gh-87595: mmap.size() now returns the size on Unix for anonymous memory

Doc/library/mmap.rst

@@ -281,6 +281,11 @@ To map anonymous memory, -1 should be passed as the fileno along with the length
       Return the length of the file, which can be larger than the size of the
       memory-mapped area.
 
+      .. versionchanged:: 3.10
+         For a mapping of anonymous memory, the size is now returned on both
+         Unix and Windows. Previously, the size would be returned on Windows
+         and an :exc:`OSError` would be raised on Unix.
+
 
    .. method:: tell()
 

Doc/whatsnew/3.10.rst

@@ -1098,6 +1098,12 @@ Changes in the Python API
   also inherits the current builtins.
   (Contributed by Victor Stinner in :issue:`42990`.)
 
+* The :meth:`~mmap.mmap.size` method of the :class:`mmap.mmap` class
+  now returns the size of an anonymous mapping on both Unix and Windows.
+  Previously, the size would be returned on Windows and an :exc:`OSError` would
+  be raised on Unix.
+  (Contributed by Zackery Spytz in :issue:`43429`.)
+
 CPython bytecode changes
 ========================
 

Lib/test/test_mmap.py

@@ -405,6 +405,7 @@ def test_anonymous(self):
             b = x & 0xff
             m[x] = b
             self.assertEqual(m[x], b)
+        self.assertEqual(m.size(), PAGESIZE)
 
     def test_read_all(self):
         m = mmap.mmap(-1, 16)

Misc/NEWS.d/next/Library/2021-03-07-16-31-36.bpo-43429.Koa0mf.rst

@@ -0,0 +1,4 @@
+The :meth:`~mmap.mmap.size` method of the :class:`mmap.mmap` class now
+returns the size of an anonymous mapping on both Unix and Windows.
+Previously, the size would be returned on Windows and an :exc:`OSError`
+would be raised on Unix.

Modules/mmapmodule.c

@@ -458,6 +458,9 @@ mmap_size_method(mmap_object *self,
 
 #ifdef UNIX
     {
+        if (self->fd == -1) {
+            return PyLong_FromSsize_t(self->size);
+        }
         struct _Py_stat_struct status;
         if (_Py_fstat(self->fd, &status) == -1)
             return NULL;