itemsize == {{sizeof_dtype}}:
                     memslice = {{coerce_from_py_func}}(arg, 0)
                     if memslice.memview:
-                        __PYX_XDEC_MEMVIEW(&memslice, 1)
+                        __PYX_XCLEAR_MEMVIEW(&memslice, 1)
                         # print 'found a match for the buffer through format parsing'
                         %s
                         break
@@ -493,7 +493,7 @@ class FusedCFuncDefNode(StatListNode):
                 ctypedef struct {{memviewslice_cname}}:
                     void *memview
 
-                void __PYX_XDEC_MEMVIEW({{memviewslice_cname}} *, int have_gil)
+                void __PYX_XCLEAR_MEMVIEW({{memviewslice_cname}} *, int have_gil)
                 bint __pyx_memoryview_check(object)
             """)
 

Cython/Compiler/MemoryView.py

@@ -100,6 +100,12 @@ def put_acquire_memoryviewslice(lhs_cname, lhs_type, lhs_pos, rhs, code,
 
 def put_assign_to_memviewslice(lhs_cname, rhs, rhs_cname, memviewslicetype, code,
                                have_gil=False, first_assignment=False):
+    if lhs_cname == rhs_cname:
+        # self assignment is tricky because memoryview xdecref clears the memoryview
+        # thus invalidating both sides of the assignment. Therefore make it actually do nothing
+        code.putln("/* memoryview self assignment no-op */")
+        return
+
     if not first_assignment:
         code.put_xdecref(lhs_cname, memviewslicetype,
                          have_gil=have_gil)

Cython/Compiler/ModuleNode.py

@@ -1841,7 +1841,7 @@ class ModuleNode(Nodes.Node, Nodes.BlockNode):
             code.putln("Py_CLEAR(p->%s.obj);" % entry.cname)
 
         if cclass_entry.cname == '__pyx_memoryviewslice':
-            code.putln("__PYX_XDEC_MEMVIEW(&p->from_slice, 1);")
+            code.putln("__PYX_XCLEAR_MEMVIEW(&p->from_slice, 1);")
 
         code.putln("return 0;")
         code.putln("}")

Cython/Compiler/PyrexTypes.py

@@ -1099,7 +1099,7 @@ class MemoryViewSliceType(PyrexType):
     # decref however did look to always apply for memoryview slices
     # with "have_gil" set to True by default
     def generate_xdecref(self, code, cname, nanny, have_gil):
-        code.putln("__PYX_XDEC_MEMVIEW(&%s, %d);" % (cname, int(have_gil)))
+        code.putln("__PYX_XCLEAR_MEMVIEW(&%s, %d);" % (cname, int(have_gil)))
 
     def generate_decref(self, code, cname, nanny, have_gil):
         # Fall back to xdecref since we don't care to have a separate decref version for this.

Cython/Utility/MemoryView.pyx

@@ -56,7 +56,7 @@ cdef extern from *:
         Py_ssize_t suboffsets[{{max_dims}}]
 
     void __PYX_INC_MEMVIEW({{memviewslice_name}} *memslice, int have_gil)
-    void __PYX_XDEC_MEMVIEW({{memviewslice_name}} *memslice, int have_gil)
+    void __PYX_XCLEAR_MEMVIEW({{memviewslice_name}} *memslice, int have_gil)
 
     ctypedef struct __pyx_buffer "Py_buffer":
         PyObject *obj
@@ -987,7 +987,7 @@ cdef class _memoryviewslice(memoryview):
     cdef int (*to_dtype_func)(char *, object) except 0
 
     def __dealloc__(self):
-        __PYX_XDEC_MEMVIEW(&self.from_slice, 1)
+        __PYX_XCLEAR_MEMVIEW(&self.from_slice, 1)
 
     cdef convert_item_to_object(self, char *itemp):
         if self.to_object_func != NULL:

Cython/Utility/MemoryView_C.c

@@ -113,9 +113,9 @@ static CYTHON_INLINE int __pyx_sub_acquisition_count_locked(
 #define __pyx_get_slice_count_pointer(memview) (memview->acquisition_count_aligned_p)
 #define __pyx_get_slice_count(memview) (*__pyx_get_slice_count_pointer(memview))
 #define __PYX_INC_MEMVIEW(slice, have_gil) __Pyx_INC_MEMVIEW(slice, have_gil, __LINE__)
-#define __PYX_XDEC_MEMVIEW(slice, have_gil) __Pyx_XDEC_MEMVIEW(slice, have_gil, __LINE__)
+#define __PYX_XCLEAR_MEMVIEW(slice, have_gil) __Pyx_XCLEAR_MEMVIEW(slice, have_gil, __LINE__)
 static CYTHON_INLINE void __Pyx_INC_MEMVIEW({{memviewslice_name}} *, int, int);
-static CYTHON_INLINE void __Pyx_XDEC_MEMVIEW({{memviewslice_name}} *, int, int);
+static CYTHON_INLINE void __Pyx_XCLEAR_MEMVIEW({{memviewslice_name}} *, int, int);
 
 
 /////////////// MemviewSliceIndex.proto ///////////////
@@ -512,7 +512,7 @@ __Pyx_INC_MEMVIEW({{memviewslice_name}} *memslice, int have_gil, int lineno)
     }
 }
 
-static CYTHON_INLINE void __Pyx_XDEC_MEMVIEW({{memviewslice_name}} *memslice,
+static CYTHON_INLINE void __Pyx_XCLEAR_MEMVIEW({{memviewslice_name}} *memslice,
                                              int have_gil, int lineno) {
     __pyx_atomic_int_type old_acquisition_count;
     struct {{memview_struct_name}} *memview = memslice->memview;

tests/memoryview/memoryview.pyx

@@ -157,6 +157,7 @@ def assignmvs():
     cdef int[:] mv3
     mv1 = array((10,), itemsize=sizeof(int), format='i')
     mv2 = mv1
+    mv1 = mv1
     mv1 = mv2
     mv3 = mv2