bpo-40882: Fix a memory leak in SharedMemory on Windows (GH-20684)

In multiprocessing.shared_memory.SharedMemory(), the temporary view
returned by MapViewOfFile() should be unmapped when it is no longer
needed.
(cherry picked from commit 85c128e)

Co-authored-by: Zackery Spytz <zspytz@gmail.com>

Lib/multiprocessing/shared_memory.py

@@ -173,7 +173,10 @@ def __init__(self, name=None, create=False, size=0):
                     )
                 finally:
                     _winapi.CloseHandle(h_map)
-                size = _winapi.VirtualQuerySize(p_buf)
+                try:
+                    size = _winapi.VirtualQuerySize(p_buf)
+                finally:
+                    _winapi.UnmapViewOfFile(p_buf)
                 self._mmap = mmap.mmap(-1, size, tagname=name)
 
         self._size = size

Misc/NEWS.d/next/Windows/2020-06-06-15-10-37.bpo-40882.UvNbdj.rst

@@ -0,0 +1,2 @@
+Fix a memory leak in :class:`multiprocessing.shared_memory.SharedMemory` on
+Windows.

Modules/_winapi.c

@@ -1402,6 +1402,30 @@ _winapi_MapViewOfFile_impl(PyObject *module, HANDLE file_map,
     return address;
 }
 
+/*[clinic input]
+_winapi.UnmapViewOfFile
+
+    address: LPCVOID
+    /
+[clinic start generated code]*/
+
+static PyObject *
+_winapi_UnmapViewOfFile_impl(PyObject *module, LPCVOID address)
+/*[clinic end generated code: output=4f7e18ac75d19744 input=8c4b6119ad9288a3]*/
+{
+    BOOL success;
+
+    Py_BEGIN_ALLOW_THREADS
+    success = UnmapViewOfFile(address);
+    Py_END_ALLOW_THREADS
+
+    if (!success) {
+        return PyErr_SetFromWindowsErr(0);
+    }
+
+    Py_RETURN_NONE;
+}
+
 /*[clinic input]
 _winapi.OpenFileMapping -> HANDLE
 
@@ -2095,6 +2119,7 @@ static PyMethodDef winapi_functions[] = {
     _WINAPI_READFILE_METHODDEF
     _WINAPI_SETNAMEDPIPEHANDLESTATE_METHODDEF
     _WINAPI_TERMINATEPROCESS_METHODDEF
+    _WINAPI_UNMAPVIEWOFFILE_METHODDEF
     _WINAPI_VIRTUALQUERYSIZE_METHODDEF
     _WINAPI_WAITNAMEDPIPE_METHODDEF
     _WINAPI_WAITFORMULTIPLEOBJECTS_METHODDEF