bpo-45621: Small changes to mmap (GH-29247)

* Small tidy-ups / comments
* Use randomized names when testing tagged mmaps to avoid any risk of parallel tests treading on each others' toes

Modules/mmapmodule.c

@@ -32,7 +32,6 @@
 
 #ifdef MS_WINDOWS
 #include <windows.h>
-#include <winternl.h>
 static int
 my_getpagesize(void)
 {
@@ -505,21 +504,6 @@ mmap_resize_method(mmap_object *self,
     }
 
     {
-        /*
-        To resize an mmap on Windows:
-
-        - Close the existing mapping
-        - If the mapping is backed to a named file:
-            unmap the view, clear the data, and resize the file
-            If the file can't be resized (eg because it has other mapped references
-            to it) then let the mapping be recreated at the original size and set
-            an error code so an exception will be raised.
-        - Create a new mapping of the relevant size to the same file
-        - Map a new view of the resized file
-        - If the mapping is backed by the pagefile:
-            copy any previous data into the new mapped area
-            unmap the original view which will release the memory
-        */
 #ifdef MS_WINDOWS
         DWORD error = 0, file_resize_error = 0;
         char* old_data = self->data;
@@ -567,6 +551,11 @@ mmap_resize_method(mmap_object *self,
             self->tagname);
 
         error = GetLastError();
+        /* ERROR_ALREADY_EXISTS implies that between our closing the handle above and
+        calling CreateFileMapping here, someone's created a different mapping with
+        the same name. There's nothing we can usefully do so we invalidate our
+        mapping and error out.
+        */
         if (error == ERROR_ALREADY_EXISTS) {
             CloseHandle(self->map_handle);
             self->map_handle = NULL;